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The Clear Creek mine, San Benito County, California: a unique mercury locality.

From the first discovery in 1959 until the present, the abandoned Clear Creek mine has yielded an exceptional suite of rare and unusual mercury-bearing minerals unequaled in the world. Among those recently described as new are aurivilliusite, clearcreekite, deanesmithite, edgarbaileyite, edoylerite, hanawaltite, peterbaylissite, wattersite, szymanskiite, tedhadleyite and vasilyevite. In addition, seven new mercury-bearing species are currently under study. Other mercury-bearing minerals identified from the mineralized veins include montroydite, schuetteite, calomel, gianellaite, mosesite, terlinguaite, eglestonite, metacinnabar, cinnabar, native mercury and the second world occurrence of donharrisite.

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INTRODUCTION

The Clear Creek mercury mine has produced some of the rarest and most exotic mercury-bearing minerals known. Once worked for its native mercury and cinnabar, this small, elongated mineralized zone has produced, to date, 29 mercury-bearing minerals, the majority of which are new to the mineral science. This wealth of rare mercury-bearing minerals prompted our study of this old, long-abandoned mine to better understand its complex mineralogy, geochemistry and paragenesis.

LOCATION

One of several mines in the old Flint Group, the Clear Creek mine is located in the Clear Creek drainage area of the New Idria mining district, San Benito County, California. This district is one of the oldest and best known mining districts in California, a state famous for its mercury deposits. The exact location of the Clear Creek mine is 120[degrees]44'12" W. and 36[degrees]23'3" N., or E1/2 of the SW1/4 of section 2, T. 18S., R. 11E., Mount Diablo Meridian, on the Idria 7 1/2-minute Quadrangle map produced by the U.S. Geological Survey. The mine workings are located in the barren hills above the north bank of Clear Creek, at an elevation of approximately 1235 meters.

Three distinct workings comprise the Clear Creek mine. The northernmost workings, which now consist of an elongated pit, is the Clear Creek mine proper and is referred to here as the "upper workings." About 300 meters south of this upper mineralized zone are the "middle workings," a small group of cliff faces and pits. Further below this locality are the southernmost or "lower workings," often referred to as the Clear Creek claim (120[degrees]43'58" W. and 36[degrees]22'59" N., or in the NW 1/4 NE 1/4 sec. 11, T. 18S., R. 11E.), approximately 100 meters south of, and 100 meters lower in elevation than, the upper workings. It is at these southernmost workings that the majority of the recently described rare mercury minerals occur. For this paper, the name "Clear Creek mine" is used to describe all three mineralized areas collectively.

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HISTORY

Mining History

Historical accounts of the mines in the Clear Creek drainage area of the New Idria mining district are given by Bradley (1918), Eckel and Myers (1946) and Linn (1968). In a district as old as New Idria, it is not surprising that many mines and prospects have changed names several times and their production records have been lost or were never recorded.

The New Idria mercury deposit, destined to become second only to the New Almaden deposit amongst North American mercury producers, was discovered about 1853 by two Mexican prospectors who had first explored a small body of chromite in the mistaken belief that it was silver ore. They also mistook the cinnabar in the New Idria outcrops for a silver mineral. Its true identity was soon learned and the first mercury was produced in 1854, though not by the original discoverers. Beginning with the recognition of cinnabar at New Idria in 1853, nearly a score of smaller mercury-bearing deposits were discovered in the Clear Creek area to the southwest.

These smaller deposits were worked by Mexican miners using hand tools and small-charge explosives. Where practical, the ore was processed at each mine in crude retorts fired by local timber. The expelled mercury vapors were then condensed by air or water cooling, depending on the availability of water. The influence of the early Mexican miners in the district is revealed by the names of these mines, which include the Anita, Chiquita, Del Mexico, Don Juan, Don Miguel, Esmeralda, Hernandez, Picacho, Mexican, Monterey, Ramirez, Rita, Santa Margarita, San Benito and Spanish. Most of these mines were later renamed or merged with other mines, and are currently idle and (in most cases) abandoned. At present no mercury mining is being conducted in the district.

The mercury-bearing zones in the Clear Creek area south of New Idria are situated along a northwest-southeast trend of randomly exposed silica-carbonate rock, and include the Clear Creek, Red Rock, Alpine, Fourth of July, Andy Johnson and Picacho mines. Most of these bodies are very shallow and nearly all the ore produced was taken from open cuts or boulders on the surface. Eckel and Myers (1946) state that the shallowness of the orebodies, and the failure of several extensive exploratory adits to encounter anything but unaltered serpentinite, are due to the fact that the shear zone dips to the southwest at low angles. Many of the tabular outcrops thus represent discontinuous remnants perched on hill slopes. The Clear Creek mine is situated within the northwestern part of this prominent zone of ocherous, silicified rock which strikes approximately north 30[degrees] west.

Records for the Flint Group show the discovery as pre-1880 (Bradley, 1918), with the last year of recorded production as 1942. Following this date, no records of production have been published. Operation of these mines has been highly sporadic, and production was minor over the history of the district. Linn (1968) estimates that the total combined production of all these Flint Group mines is less than 10,000 flasks of mercury (one flask contains 34.5 Kg of mercury).

In 1955, the Clear Creek mine was renamed the Morning Star mine by a Mr. Nepper, a tool sharpener employed at the New Idria mine. During the following ten years or so, he removed the overburden above the two adits by bulldozing along the mineralized zone. With the aid of several employees, he selectively sorted the ore and transported it to New Idria for processing. It is reported that the ore he removed resulted in the recovery of about 100 flasks of mercury. During this same period of time, Nepper also worked the Alpine mine, which can be seen across Clear Creek canyon at about the same elevation as the Clear Creek mine.

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The Clear Creek mine was described by Bradley (1918) as being operated through two tunnels with about 200 meters of total workings. Today, the upper workings consist of an elongate open pit 20 meters wide, 10 meters deep and about 100 meters long. Examination of the remaining mine tailings indicate that the original workings consisted of two obliquely intersecting tunnels at different elevations. The intersection point of the tunnels was apparently at the center of the orebody. Few remnants of the original retort remain. There is no evidence of ore processing at the middle workings and all that remains of the retort at the lower workings is a small burnt-ore pile and a few bricks.

Numerous other small, unnamed, surface workings and scattered piles of retorted ore are visible along the ore trend from the Clear Creek mine down slope to Clear Creek proper. Most of these early workings were of the "pick and shovel" type and produced little ore.

Collecting History

The history of mineral collecting at the Clear Creek mine and the surrounding areas extends back to at least 1959 with the discovery by the late Mr. Edward H. Oyler of a mercury mineral that eventually was named in his honor, edoylerite. Mr. Oyler collected extensively in this area for over twenty-five years. In addition to edoylerite, other newly described minerals first collected from the Clear Creek mine by Mr. Oyler include clearcreekite, deanesmithite, edgarbaileyite, hanawaltite, peterbaylissite, szymanskiite, wattersite and a mineral similar to edoylerite that is under study.

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Because these newly described mercury-bearing minerals are very scarce within the thin quartz veins, field identification is tenuous at best, due to their diverse habits, size and colors. The rarer mercury-bearing minerals are best identified by X-ray powder-diffraction methods because each species has a unique diffraction pattern, even though many share very similar megascopic properties. For example, edgarbaileyite occurs as massive lumps, spheroids, thin coatings and complex angular aggregates of minute crystals with colors ranging from bright yellow through dark green to black. This entire color variation can often be seen in a single sample. Like edgarbaileyite, montroydite can also exhibit various habits and colors ranging from thick bladed crystals to thin hair-like needles in colors ranging from pale yellow to deep reddish brown to black.

Our interest in the Clear Creek mine was initiated in 1990 by the description of edgarbaileyite, the first of the rare mercury-bearing minerals that was formally described. Early trips to the area yielded small, rich hand-samples containing a plethora of varicolored mercury mineralization. Extensive macroscopic and energy-dispersive spectrometry (EDS) examinations were used to make the initial identification of the more common mercury minerals. As additional collecting was done, we became more familiar with the diverse habits and colors displayed by these minerals. Because of this diversity, X-ray powder-diffraction utilizing Debye-Scherrer cameras was the principal method used for identification, and this further aided subsequent macroscopic identification. As our experience grew, several unidentified phases were recognized, resulting in the discovery of an additional nine new mercury-bearing phases that are currently under study or have been described.

To best observe the mercury mineralization within the large silicified field rocks, which are often quite hard, they must first be reduced by sledgehammer, followed by further reduction in volume using a hydraulic wedge breaker. This is followed by carefully opening the thin mercury-bearing quartz veins using a small trimmer. These final samples, which range in size from two to four centimeters, are then examined under a fiber-optic illuminated binocular microscope using up to 40X magnification. The freshly exposed surfaces often contain as many as six different mercury minerals, ranging in habit from individual crystals to subhedral and anhedral masses, and also intimate mixtures displaying various colors. Also, along these thin quartz veins small voids can be encountered which individually can host different associations of mercury minerals. The resulting identified minerals are then placed in individual plastic boxes to prevent damage and to reduce the potential danger from exposure to native mercury and its vapors. Those minerals that are known to be light-sensitive are put in black plastic boxes.

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GEOLOGICAL SETTING

The geological features of the New Idria serpentinite body and surrounding formations have been described by Eckel and Myers (1946), Linn (1968), Fox (1983), Studemeister (1984), and Coleman (1986). This large, elongate, tectonically emplaced serpentinite body, which measures about 19 km long by 6 km wide by 5 km deep, intrudes parts of the Panoche Formation (Upper Cretaceous) and the Franciscan Complex (late Mesozoic to early Tertiary), and forms the core of a large anticline that lies between the San Andreas fault and Coast Ranges to the west and the Great Valley to the east.

Tertiary- and Mesozoic-aged sedimentary rocks surrounding the serpentinite body have been folded into a series of anticlines and synclines whose axes form acute angles with the northwest trending San Andreas fault. The contact of the serpentinite with the surrounding sediments is marked by high-angle faults and shear zones that record the upward tectonic movement of the New Idria serpentinite body. The structural position of the New Idria serpentinite body suggests that it may represent serpentinized peridotites that originally made up part of the Late Jurassic-aged Pacific oceanic crust. Post Jurassic-aged subduction of the peridotites emplaced them under the growing Franciscan accretionary wedge. Following serpentinization, a northwest-southeast fault formed within the serpentinite at depth. This fault zone, which now lies nearly parallel to the boundary of the western edge of the serpentinite body with the Franciscan Complex, was then silicified by fluids along a trend from what is now the Clear Creek mine at the northern end to the Picacho mine at the southern end.

Studemeister (1984) suggested that prograde metamorphism of Mesozoic-aged sediments released mercury-bearing fluids that migrated along fractures in the silicified serpentinite. He stated that these sediments were the principal source for the mercury-bearing fluids in the majority of mercury mines within the New Idria district, although he presented no geochemical evidence.

MINERALOGY

Besides the accessory minerals normally encountered in serpentinite, the silica-carbonate replacement rocks host a number of rare mercury-bearing minerals, most of which are unique to the Clear Creek mine. This uniqueness is the principal reason for our extensive examination of the mine and its mineralogy.

Examination of the mercury-bearing rocks at the nearby Red Rock, Fourth of July, and Andy Johnson mines has revealed only native mercury and cinnabar. Rare coatings of one of two new unknown mercury silicate minerals were discovered at the Picacho mine; edgarbaileyite has been identified with cinnabar from the Alpine mine (Ed Oyler, personal communication).

Mercury Minerals

Aurivilliusite [Hg.sup.1+][Hg.sup.2+]OI

A subhedral-shaped thin sub-metallic patch coating quartz collected from the lower workings was determined to contain two new mercury-bearing phases by XRD tests. This first phase, aurivilliusite, was formally described by Roberts et al. (2003a); it is intimately associated with CCUK-15, a similar mercury oxy-halide.

Aurivilliusite is dark gray-black with a dark red-brown streak. During work on this sample, aurivilliusite was subsequently identified with the synthetic compound [Hg.sup.1+][Hg.sup.2+]OI by comparison of unit cell and X-ray powder diffraction data. This natural phase is extremely rare at this locality, with only one confirmed specimen known. The artificial compound was synthesized by Stalhandske et al. (1985) at 450[degrees]C from a mixture of HgO and [Hg.sub.2][I.sub.2].

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The aurivilliusite structure consists of infinite zigzag -[Hg.sup.2+]-O-[Hg.sup.2+] chains, running in the [010] direction, which are condensed by [[Hg.sup.1+]-[Hg.sup.1+]] groups to form folded layers. These layers are further connected by weak [Hg.sup.2+]-O bonds to form a three-dimensional structure. The iodine atoms are situated within cavities in the layers and are weakly bonded to [Hg.sup.2+].

Calomel [Hg.sub.2.sup.1+][Cl.sub.2]

Calomel is very rare at the Clear Creek mine, and has been found only in small amounts at the lower workings. There it occurs as massive coatings and as clear to pale yellow striated crystals associated with native mercury, montroydite, cinnabar and, very rarely, hanawaltite. It can be distinguished by its red fluorescence under short-wave ultraviolet light. The crystals usually do not exceed 0.1 mm and are often much smaller. On some specimens, calomel shows evidence of dissolution, resulting in a frosted appearance.

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Cinnabar HgS

Cinnabar is the major ore of mercury at the Clear Creek mine. It occurs mainly as massive vein-fillings with quartz in the highly silicified serpentinite (lower workings), or as films along shear surfaces in opalized serpentinite (upper workings). Euhedral to subhedral crystals are uncommon in the thicker parts of the veins. Rare acicular hairs of cinnabar occur in open cavities along some quartz seams. Most crystals of cinnabar show typical forms, although very unusual growth habits (loops, rings and curved forms) have been observed. In the upper and middle workings, cinnabar is confined to the fracture surfaces of the low-grade opalized serpentinite, and is often associated with pods of native mercury and rare edgarbaileyite, wattersite and CCUK-8, a hydrous mercury chromate-sulfide similar to edoylerite in composition.

In the rocks of the lower workings, which have been extensively silicified, cinnabar is often localized within quartz veins, although usually it is confined to the silica-carbonate-quartz margins. In parts of these veins, cinnabar often shows the effects of chemical dissolution by post-depositional low pH hydrothermal solutions. This has resulted in the formation of several unique and rare mercury minerals, including deanesmithite, edoylerite and CCUK-8. Cinnabar and pecoraite have been observed as alteration products of rare donharrisite associated with contemporary cinnabar in samples from the lower workings.

Clearcreekite [Hg.sub.3.sup.1+](C[O.sub.3])(OH) * 2[H.sub.2]O

The very rare hydrous mercury carbonate mineral, clearcreekite (Roberts et al., 2001), occurs as two small crystals with a pale greenish-yellow color that were discovered from the lower workings of the Clear Creek mine by Edward H. Oyler. They are subhedral, tabular {100} with a maximum dimension of 0.17 mm and generally are similar in habit to terlinguaite crystals found at the locality. Associated minerals on the sample include edoylerite and cinnabar, although these are not near the clearcreekite crystals. Crystal structure data indicates that it is polymorphous with peterbaylissite but that the two structures are very different.

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Deanesmithite [Hg.sub.2.sup.1+][Hg.sub.3.sup.2+]([Cr.sup.6+][O.sub.4])O[S.sub.2]

Deanesmithite was discovered in 1988 by Edward H. Oyler from a float rock at the lower workings of the Clear Creek mine. Megascopically, it resembles montroydite but is much redder in color. A preliminary X-ray powder-diffraction examination proved this mineral to be new. Its full description has been reported by Roberts et al. (1993). Szymanski and Groat (1997) have shown that the crystal structure of deanesmithite exhibits numerous similarities to the structure of wattersite.

Deanesmithite is generally associated with edoylerite and anhedral crystalline masses and powdery coatings of cinnabar. The crystals occur as fan-shaped elongate [001] prismatic aggregates up to 1 mm long, and as isolated bladed to tabular [010] aggregates of crystals. Forms reported by Roberts et al. (1993) include {100}, {310}, {001}, {510}, {011}, {010}, {210}, {310}, {410}, {510}, {610}, {110}, {320}, {210}, {023}, {032}, and {101}.

When fresh, it is orange-red and is distinctive against the deep red cinnabar. On weathering, it appears brown, with a submetallic luster. Fine radiating clusters of deanesmithite have also been identified in cavities within the silica-carbonate host rock, where the carbonate minerals have been leached by acidic fluids. In several samples, fine sprays of edoylerite have been identified in adjacent leached cavities. Deanesmithite rarely has been observed coated by CCUK-8.

Donharrisite [Ni.sub.8][Hg.sub.3][S.sub.9]

Donharrisite occurs as rare bronze metallic masses associated with cinnabar along thin veins in the silica-carbonate rocks. One specific instance showed it together with cinnabar cutting across a quartz vein devoid of mercury mineralization. Textural observations suggest that donharrisite formed prior to or contemporaneously with cinnabar but from the same sulfide-rich fluids. Rims of rare secondary cinnabar and pecoraite surround several of the partially oxidized donharrisite masses.

Donharrisite was suspected after an EDS examination and confirmed by X-ray powder-diffraction; it represents the second world occurrence. This rare sulfide of mercury and nickel was initially characterized by Paar et al. (1989) from a museum sample collected before 1834, probably from the former Erasmus mine, Leogang, Salzburg Province, Austria. There, it is associated with cinnabar, native mercury, sphalerite, tennantite, chalcopyrite, polydymite and pyrite.

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Edgarbaileyite [Hg.sub.6.sup.1+][Si.sub.2][O.sub.7]

Edgarbaileyite, the first known natural silicate of mercury, was discovered by Leo Rosenhahn in 1963 at the Socrates mine (type locality), Sonoma County, California and described by Roberts et al., 1990a. In 1972. Edward H. Oyler collected similar-looking material from the lower workings of the Clear Creek mine and this was submitted for X-ray identification. It was found to be identical to the material from the Socrates mine. In addition, the same mineral was identified by the late Dr. Alan J. Criddle on a Terlingua, Texas specimen in The Natural History Museum, London, England. The mineral also has been identified from the Alpine mine, just south of the Clear Creek mine, associated with cinnabar, montroydite and native mercury in quartz-lined cavities.

Angel et al. (1990) reported the crystal structure of edgarbaileyite from the Texas locality and established the stoichiometry from the structure determination.

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At the Clear Creek mine, edgarbaileyite occurs at all three workings as cryptocrystalline masses, velvety spheres, and rarely as complex aggregates of twinned platy microcrystals. The mineral is extremely variable in color. Freshly exposed material is lemon-yellow to orangish-yellow, and exposed surfaces are varicolored from dark olive-green, to a lighter yellowish-green, to a dark green-brown grading into black. Very fine nodular masses occur lining quartz cavities in the lower workings. It is generally associated with most of the described mercury-bearing minerals, especially native mercury, wattersite, montroydite and etched cinnabar. In some specimens edgarbaileyite has been observed covering the linings of microcavities, where it has been subsequently covered by quartz and then by a second generation of edgarbaileyite. At the upper workings, it is often seen as a smooth coating over highly corroded cinnabar and is associated with small spheres of native mercury.

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Edoylerite [Hg.sub.3.sup.2+]([Cr.sup.6+][O.sub.4])[S.sub.2]

Edoylerite was discovered by Edward H. Oyler from the lower workings of the Clear Creek mine and described by Erd et al. (1993). It typically occurs as acicular to stellate, and as equant prismatic crystals on and around corroded masses of cinnabar. Crystal forms reported by Erd et al. (1993) include {010}, {111}, {001}, and {101}. It also has been found as isolated groups of divergent, acicular crystals lining cavities in porous silica-carbonate rock associated with individual crystals of second-generation cinnabar. Occasionally, it is associated with deanesmithite and postdates it in the same micro-environment. Other close associations include native mercury, montroydite and edgarbaileyite. The color ranges from canary yellow to orange-yellow as the crystals become thicker. It can also have a slight greenish color in some of the more massive material.

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At both the lower and upper workings, it can be found intimately associated with CCUK-8, a hydrous mercury chromate sulfide with similar composition, but edoylerite always precedes it in formation. Superb specimens have been discovered at the lower workings and are very attractive, especially when associated with water-clear quartz crystals.

The crystal structure of edoylerite has been reported by Burns (1999) and contains three symmetrically distinct [Hg.sup.2+] sites, each of which is strongly bonded to two S anions to form approximately linear S-Hg-S clusters. These clusters link to form crankshaft-type chains composed of eight-membered rings of alternating [Hg.sup.2+] and S that are parallel to [101]. [Hg.sup.2+] cations are weakly bonded to additional anions, resulting in distorted-octahedral coordinations in two cases and a pentagonal bipyramidal coordination in the other case. The [Hg.sup.2+] polyhedral link to each other and to Cr[O.sub.4] tetrahedra by sharing of polyhedron edges and corners, resulting in a heteropolyhedral framework structure.

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Eglestonite [Hg.sub.6.sup.1+][Cl.sub.3]O(OH)

Eglestonite occurs as deeply striated cubic and dodecahedral crystals with a blocky habit, and as thinly elongated spears with apparent twinning. The color ranges from deep lemon-yellow to orange-yellow to black, with the latter especially evident in thicker sections. Upon exposure to light, eglestonite may darken to black, though it is often that color in fresh specimens. It is most easily recognized by its greasy luster. Eglestonite is most often associated with edgarbaileyite, native mercury, cinnabar and wattersite coating quartz cavities. Slender crystals often protrude through spheres of native mercury or occur as free crystals extending across spherical cavities in quartz. Mereiter et al. (1992) have recently revised the chemical formula for eglestonite.

Gianellaite [Hg.sub.4.sup.2+][N.sub.2](S[O.sub.4])

Massive gianellaite has been identified from the lower workings by X-ray powder-diffraction and microchemical methods. It has been observed as isolated masses to 0.08 mm within quartz vein cavities, and is associated with terlinguaite and cinnabar. It is yellow to greenish-yellow to black upon exposure to light. Although the composition and crystal structure of gianellaite are similar to that of mosesite, the two minerals have not been observed together. The mineral was first described by Tunell et al. (1977) from the Mariposa mine, Terlingua district, Brewster County, Texas, where it is associated with terlinguaite, calomel, montroydite, native mercury and cinnabar.

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Hanawaltite [Hg.sub.6.sup.1+][Hg.sup.2+][[Cl.sub.1.47](OH)[.sub.0.55]][O.sub.3]

The specimen on which hanawaltite was originally discovered was collected in 1962 by Edward H. Oyler and was identified as a new mineral in 1991. Its description and crystal structure formally reported by Roberts et al. (1996). This holotype specimen was collected from a float rock that was situated approximately 15 meters west of the original edoylerite discovery. Hanawaltite resembles wattersite and varies from black to very dark brown-black. The hanawaltite-bearing area on the original sample measures 8 X 2 mm and is located on a quartz-rich fracture. The mineral occurs as isolated subhedral crystals that are intimately associated with greenish-brown resinous calomel, globules of native mercury, cinnabar and varicolored montroydite.

Two additional hanawaltite samples were discovered from the lower workings during our study. The host rock is a brecciated silica-carbonate rock composed principally of ferroan magnesite and quartz. One of these two samples was used to improve the R index and the standard deviations of the atomic positions. The revised results are reported by Grice (1999).

The hanawaltite structure has two distinct layers; a [Hg.sup.1+]-Cl-O layer consisting of ribbons of [Hg-Hg][.sup.1+] dimers resembling the configuration in calomel structure and a [Hg.sup.2+]-[Hg.sup.1+]-Cl-O layer with alternating mercurous and mercuric oxychloride chains.

Mercury Hg

Bright silvery droplets and coatings of native mercury fill cavities and veins throughout the mine, often in volumes exceeding 1 ml. It is generally associated with cinnabar, edgarbaileyite, eglestonite, montroydite, mosesite, szymanskiite and wattersite. As native mercury is highly mobile, precise paragenetic relations can be difficult to establish.

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Most native mercury has been found either filling isolated cavities in quartz veins or associated with cinnabar as vein fillings. This native mercury shows a strong bond with the quartz that is difficult to break. When a native mercury-containing cavity is split open, quartz fragments will also adhere to the mercury globules. However, when purified mercury is placed in one of these cavities it will not adhere to the quartz. This strong affinity for quartz is not unique to Clear Creek native mercury, and has been observed at other localities. In many instances, individual spheres of mercury show surface oxidation colors that range from pale gray to bronze. Historically, the Clear Creek mine was noted for its abundance of native mercury.

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Metacinnabar Hg(S,Se)

Black coatings of metacinnabar, some of it selenian, have been identified at the lower workings by XRD where it is associated with cinnabar. It is very uncommon at the deposit, in marked contrast to the New Idria mine, where it is more abundant.

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Montroydite HgO

Montroydite is a common mineral at all of the workings. It occurs as spear-shaped crystals and hair-like mats of needles that are usually flattened, as radiating groups of crystals and as blocky irregularly-shaped masses of fine-grained crystals. The color ranges from reddish-brown to bronze, yellowish-brown, pale yellow, reddish-orange and black. It is most commonly observed covering or attached to wattersite, native mercury, edgarbaileyite, eglestonite and cinnabar. Occasionally, it is found as hard shells that are epimorphous after native mercury.

Mosesite [Hg.sub.2.sup.2+]N(Cl,I,Br) * [H.sub.2]O

Mosesite is a very rare mineral and is found only at the lower workings. It most often occurs as a coating on clear quartz crystals within closed cavities associated with native mercury and CCUK-10, a mercury nitrogen-halide. Crystals are rare but, when encountered, show the forms {001}, {012}, {112} and either the forms {114} or {116}. Most crystals are complex intergrowths. Canfield (1913) observed the forms {001}, {111}, {116}, {114} and {112} on crystals from Terlingua, Texas. Bird (1932) observed {011} on crystals from Nevada. Other anions substitute readily for chloride in mosesite, as discussed by Switzer et al. (1953). The substitution of [Cl.sup.-] [right arrow] 1/2(S[O.sub.4])[.sup.2-] gives the sulfate-dominant analog gianellaite (see above). EDS analysis of the deep red "mosesite" shows iodide prevalent over chloride and bromide. Srinath et al. (1951) have shown that Millons's base ([Hg.sub.2]NOH * 2[H.sub.2]O) was easily converted to a series of ion-exchange compounds, including [Hg.sub.2]NI. This implies that the different anion compositions of mosesite-like minerals may either reflect fluid compositions at the time of growth or subsequent ion exchange. Rouse (1975) reports additional data for mosesite which includes a supercell with a = 28.8 [Angstrom], a factor of 8 times larger than that of the cubic cell. There is also a second subcell with a/12 = 2.38 [Angstrom].

Peterbaylissite [Hg.sub.3.sup.1+](C[O.sub.3])(OH) * 2[H.sub.2]O

This basic carbonate of mercury (which is chemically identical to clearcreekite) was described by Roberts et al. (1995) from a single sample collected by Edward H. Oyler in 1960 and later identified as a new mineral in 1985. Peterbaylissite occurs both as isolated and clustered crystals on a secondary yellow-brown crust composed of ferroan magnesite and quartz. Individual crystals range in size from 20 to 200 microns. The crystals are subhedral to euhedral, and are somewhat elongate, possessing a wedge-like shape. The mineral is opaque and black to very dark red-brown with a dark brown-black streak. Associated mercury-bearing minerals on the holotype sample are cinnabar, metacinnabar and native mercury. Roberts et al. (1995) state that peterbaylissite probably formed as an alteration product of pre-existing mercury minerals such as cinnabar.

Schuetteite [Hg.sub.3.sup.2+](S[O.sub.4])[O.sub.2]

A single specimen of schuetteite-bearing vein material was discovered with small individual cinnabar masses, some of which have partially been replaced with gray CCUK-14, a rare mercury silicate. Dispersed between, and sometimes surrounding these individual cinnabar masses are orange-yellow masses of microcrystalline schuetteite. The mineral's identity was confirmed by X-ray powder-diffraction methods. Bailey et al. (1959) have described a number of schuetteite occurrences in the western United States and give a thorough description and discussion of the origin of schuetteite.

Szymanskiite [Hg.sub.16.sup.1+](Ni, Mg)[.sub.6](C[O.sub.3])[.sub.12](OH)[.sub.12]([H.sub.3]O)[.sub.8.sup.1+] * 3[H.sub.2]O

Szymanskiite was described by Roberts et al. (1990b) from a specimen discovered at the lower workings of the Clear Creek mine. It was first encountered by Richard C. Erd during megascopic examination and routine X-ray powder-diffraction characterization of mercury-bearing minerals collected in 1972 by Mr. Oyler. Initially, the mineral was thought to be a small spray of stibnite in a tiny vug of a hand specimen composed principally of blackish chert and white, well-crystallized quartz. After part of this spray was crushed for confirmatory X-ray analysis, it was found to have a core of a clear, light sky-blue crystalline material. The mineral is surrounded but not overgrown by massive, amber edgarbaileyite, which in turn has mm-size needles of red montroydite perched on it.

Individual crystals of szymanskiite are acicular to prismatic, euhedral to subhedral. They are elongate [0001], with striations parallel to [0001] on {10[bar.1]0}. Forms observed are {10[bar.1]0} and {0001}

Szymanskiite, as sky-blue, small anhedral grains and acicular, terminated hexagonal crystals, has been recently discovered on quartz in silicified serpentinite cavities from the upper workings, and is closely associated with native mercury, pecoraite and edgarbaileyite. A small cinnabar-lined cavity containing two crystalline masses of szymanskiite was also discovered in the lower workings in 1996.

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The crystal structure of szymanskiite was solved by Szymanski and Roberts (1990). The structure contains [Hg.sup.1+] in near-linear coordination of -O-Hg-Hg-O, as well as a second chain containing [Hg.sup.1+] atoms and trios of oxygen atoms in a face-sharing column of the form -Hg-[O.sub.3]-Hg-Hg-[O.sub.3]-Hg. The ordered part of the structure contains these mercury coordination chains, as well as (Ni,Mg)-O distorted octahedra, held together by carbonate groups. Between the clearly defined tubular walls of the structure, there are very large tunnels within which the remaining portion of the structure is disordered. The structure can be regarded as a non-silicate zeolite.

Tedhadleyite [Hg.sup.2+][Hg.sub.10.sup.1+][O.sub.4][I.sub.2]([Cl.sub.1.2][Br.sub.0.8])

Tedhadleyite, formally described by Roberts et al. (2002), is the first of the new mercury oxy-halide minerals rich in iodide discovered during this study. It was discovered in 1994 during a routine X-ray powder diffraction of a submitted sample containing an elongated dark brown material in a quartz cavity from the lower workings. Initial EDS showed major mercury and iodine with lesser amounts of chlorine and bromine. It is an extremely rare constituent at the Clear Creek claim and occurs as a single spheroidal hollow mass that appears to be pseudomorphous after native mercury.

A paper describing the crystal structure of tedhadleyite is in preparation. Preliminary aspects of the structure consist of strong covalently bonded O-Hg-O and O-Hg-Hg-O groups and complex slabs parallel to (010), with halogen sites midway between adjacent slabs. Square rings consisting of four [Hg-Hg][.sup.2+] dimers are joined to four more rings via two types of linkage: O-Hg-O and O-Hg-Hg-O (Mark A. Cooper, personal communication, 2003).

Terlinguaite [Hg.sup.1+][Hg.sup.2+]OCl

Terlinguaite is very rare in the deposit and only a few specimens have been identified. It generally occurs as dark-brown masses of subhedral crystals, although at least two euhedral, prismatic crystals have been found associated with eglestonite and montroydite in the lower workings. Under the microscope it is difficult to identify when it is mixed with montroydite.

Vasilyevite [Hg.sub.20.sup.1+][O.sub.6][I.sub.3]([Br.sub.1.6][Cl.sub.1.4])[(C[O.sub.3])[.sub.0.8][S.sub.0.2]]

Vasilyevite is an extremely rare constituent at the Clear Creek claim and has been formally described by Roberts et al. (2003b). To date this somewhat inconspicuous mineral has been identified on five micromount specimens that were collected in the mid-1990's and occurs within less than 5.0 mm-sized or less quartz-lined microcavities in centermeter-sized quartz veins. The cavities containing the new mineral are typically monomineralic. In four of the micromounts, vasilyevite occurs as anhedral, almost cryptocrystalline, masses, less than 0.5 mm in size. It is silvery-gray to black to dark red-black with a red-brown streak. Outward form and color can be easily mistaken for either tedhadleyite or aurivilliusite, both of which are extremely rare constituents at the mine. It is opaque and brittle, with an adamantine to metallic luster, and is nonfluorescent.

Cooper and Hawthorne (2003) have determined the crystal structure for vasilyevite. Other mercury minerals with structures most resembling that of vasilyevite are poyarkovite, [[Hg.sub.2]]OCl, and eglestonite, [[Hg.sub.2]][.sub.3][Cl.sub.3][O.sub.2]H, although the additional topological complexity of vasilyevite arises from the fact that polymerization in the c direction involves a crankshaft configuration rather than a linear motif.

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Wattersite [Hg.sub.4.sup.1+][Hg.sup.2+]([Cr.sup.6+][O.sub.4])[O.sub.2]

This mineral was identified as a new species by Richard C. Erd on specimens collected by Edward H. Oyler from the lower workings of the Clear Creek mine and formally described by Roberts et al. (1991). Wattersite is commonly associated with native mercury, edgarbaileyite, montroydite and eglestonite. It is one of the few minerals from the mine that is well crystallized, occurring as sharp crystals with a bright metallic luster, often as single or multiple groups. Individual crystals are dark reddish-brown, but masses tend to appear black. Deep-red internal reflections are characteristic of wattersite, especially when seen on thin edges.

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Wattersite has been identified in three different forms: (1) as thin shell-like aggregates in vugs; (2) as deeply etched crystals on cinnabar, coated with CCUK-8; and (3), as small discrete crystals or crystal aggregates within cavities along mineralized quartz veins. It has been observed attached to edgarbaileyite and associated with both native mercury and montroydite, and to a lesser extent, cinnabar. Individual crystals are prismatic and elongate parallel to [001]. Twinning is common and is found as simple contact twinning on {100}. The twin axis is [001]. Roberts et al. (1991) reported sixteen different crystal forms for wattersite: {110}, {010}, {310}, {130}, {021}, {100}, {101}, {403}, {011}, {342}, {311}, {001}, {111}, {742}, {112}, and {312}.

Groat et al. (1995) have described the crystal structure of wattersite. The structure may be visualized as consisting of edge-sharing Hg octahedra with projecting Cr[O.sub.4] tetrahedra forming zigzag infinite chains of composition [HgCr[O.sub.6]][.sup.6-] that extend parallel to [001]. These chains are linked by O-Hg bonds to the Hg-Hg dimers, which form gently modulated sheets parallel to (100).

[FIGURE 44 OMITTED]

Wattersite also has been identified from the Challenge deposit, near Emerald Lake, southwest of Redwood City. San Mateo County, California (R. C. Erd, personal communication, 1997).

CCUK-8 [Hg.sup.2+]-Cr[O.sub.4]-S-n[H.sub.2]O?

CCUK-8, a new, undescribed hydrous mercury chromate-sulfide, occurs as powdery lemon-yellow massive crusts or as micron-sized, prismatic crystalline aggregates and masses. Poorly developed subhedral to euhedral crystals, up to 0.1 mm long and less than 1 micron in cross section, have recently been found associated with cinnabar from the upper workings of the Clear Creek mine. Some of the larger crystals appear turbid, suggesting partial dehydration. The luster of the fine hair-like crystals is always dull. The mineral is often associated with edoylerite and dull (etched) cinnabar. It also has been found in association with etched wattersite at the upper workings. CCUK-8 seems to be one of the last mercury minerals to have formed during the chromate-rich hydrothermal event. The majority of CCUK-8 at the Clear Creek mine is probably of the dehydrated variety.

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This mineral also has been identified from the Vaughn mine, east of Hollister, San Benito County, California, where it occurs associated with cinnabar in a hydrothermally altered serpentinite (R. C. Erd, personal communication, 1997).

CCUK-10 Hg-N-I-(Cl, Br)

This very rare mercury nitrogen-halide mineral was initially discovered by the late John L. Parnau (1906-1990) in the 1960's following field collecting from the lower workings. Unfortunately, this specimen was never submitted for identification but has been preserved in a private collection. During 1993, our collecting efforts revealed additional samples from a quartz vein in float rock, situated about 10 meters west of the original edoylerite discovery at the lower workings. This mineral is almost always associated with CCUK-18, the iodine analog of mosesite, but postdates it. It varies from bright yellow through various shades of yellow-orange to dark red-orange. The mineral has orthorhombic symmetry and a very large cell volume. It is found both as simple terminated crystals with an acicular radiating habit filling the spaces between clear quartz crystals and as fine acicular, slightly flattened crystals lining voids in massive quartz. Initial X-ray crystal structure data suggest that the mineral possesses a very complex structure.

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CCUK-12 Hg-O-I-(Cl, Br)

This very rare mercury-bearing oxy-halide, designated CCUK-12, was collected in 1994 from the lower workings. It occurs as pale green, spear-shaped crystals associated with montroydite on a single specimen. Since insufficient material is available for complete characterization, no further work has been attempted on it as yet. Its X-ray powder-diffraction pattern does not match any mercury-bearing phase known to date; the structure is c-centered monoclinic. Preliminary EDS tests confirm the presence of iodine, chlorine and bromine.

CCUK-13 Hg silicate

A light pink unknown silicate of mercury has been observed as a microcrystalline coating (or replacement) on cinnabar associated with edgarbaileyite and montroydite from the lower workings. On one sample, it is coated by schuetteite. The cinnabar associated with CCUK-13 shows the effects of etching by low-pH hydrothermal fluids. A preliminary EDS analysis, using a light-element detector, shows only mercury, silicon and oxygen. The X-ray diffraction-pattern does not match that of any mercury-bearing phase known to date. Only a few specimens are known.

CCUK-14 Hg silicate

A second unknown silicate of mercury was recently discovered as a thin gray film replacing cinnabar and rarely coating CCUK-8 on cinnabar at the lower workings. On one specimen it is closely associated with schuetteite. It also has been identified at the Picacho mine, about 3.5 km to the south, replacing cinnabar on a single quartz specimen. The X-ray diffraction-patterns for both localities are identical and match no mercury-bearing phase known to date. No crystals so far have been discovered. A preliminary EDS analysis indicated only mercury, silicon and oxygen. Only a few specimens are known. The extremely fine-grained nature of the mineral precludes any attempt to further characterize it at this time.

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CCUK-15 [Hg.sub.10.sup.1+][Hg.sub.3.sup.2+][O.sub.6][I.sub.2](Cl, Br)[.sub.2]

This very rare mercury oxy-halide was discovered as an inclusion during the X-ray powder diffraction study of the now approved mineral, aurivilliusite (Roberts et al., 2003a). Both minerals are closely associated with native mercury, cinnabar and edgarbaileyite on quartz. CCUK-15 is megascopically indistinguishable from aurivilliusite, tedhadleyite, and vasilyevite. In general, CCUK-15 most closely resembles a metallic sulfide or sulphosalt in appearance. It is opaque with a metallic luster, brittle, has an uneven fracture and is non-fluorescent under both long- and short-wave ultraviolet light. The provisional formula is based on quantitative electron-microprobe analyses.

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CCUK-18 [Hg.sub.2.sup.2+]N(I, Cl, Br) * [H.sub.2]O

This unknown has the X-ray powder diffraction pattern closely resembling mosesite but chemically is rich in I by EDS. It is generally found closely associated with CCUK-10 or isolated on the surfaces of quartz seams. Divergent deep red crystal groups that show a growth habit along one axis have been observed in quartz cavities. Initial X-ray diffraction patterns attempted during structure work showed severe streaking, indicative of inhomogeneous strain of the structure and/or short-range ordering, presumably due to the large size of the I atom. Annealing of the sample with increasing temperatures failed to improve the diffraction pattern.

[FIGURE 55 OMITTED]

Associated Minerals

The majority of the accessory minerals were identified by Richard C. Erd using powder-diffraction and optical methods during the years prior to our involvement in the study of the Clear Creek mine.

Barite BaS[O.sub.4]

Rare crystals of barite were found associated with cinnabar at the lower workings. The barite seems to be of late-stage origin.

Calcite CaC[O.sub.3]

Calcite has been identified by powder-diffraction methods from the silica-carbonate host rock of the Clear Creek mine.

Dolomite CaMg(C[O.sub.3])[.sub.2]

Dolomite is a common constituent of the silica-carbonate rock at the Clear Creek mine.

Eskolaite [Cr.sub.2][O.sub.3]

Eskolaite occurs as massive green alteration halos around magnesiochromite grains in the silicified serpentinite rock margins with the silica-filled fractures. No crystals have been noted.

Goethite FeO(OH)

Minute spherical groups of goethite have been identified from the lower workings.

Gypsum CaS[O.sub.4] * 2[H.sub.2]O

Small amounts of gypsum associated with huntite have been identified in the silica-carbonate rock of the Clear Creek mine.

Hematite [Fe.sub.2][O.sub.3]

Rare hematite pseudomorphs after pyrite occur on quartz from the lower workings. The interpenetrating cubes are coated with both massive and crystals of cinnabar, indicating their position in the paragenesis.

Huntite Ca[Mg.sub.3](C[O.sub.3])[.sub.4]

Huntite has been identified on magnesite from the Clear Creek mine associated with gypsum.

Hydromagnesite [Mg.sub.5](C[O.sub.3])[.sub.4](OH)[.sub.2] * 4[H.sub.2]O

Hydromagnesite is a common constituent of the serpentinite surrounding the mercury-mineralized silica-carbonate rock.

Jarosite [K.sub.2][Fe.sub.6](S[O.sub.4])[.sub.4](OH)[.sub.12]

Massive coatings of yellowish-brown jarosite have been identified from the lower workings by XRD. The mineral is not common and is confined to those rocks that are in the more acid environment.

Magnesiochromite (Mg,Fe)[Cr.sub.2][O.sub.4]

Ferroan magnesiochromite is common both as anhedral masses and as octahedral crystals in the serpentinite. It is jet black and occasionally weakly magnetic. Many of the grains have been tectonically fractured and filled with cinnabar, but no visible reaction rims have been identified. Hydrothermal fluids have altered the mineral to eskolaite, which, upon further oxidation, has provided the chromate that formed the rare mercury-bearing chromates.

Magnesite (Mg,Fe)C[O.sub.3]

Ferroan magnesite occurs abundantly as white to light-brown porcelainous masses above the upper workings on the ridge and also as a major constituent of the silica-carbonate rock in the area.

Magnetite [Fe.sub.3][O.sub.4]

Some pyrite grains in silicified serpentinite matrix were observed to be rimmed or replaced by a black submetallic mineral. EDS analysis confirmed the sulfide as pyrite, and the black mineral to be an Fe oxide devoid of Mg or Cr. The black mineral was magnetic. Magnetite was presumably formed as an early product of pyrite oxidation, and the hematite pseudomorphs also observed may have been produced by further oxidation of precursor magnetite.

Melanophlogite Si[O.sub.2] + stabilizing gas molecules

Melanophlogite is a clathrate polymorph of silica stabilized by molecules of gases such as [N.sub.2], C[O.sub.2] and C[H.sub.4] (Nakagawa et al., 2001) trapped within the crystal structure. It has been identified only from float boulders in the open cut of the upper workings (Dunning and Cooper, 2002). It occurs both as clear brilliant cubes and as small barrel-shaped aggregates pseudomorphic after an unknown mineral. It occurs principally coating open spaces in silicified serpentinite. No modifying forms have been identified so far. The crystals range in size up to 1 mm along an edge, and interpenetrating growths are common, although they are not as common as at other California localities. The mineral has partly inverted to chalcedony. Several small specimens have been recovered from rocks in the bottom of the open cut.

Millerite NiS

Millerite is ubiquitous but rare in the area and occurs as capillary crystals in quartz.

Montmorillonite (Na,Ca)[.sub.0.3](Al,Mg)[.sub.2][Si.sub.4][O.sub.10](OH)[.sub.2] * n[H.sub.2]O

Fine-grained montmorillonite has been identified by XRD from both the upper and lower workings.

Nimite (Ni,Mg,Fe)[.sub.5]Al([Si.sub.3]Al)[O.sub.10](OH)[.sub.8]

Nimite, a nickel chlorite, has been identified in the silica-carbonate rocks of the Clear Creek mine.

Opal Si[O.sub.2] * n[H.sub.2]O

Opal occurs as vein-filling masses throughout the deposit, especially in the upper workings.

Pecoraite (Ni,Mg)[.sub.3][Si.sub.2][O.sub.5](OH)[.sub.4]

[FIGURE 56 OMITTED]

Light apple-green spheroidal masses and minute spear-shaped crystals of pecoraite occur sparingly in the upper and lower workings, usually attached to quartz or cinnabar. It is generally associated with native mercury, edgarbaileyite and szymanskiite, and apparently formed by alteration of Ni-bearing sulfides (millerite and donharrisite), with which it is closely associated.

Pyrite Fe[S.sub.2]

Pyrite is an uncommon mineral in the mine and occurs as tiny octahedrons, cubes and masses in veins of quartz. Hematite pseudomorphs after cubes of pyrite coated with cinnabar occur very rarely. It has been very rarely seen replacing magnetite in the silicified serpentinite.

Quartz Si[O.sub.2]

Quartz occurs as tiny vein-filling crystals, crystalline aggregates and chalcedonic masses. Some brown-white color-banded chalcedony has been found in the upper workings, as well as green botryoids with cinnabar inclusions which may be pseudomorphs after melanophlogite. The vein fillings are always small, and display several separate depositional episodes. The massive material is often pale-green and is highly fractured, with very thin layers of cinnabar lining fracture surfaces.

Reevesite [Ni.sub.6][Fe.sub.2](C[O.sub.3])(OH)[.sub.16] * 4[H.sub.2]O

Light yellowish-green flakes of reevesite occur within the serpentinite adjoining the silica-carbonate rock of the Clear Creek mine.

Rozenite FeS[O.sub.4] * 4[H.sub.2]O

White coatings of rare rozenite on cinnabar have been identified from the lower workings by XRD.

Sepiolite [Mg.sub.4][Si.sub.6][O.sub.15](OH)[.sub.2] * 6[H.sub.2]O

Fine-grained, off-white sepiolite has been identified from much of the silica carbonate rock at the lower workings. It was identified by XRD.

Sulfur S

Rare massive sulfur has been identified in cavities of the mercury-bearing samples. It is associated with cinnabar only from the lower workings.

Talc [Mg.sub.3][Si.sub.4][O.sub.10](OH)[.sub.2]

Talc has been identified by XRD from the silica carbonate rocks of the lower workings.

Todorokite (Mn,Ca,Mg)[Mn.sub.3][O.sub.7] * [H.sub.2]O

Black coatings of todorokite are common throughout the silica carbonate rocks at the upper and lower workings. It was identified by XRD.

PARAGENESIS

Post-Serpentinite Mercury Mineralization

The mercury deposits of the Clear Creek area occur in altered rocks that are clearly related to the New Idria serpentinite intrusive. These rocks were formed at depth by direct replacement of serpentinite at low temperature and high pressure by silica- and carbonate-rich hydrothermal fluids (Barnes et al., 1973) and were probably derived from silica in the lower marine sediments heated by Miocene-Pliocene-aged volcanics of the southern Diablo Range.

The silica-carbonate rocks contain variable amounts of coarsely crystalline quartz and its varietal forms, chalcedony and opal, with associated carbonate minerals (magnesite, dolomite, ankerite and calcite) and abundant microcrystals and grains of ferroan magnesiochromite and magnetite. The latter minerals formed within the serpentinite and remained in place during silicification of the host serpentinite. Initial mercury mineralization, in the form of cinnabar and perhaps minor native mercury, was confined to fracture networks within the silica-carbonate rock and along the softer contact margins of the partly altered serpentinite on either side of the highly silicified zone.

The main orebody of the Clear Creek mine (upper workings) consists of a partly silicified (opalized) green serpentinite composed of highly fractured meter-sized or larger blocks which contain thin veins of cinnabar associated with native mercury. Because of the low degree of silicification, the Hg-bearing fluids easily migrated along the shear planes of the opalized serpentinite, depositing the cinnabar and minor native mercury. Subsequent to this initial mineralization, small amounts of montroydite, eglestonite, wattersite, deanesmithite, edoylerite, edgarbaileyite, CCUK-8, and szymanskiite formed.

Although much smaller in size, the middle workings have exposed rocks very similar to those at the upper workings. Here, the opalized serpentinite rocks contain thin coatings and veins of cinnabar with isolated native mercury and small quantities of eglestonite and edgarbaileyite.

At the lower workings, the degree of silicification is much greater and consists of quartz interlayered with both ferroan magnesite and thin veins of opalized serpentinite. The silica-carbonate rock is characterized by a complex, highly developed fracture system. Cross fracturing is very common, with the fractures generally less than 10 mm wide. These fractures contain several generations of silica ranging from homogeneous chalcedony to banded chalcedony (agate) to drusy quartz. The chalcedony generally occurs along the center of the veins following the formation of earlier quartz. Based on extensive macroscopic examination of numerous field samples, mercury-bearing fluids initially deposited cinnabar and possibly native mercury, with minor metacinnabar and donharrisite along the margins of this fracture system. Mercury minerals identified from the lower workings that probably formed after the cinnabar include eglestonite, hanawaltite, peterbaylissite, clearcreekite, calomel, terlinguaite, wattersite, edoylerite, deanesmithite, szymanskiite, and seven unnamed minerals.

The difference in carbonate content of the mineralized host rocks between the upper and lower workings was determined to establish an approximate degree of silicification between the two locations. Analysis of typical hand samples of mineralized silica-carbonate rock from the upper workings gave an average of 30 weight percent carbonate. A somewhat lower average of 22 weight percent was determined for typical hand samples from the lower workings, representing a higher degree of silicification.

Paragenetic Sequence

Our initial picture of the Clear Creek deposit paragenesis was deduced from the various mineral associations observed, their locations along the veins, and the relationships between mineral species within the micro cavities.

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Establishing a paragenetic model for the Clear Creek mine was complicated by the diverse association of minerals in the numerous microvoids and thin lenticular veins on any give field sample. Also, the majority of mineralized rocks examined were found to be float material (lower workings) or rocks removed from the ore zone during mining operations (upper workings). The diverse assemblage of rare minerals identified at the lower workings has been found in isolated float rocks generally located near the bottom and southwestern portions of the outcrop. Rocks on the northern side of the outcrop contain only unaltered cinnabar and minor mercury. None of the newly described mercury minerals has been found in rocks exposed in situ despite extensive searching.

Three discrete phases of mineralization have been recognized from cross-cutting relationships. The first general episode recognized was a fracturing of the silica-carbonate rock which allowed the channeling of mercury-rich fluids and subsequent precipitation of primary cinnabar, with minor donharrisite, metacinnabar and probably native mercury along the fracture margins. Some cinnabar and possible native mercury also permeated into the lower grade silicified serpentinite wall margins. Generally, little silica accompanied this event.

Following this initial mercury mineralization, there was a second period of fracturing which resulted in cross and parallel fractures to the first set of fractures, with subsequent channeling of silica-rich fluids, with possible remobilization of earlier deposited mercury-bearing phases.

Later, a third tectonic event re-fractured the older silica-rich veins with yet more deposition of minor cinnabar and possibly native mercury. Localized chemical reaction of pre-existing cinnabar and native mercury with invasive fluids occurred along selected portions of the deposit. Some mercury from the cinnabar and mercury combined with cations and anions present in the fluids to form the many rare mercury minerals localized within the quartz veins and adjoining leached silica-carbonate rock.

When these mineralized silica veins are opened, they often reveal abundant native mercury and cinnabar, with minor edgarbaileyite, eglestonite, montroydite and wattersite filling irregular cavities. The most interesting aspect of these cavities is that each one often shows a different combination of minerals even though the cavities generally are separated by only a few millimeters. These diverse micro-environments were probably formed during solidification of the silica-rich veins and represent one of the unusual aspects of the deposit. The observed associations suggest that reaction of earlier mercury minerals occurred with one or more fluid events to produce different suites of secondary minerals in different cavities. Because of the diverging bulk compositions of the various cavities, the mercury mineral suite cannot be represented as a single paragenetic progression, but rather as a network of pathways in which a variety of fluids react with a range of possible mineral associations at each stage.

The earliest phase of sulfide mineralization appears to have consisted of sulfides millerite, cinnabar, donharrisite and metacinnabar along the quartz veins. There were at least three separate periods of cinnabar formation generally associated with quartz veining. Rare donharrisite is restricted to thin veins associated with cinnabar. Subsequent decomposition of donharrisite resulted in secondary cinnabar and pecoraite. There was no evidence that it formed during either of two later distinguishable cinnabar crystallization episodes.

Some native mercury may also have precipitated very early in the mineralization sequence, contemporaneously with cinnabar. However, most of the mercury was likely derived from primary cinnabar by subsequent disproportionation or oxidation.

The majority of the rare mercury minerals are likely rather late, low-temperature precipitates. These minerals include montroydite (e.g. shells pseudomorphing mercury droplets) and oxy-chlorides such as eglestonite and terlinguaite, which appear to be derived by oxidation of mercury and reaction with [OH.sup.-] and [Cl.sup.-]. Edoylerite, deanesmithite and CCUK-8 appear to have formed by direct oxidation and reaction of cinnabar with chromate-rich fluid. Some CCUK-8 is relatively late on the basis of its epimorphic relationship to deanesmithite and edoylerite. Reaction of the chromate-bearing fluid in mercury/montroydite-bearing cavities formed wattersite. Edgarbaileyite would have formed from mercury by reaction with silica; it occurs in several generations in some cavities. Additional edgarbaileyite, as well as CCUK-13 and CCUK-14, are the silicates that appear to have formed by reaction of silica with cinnabar. The nitrides mosesite and CCUK-18 are associated with mercury/montroydite but gianellaite with cinnabar, which suggests that an ammonium-bearing fluid also reacted with both metal/oxide cavities and with sulfide cavities.

The (Br,I)-free chloride minerals appear to be derived from mercury and/or montroydite. Hanawaltite shows evidence of being an alteration product of earlier calomel. These latter minerals and terlinguaite are rare in comparison with eglestonite. A different fluid, richer in iodide with lesser chloride and bromide, may have been implicated in forming the iodide-rich minerals aurivilliusite, tedhadleyite, vasilyevite, CCUK-12 and CCUK-15 from mercury/montroydite. The association of CCUK-10 and vasilyevite with mosesite and its I-dominant analog CCUK-18 suggest that the iodide fluid postdates the ammonium fluid, and that CCUK-18 may have formed from [Cl.sup.-] dominant mosesite by ion exchange as evidenced by red halos of CCUK-18 replacing mosesite on some samples. Particularly late-forming minerals include the sulfate schuetteite and associated silicates CCUK-13 and CCUK-14, which postdate cinnabar and CCUK-8 on edoylerite. The carbonates also appear to have been very late in the sequence since they have not been observed to be overgrown by any other mercury minerals.

DISCUSSION

Source Rocks of Mercury

Krupp (1988) states that the low geochemical abundance of mercury in most common rock types makes it necessary that large source volumes are available from which the element can be extracted in order to form an economic deposit. The underlying sedimentary rocks of the Panoche Formation (Upper Cretaceous) and the Franciscan Complex (late Mesozoic to early Tertiary) are considered to be the principal source for the mercury mineralization of the New Idria district (Wells and Ghiorso, 1988; Studemeister, 1984; Moiseyev, 1971; Linn, 1968). Limited data suggest that recent marine clays contain many times the mercury content of average igneous rocks (Aidin'yan et al., 1963). Krupp (1988) considers organic carbon-rich sediments, and in particular black shales, as having an order of magnitude higher mercury content than most other rock types that are frequently found in mercury ore districts. Such lithologies are likely to have been the source of the mercury at Clear Creek.

White (1957) and White and Roberson (1962) have suggested that mercury may be expelled from sedimentary rocks subjected to high temperatures. Abundant evidence indicates that mercury becomes concentrated in shallow epithermal deposits but is rare in mesothermal and hypothermal deposits except as broad geochemical halos that argue for its high mobility at elevated temperature (Barnes, 1967). Moiseyev (1971) presents a comparative study based on a non-genetic classification of mercury ore deposits from around the world and suggests that mercury is derived from sediments rather than from magmas and is mobilized by volcanic heat. Hence, the overall consensus is that Hg initially resides in sediments but is readily remobilized on heating due to burial or volcanic/tectonic activity. The transporting fluid could be interstitial connate water driven out by compaction, or metamorphic water released by reconstitution of hydrous minerals. Either would have been possible during the emplacement of the New Idria serpentinite body and its subsequent localized replacement by silica-rich solutions.

Mercury Transportation Mechanisms

The extreme insolubility of cinnabar and its higher-temperature polymorph metacinnabar is a potential difficulty in producing rich deposits of mercury minerals, since only vanishingly small amounts of mercury can remain dissolved in anoxic fluids that contain moderate sulfide concentrations. However, Dickson (1964) demonstrated that reduced solutions rich enough in sulfur to form sulfide complexes of mercury can dissolve appreciable amounts of mercury which can then be precipitated as cinnabar by cooling towards a solubility minimum at 100[degrees]C, or as a result of disassociation of the complexes on mixing with sulfur-poor waters. This is the most likely means by which mercury was introduced into the host rocks at Clear Creek, since the earliest mercury minerals to form appear to have all been sulfides: cinnabar or metastable metacinnabar, and donharrisite. The essential Ni content of donharrisite and the presence of mercury-free secondary Ni sulfides (millerite) suggest reaction between a fluid bearing Hg and S and a fluid containing Ni derived from the serpentinite. Alternative possibilities are that the mercury was transported in a less sulfidic or somewhat more oxidizing fluid as either the neutral atomic species or as organic complexes. Association of mercury mineralization with hydrocarbons is well known (Dickson and Tunell, 1968; White, 1981) and there are experimental data indicating that mercury is highly soluble in hydrocarbons (Okouchi and Sasake, 1981; Clever and Iwamoto, 1987).

[FIGURE 57 OMITTED]

According to Krupp (1988), the solubility of metallic mercury in aqueous solutions can exceed the previously calculated solubilities of mercury as sulfide complexes by a considerable margin. The calculations of Fein and Williams-Jones (1997) support this conclusion in the appropriate Eh range, but they go on to show that in the presence of organic compounds, such as alkanes, mercury-organic complexes are even more important. Another possible transport mode discussed by Fein and Williams-Jones (1997) is partitioning of mercury into the vapor phase. Since boiling is suppressed by pressure, this is not likely to be important for deepsourced fluids.

Karpatite (native coronene, [C.sub.24][H.sub.12]) occurs in several mercury mines in the district, and elsewhere in California mercury deposits, although not at the Clear Creek mine itself. At the Picacho and Andy Johnson mines, about 5.3 kilometers to the southeast along the same silica carbonate exposure, karpatite coexists with cinnabar and predates some quartz.

Melanophlogite does occur at Clear Creek, a mineral that requires some sort of stabilizing molecules such as C[O.sub.2], [N.sub.2], and C[H.sub.4] for its formation (Nakagawa et al., 2001). However, since melanophlogite is very rare and found only at the upper workings, the stabilizing volatiles may have been restricted in their occurrence. Cinnabar has been observed coating and replacing melanophlogite, consistent with other field evidence implying early crystallization of the melanophlogite (Dunning and Cooper, 2002).

These observations imply that organic-mediated or vapor transport may have operated at some times and particular locations at Clear Creek, but the association with cinnabar suggests that sulfide complexes remained the dominant transport mode.

Source of Reacting Species

In general, it is straightforward to identify the sources for the various cations and anions that reacted to produce the observed diversity of mercury minerals at Clear Creek. Si for the silicates would have been abundantly supplied from the silica-rich fluids, veins and silicified host rock. Carbonate-rich host rock would have supplied carbonate. The cations Ni and Mg could have been derived from either serpentinite (nickel sulfides) or secondary carbonates.

Within the silicified serpentinite margins adjacent to the quartz-rich veins are abundant masses and crystals of ferroan magnesiochromite that show dark green rims. This very fine-grained dark green material was presumed to be an alteration product of ferroan magnesiochromite and was subsequently identified as eskolaite by X-ray powder-diffraction methods. Reaction of the magnesiochromite with, for instance, a carbonate-rich fluid presumably leached away the Mg and Fe, leaving the less mobile chromium oxide as a residual mineral. Finely divided eskolaite is likely the source of chromium for the several colorful mercury chromates identified in the veins.

White (1967) has recorded small but significant levels of ammonia, chlorine, iodine and bromine in the associated spring waters of certain mercury deposits within the Diablo Range of the Franciscan Formation. Ancient marine brines trapped in the underlying oceanic sediments can be rich in chlorine, iodine and bromine and afford the source for these elements identified in the several mercury halides isolated at Clear Creek.

Organic material in sediments may have contributed ammonia required for the formation of the nitride minerals. Mercury cations are soft (Pearson, 1973) and have a strong affinity for iodine and bromine as well as for ligands such as ammonia or nitride. Relatively covalent compounds in which mercury bonds to these species tend to be low in solubility and more stable relative to oxycompounds than would be the case for a harder cation. Hence, these compounds occur as minerals at Clear Creek and other localities.

Thermochemistry and Conditions of Crystallization

The relative stabilities of the major minerals cinnabar, mercury and montroydite and some dissolved Hg complexes as a function of pH and redox potential (Eh) are well known (Krauskopf, 1951; Dickson, 1964; Barnes et al., 1967; Hepler and Olofsson, 1975; Khodakovskiy et al., 1975) and relationships at 25[degrees]C are well summarized for the major species by Eh-pH diagrams such as that in Brookins (1988). The data for cinnabar, mercury, montroydite and associated dissolved species were used as a starting point for understanding the conditions at which the other secondary minerals may have formed. Some results are summarized here of geochemical speciation calculations using additional data, which are reported in detail in a companion paper (in preparation).

At moderately acid to moderately alkaline conditions (pH = 4-10), predominant dissolved mercury species in the Hg-O-H-S system are typically sulfide complexes of [Hg.sup.2+] such as Hg[S.sub.2.sup.2+] and its protonated analogs under reducing conditions, neutral [Hg.sup.0] at intermediate redox potential, and [Hg.sup.2+], Hg[OH.sup.+] or Hg(OH)[.sub.2] under oxidizing conditions. Each of these regimes has an associated mineral--cinnabar (HgS), mercury (Hg), and montroydite (HgO) respectively--in which the bonding and oxidation state of Hg are similar to those of the predominant dissolved complexes. These minerals are abundant at Clear Creek relative to the more complex mercury species, are sparingly soluble to highly insoluble, and play a major role in determining the concentration of mercury that can be carried in solution. The crossover from the regime of mercury to that of montroydite occurs as a result of mercury oxidation with increasing electrode potential: [Hg.sup.0] [right arrow] [Hg.sup.2+] + 2[e.sup.-]. However, the transition from cinnabar to the neutral species with increasing Eh is a more complex disproportionation, involving reduction of the [Hg.sup.2+] in cinnabar combined with oxidation of the sulfide to sulfate: HgS + 8O[H.sup.-] [right arrow] Hg + S[O.sub.4.sup.2-] + 4[H.sub.2]O + 6[e.sup.-]. This happens at Eh very similar to that of the sulfide-sulfate couple in the Hg-free system, [S.sup.2-] + 8O[H.sup.-] [right arrow] S[O.sub.4.sup.2-] + 4[H.sub.2]O + 8[e.sup.-]. The existence of a stability field for the neutral metal at Eh higher than that for metal sulfide but lower than that for metal oxysalts is characteristic of extremely chalcophilic metals. The diagrams of Brookins (1988) show this behavior for Hg and Cu but not for the more lithophile Fe, Co and Ni. An important consequence of this phenomenon is that montroydite and cinnabar cannot coexist at thermodynamic equilibrium, and any observed association of these two is metastable. Coexistence is observed at Clear Creek, presumably due to the difficulty in dissolving cinnabar.

The redox relationships outlined above indicate that the general evolution of conditions in the Clear Creek mineralized zones involved (1) introduction of Hg in reduced, sulfidic fluids, forming cinnabar and minor donharrisite, (2) oxidative disproportionation of cinnabar to produce mercury and (3) further oxidation to produce montroydite. Montroydite dissolves in strongly acid or alkaline solutions, so the pH must have remained with a few units of neutrality to maintain montroydite stability.

In the absence of thermodynamic data for many of the rare minerals, speciation models including only the well-known species were used to predict the dependence of the relative (but not absolute) saturation of rare solid phases as a function of Eh, pH and the concentrations of dissolved cations and anions. In the model, chemically similar minerals were most saturated at similar Eh-pH conditions, and several different Eh-pH facies of secondary minerals could be distinguished. These would include:

(1) The [Hg.sup.1+] oxy-compounds including edgarbaileyite and the carbonates peterbaylissite, szymanskiite and clearcreekite, which are confined to a narrow Eh band along the Hg-HgO buffer, and neutral to alkaline pH (unsurprising given the weakness of silicic and carbonic acids).

(2) Conversely, the halides and oxy-halides attain minimum solubility at Eh on ([Hg.sup.1+] compounds) or just above (mixed valence and [Hg.sup.2+] only) the Hg-HgO buffer. The pH for minimum solubility was always close to the acid limit of montroydite stability, ca. pH = 5.

(3) A puzzle was presented by edoylerite, deanesmithite and CCUK-8, which simultaneously contain [Hg.sup.2+] and "oxidizing" chromate anions and "reducing" sulfide, and by wattersite, with "moderately reducing" [Hg.sup.1+] combined with chromate and oxide. The model indicates that the presence of sulfide determines the Eh for the sulfide-chromates, which are most stable along the upper stability limit for cinnabar, consistent with their observed occurrence as cinnabar oxidation products. Conversely, wattersite appears to be most stable under near-neutral conditions at the Hg-HgO buffer, similar to edgarbaileyite, which again is consistent with its observed mode of occurrence.

[FIGURE 58 OMITTED]

[FIGURE 59 OMITTED]

(4) The oxide-sulfate schuetteite is most stable under acid, oxidizing conditions. In many oxidizing zones of ore deposits, low pH values are attained on oxidation of sulfide to sulfate, simply because sulfuric acid is a much stronger acid than hydrogen sulfide. Such acidification was rare and very localized at Clear Creek, as evidenced not only by the occurrence of montroydite but also by the rarity of sulfur and schuetteite and non-occurrence of the sulfide-chloride, corderoite, [Hg.sub.3][S.sub.2][Cl.sub.2]. In strongly acid conditions, oxidation of cinnabar would form sulfur: HgS [right arrow] [Hg.sup.2+] + S + 2[e.sup.-], and reaction with dissolved chloride would form corderoite, which is less stable under near-neutral conditions. It is likely that the vast amount of carbonate in the deposit acted to neutralize the acid, and kept the pH near neutral. Avoiding the development of highly acid conditions appears crucial to the stability of many of the rare and unique species found at the Clear Creek mine.
Table 1. Minerals identified from the Clear Creek mine.

Mineral              Composition

Mercury Minerals
  Aurivilliusite     [Hg.sup.1+][Hg.sup.2+]OI
  Calomel            [Hg.sub.2.sup.1+][Cl.sub.2]
  Cinnabar           HgS
  Clearcreekite      [Hg.sub.3.sup.1+](C[O.sub.3])(OH) * 2[H.sub.2]O
  Deanesmithite      [Hg.sub.2.sup.1+][Hg.sub.3.sup.2+](Cr[O.sub.4])
                       O[S.sub.2]
  Donharrisite       [Ni.sub.8][Hg.sub.3][S.sub.9]
  Edgarbaileyite     [Hg.sub.6.sup.1+]([Si.sub.2][O.sub.7])
  Edoylerite         [Hg.sub.3.sup.2+](Cr[O.sub.4])[S.sub.2]
  Eglestonite        [Hg.sub.6.sup.1+][Cl.sub.3]O(OH)
  Gianellaite        [Hg.sup.2+][.sub.4][N.sub.2](S[O.sub.4])
  Hanawaltite        [Hg.sub.6.sup.1+][Hg.sup.2+][Cl,(OH)][.sub.2]
                       [O.sub.3]
  Metacinnabar       HgS
  Montroydite        HgO
  Mosesite           [Hg.sub.2.sup.2+]N(Cl,I,Br) * [H.sub.2]O
  Mercury            Hg
  Peterbaylissite    [Hg.sub.3.sup.1+](C[O.sub.3])(OH) * 2[H.sub.2]O
  Schuetteite        [Hg.sub.3.sup.2+](S[O.sub.4])[O.sub.2]
  Szymanskiite       [Hg.sub.16.sup.1+](Ni,Mg)[.sub.6]
                       (C[O.sub.3])[.sub.12](OH)[.sub.12]
                       ([H.sub.3]O)[.sub.8.sup.1+] * 3[H.sub.2]O
  Tedhadleyite       [Hg.sub.10.sup.1+][Hg.sup.2+][O.sub.4][I.sub.2]
                       ([Cl.sub.1,2][Br.sub.0.8])
  Terlinguaite       [Hg.sup.1+][Hg.sup.2+]OCl
  Vasilyevite        [Hg.sub.20.sup.1+][I.sub.3][O.sub.6]([Br.sub.1.6]
                       [Cl.sub.1.4])[.sub.[SIGMA]=3-]
                       [(C[O.sub.3])[.sub.0.8][S.sub.0.2.sup.2-]]
  Wattersite         [Hg.sub.4.sup.1+][Hg.sup.2+](Cr[O.sub.4])[O.sub.2]
  CCUK -8            Hydrous mercury chromate-sulfide
  CCUK -10           Hg-N-I-(Cl, Br)
  CCUK -12           Mercury oxy-halide
  CCUK -13           Mercury silicate
  CCUK -14           Mercury silicate
  CCUK -15           [Hg.sub.10.sup.1+][Hg.sub.3.sup.2+][O.sub.6]
                       [I.sub.2](Cl,Br)[.sub.2]
  CCUK -18           [Hg.sub.2.sup.2+]N(I,Cl,Br) * [H.sub.2]O

Associated Minerals
  Barite             BaS[O.sub.4]
  Calcite            CaC[O.sub.3]
  Dolomite           CaMg(C[O.sub.3])[.sub.2]
  Eskolaite          [Cr.sub.2][O.sub.3]
  Goethite           FeO(OH)
  Gypsum             CaS[O.sub.4] * 2[H.sub.2]O
  Hematite           [Fe.sub.2][O.sub.3]
  Huntite            Ca[Mg.sub.3](C[O.sub.3])[.sub.4]
  Hydromagnesite     [Mg.sub.5](C[O.sub.3])[.sub.4](OH)[.sub.2] *
                       4[H.sub.2]O
  Jarosite           [K.sub.2][Fe.sub.6](S[O.sub.4])[.sub.4]
                       (OH)[.sub.12]
  Magnesiochromite   (Mg,Fe)[Cr.sub.2][O.sub.4]
  Magnesite          MgC[O.sub.3]
  Magnetite          [Fe.sub.3][O.sub.4]
  Melanophlogite     Si[O.sub.2] + C[O.sub.2] + [N.sub.2] + C[H.sub.4]
  Millerite          NiS
  Montmorillonite    (Na,Ca)[.sub.0.3](Al,Mg)[.sub.2][Si.sub.4]
                       [O.sub.10](OH)[.sub.2] * n[H.sub.2]O
  Nimite             (Ni,Mg,Fe)[.sub.5]Al([Si.sub.3]Al)[O.sub.10]
                       (OH)[.sub.8]
  Pecoraite          (Ni,Mg)[.sub.3][Si.sub.2][O.sub.5](OH)[.sub.4]
  Pyrite             Fe[S.sub.2]
  Quartz             Si[O.sub.2]
  Reevesite          [Ni.sub.6][Fe.sub.2](C[O.sub.3])(OH)[.sub.16] *
                       4[H.sub.2]O
  Rozenite           FeS[O.sub.4] * 4[H.sub.2]O
  Rutile             Ti[O.sub.2]
  Sepiolite          [Mg.sub.4][Si.sub.6][O.sub.15](OH)[.sub.2] *
                       6[H.sub.2]O
  Sulfur             S
  Talc               [Mg.sub.3][Si.sub.4][O.sub.10](OH)[.sub.2]
  Todorokite         (Mn,Ca,Mg)[Mn.sub.3][O.sub.7] * [H.sub.2]O

Mineral              Rarity  Location

Mercury Minerals
  Aurivilliusite     VR      L
  Calomel            VR      L
  Cinnabar           C       U, M, L
  Clearcreekite      VR      L
  Deanesmithite      VR      U, L
  Donharrisite       VR      L
  Edgarbaileyite     Unc     U, M, L
  Edoylerite         Unc     U, L
  Eglestonite        Unc     U, M, L
  Gianellaite        VR      L
  Hanawaltite        VR      L
  Metacinnabar       R       L
  Montroydite        C       U, M, L
  Mosesite           VR      L
  Mercury            C       U, M, L
  Peterbaylissite    VR      L
  Schuetteite        VR      L
  Szymanskiite       VR      U, L
  Tedhadleyite       VR      L
  Terlinguaite       R       L
  Vasilyevite        VR      L
  Wattersite         Unc     U, L
  CCUK -8            Unc     U, L
  CCUK -10           VR      L
  CCUK -12           VR      L
  CCUK -13           R       L
  CCUK -14           VR      L
  CCUK -15           VR      L
  CCUK -18           VR      L

Associated Minerals
  Barite             VR      L
  Calcite            Unc     U, M, L
  Dolomite           C       U, M, L
  Eskolaite          Unc     U, L
  Goethite           VR      L
  Gypsum             Unc     L
  Hematite           R       L
  Huntite            Unc     L
  Hydromagnesite     C       U, M, L
  Jarosite           R       L
  Magnesiochromite   C       U, L
  Magnesite          C       U, M, L
  Magnetite          R       L
  Melanophlogite     R       U
  Millerite          Unc     L
  Montmorillonite    C       U, L
  Nimite             Unc     L
  Pecoraite          R       U, L
  Pyrite             Unc     U, L
  Quartz             C       U, M, L
  Reevesite          Unc     L
  Rozenite           R       L
  Rutile             R       L
  Sepiolite          C       U, L
  Sulfur             R       L
  Talc               C       U, L
  Todorokite         Unc     U, L

R = Rare, VR = Very Rare, C = Common, Unc = Uncommon, U = Upper
workings, M = Middle workings, L = Lower workings


ACKNOWLEDGMENTS

We take great pleasure in dedicating this paper to the memory of the late Mr. Edward H. Oyler (1915-2003), an ardent field collector and friend, who has specialized in mercury minerals for over thirty years. Without his persistence and enduring interest in mercury minerals, especially those of the Clear Creek mine, many of the new minerals that have been described from this mine might never have been discovered. He generously donated the best or sole specimens of clearcreekite, deanesmithite, edoylerite, hanawaltite, peterbaylissite and wattersite for their scientific study. It seems most appropriate to honor him in this way for his long and dedicated service to the science of mineralogy.

Richard C. Erd (U.S. Geological Survey, retired) suggested that a formal study be pursued to better understand the unique geochemical conditions and paragenesis of the mine. He graciously made available many references and all his notes on the mine and minerals so they could be incorporated into this study. We sincerely thank him for his continued encouragement and interest in this effort.

Andrew C. Roberts (Geological Survey of Canada, retired) provided numerous X-ray powder-diffraction analyses of minute crystals and masses that we thought were new or unusual. His timely identifications gave us the encouragement to continue the enormous task of reducing in size many hundred kilograms of rock samples in the hope of finding a single rare or new mineral.

We sincerely thank Susan Dunning for an early grammatical review of this paper that resulted in many improvements to its sentence structure.

Dan Behnke provided color photos for several of the mercury minerals. We sincerely thank him for his photographic skills.

Critical technical reviews were provided by Richard C. Erd, Andrew C. Roberts, Frank C. Hawthorne, Lee A. Groat, George W. Robinson and Edward Grew. We sincerely thank each of them for their candid thoughts and constructive comments that greatly improved the technical flow, accuracy and sanity of this paper.

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ROBERTS, A. C., ERCIT, T. S., ERD, R. C., and OSCARSON, R. L. (1990b) Szymanskiite, [Hg.sub.16.sup.1+](Ni,Mg)[.sub.6](C[O.sub.3])[.sub.12](OH)[.sub.12]([H.sub.3]O)[.sub.8.sup.1+] * 3[H.sub.2]O, a new mineral from the Clear Creek claim, San Benito County, California. Canadian Mineralogist, 28, 703-707.

ROBERTS, A. C., BONARDI, M., ERD, R. C., CRIDDLE, A. J., and LE PAGE, Y. (1991) Wattersite, [Hg.sub.4.sup.1+][Hg.sup.2+][Cr.sup.6+][O.sub.6], a new mineral from the Clear Creek claim, San Benito County, California. Mineralogical Record, 22, 269-272.

ROBERTS, A. C., SZYMANSKI, J. T., ERD, R. C., CRIDDLE, A. J., and BONARDI, M. (1993) Deanesmithite, [Hg.sub.2.sup.1+][Hg.sub.3.sup.2+][Cr.sup.6+][O.sub.5][S.sub.2], a new mineral from the Clear Creek claim, San Benito County, California. Canadian Mineralogist, 31, 787-793.

ROBERTS, A. C., ERCIT, T. S., GROAT, L. A., CRIDDLE, A. J., ERD, R. C., and WILLIAMS, R. S. (1995) Peterbaylissite, [Hg.sub.3.sup.1+](C[O.sub.3])(OH) * 2[H.sub.2]O, a new mineral species from the Clear Creek claim, San Benito County, California. Canadian Mineralogist, 33, 47-53.

ROBERTS, A. C., GRICE, J. D., GAULT, R. A., CRIDDLE, A. J., and ERD, R. C. (1996) Hanawaltite, [Hg.sub.6.sup.1+][Hg.sup.2+][Cl,(OH)][.sub.2][O.sub.3]--A new mineral from the Clear Creek claim, San Benito County, California: Description and crystal structure. Powder Diffraction, 11, 45-50.

ROBERTS, A. C., GAULT, R. A., RAUDSEPP, M., ERCIT, T. S., ERD, R. C., MOFFATT, E. A., and STIRLING, J. A. R. (2001) Clearcreekite, a new polymorph of [Hg.sub.3.sup.1+](C[O.sub.3])(OH) * 2[H.sub.2]O, from the Clear Creek claim, San Benito County, California. Canadian Mineralogist, 39, 779-784.

ROBERTS, A. C., COOPER, M. A., HAWTHORNE, F. C., CRIDDLE, A. J., STIRLING, J. A. R., and DUNNING, G. E. (2002) Tedhadleyite, [Hg.sup.2+][Hg.sub.10.sup.1+][O.sub.4][I.sub.2](Cl,Br)[.sub.2], a new mineral from the Clear Creek claim, San Benito County, California. Canadian Mineralogist 40, 909-914.

ROBERTS, A. C., STIRLING, J. A. R., CRIDDLE, A. J., DUNNING, G. E., and SPRATT, J. (2003a) Aurivilliusite, [Hg.sup.2+][Hg.sup.1+]OI, a new mineral species from the Clear Creek claim, San Benito County, California, Mineralogical Magazine, 68, 241-245.

ROBERTS, A. C., COOPER, M. A., HAWTHORNE, F. C., STIRLING, J. A. R., WERNER, H. P., STANLEY, C. J., DUNNING, G. E., and BURNS, P. C. (2003b) Vasilyevite, ([Hg.sub.2])[.sub.10.sup.2+][O.sub.6][I.sub.3]Cl(C[O.sub.3]), a new mineral species from the Clear Creek claim, San Benito Co., California. Canadian Mineralogist, 41, 1167-1172.

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SZYMANSKI, J. T., and ROBERTS, A. C. (1990) The crystal structure of szymanskiite, a partly disordered (Hg-Hg)[.sup.2+] * (Ni,Mg)[.sup.2+] hydronium-carbonate-hydroxide-hydrate. Canadian Mineralogist, 28, 709-718.

SZYMANSKI, J. T., and GROAT, L. A. (1997) The crystal structure of deanesmithite, [Hg.sub.2.sup.1+][Hg.sub.3.sup.2+][Cr.sup.6+][O.sub.5][S.sub.2]. Canadian Mineralogist, 35, 765-772.

TUNELL, G., FAHEY, J. J., DAUGHERTY, F. W., and GIBBS, G. V. (1977) Gianellaite, a new mercury mineral. Neues Jahrbuch fur Mineralogie, Monatshefte, 130, 119-131.

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Gail E. Dunning

773 Durshire Way

Sunnyvale, California 94087

Email: gedunning@juno.com

Ted A. Hadley

907 Anaconda Way

Sunnyvale, California 94087

John Magnasco

1851 McBain Avenue

San Jose, California 95125

Andrew G. Christy

Department of Geology

Australian National University

Canberra, ACT 0200 Australia

Joseph F. Cooper, Jr.

430 Van Ness Avenue

Santa Cruz, California 95060
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Author:Dunning, Gail E.; Hadley, Ted A.; Magnasco, John; Christy, Andrew G.; Cooper, Joseph F., Jr.
Publication:The Mineralogical Record
Geographic Code:1U9CA
Date:Jul 1, 2005
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