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Cubic magnetite crystals: from Balmat, New York.

In 1991 a sphalerite mining operation near Balmat, New York, encountered an altered area containing cubic to tetrahexahedral magnetite crystals in halite-filled veins. The large, lustrous, morphologically unique magnetite crystals rank among the finest known examples of the species.


In 1991, the Zinc Corporation of America's mining operation to recover sphalerite from the stratabound Fowler orebody near Balmat in St. Lawrence County, New York, encountered an altered area of unusual mineralogy. Magnetite crystals were found there in two modes: (1) lining halite-filled veins along with anhydrite, sphalerite, and calcite, and (2) frozen in a matrix of halite and talc together with pods of zoned massive sulfides. The magnetite crystals have unusual morphology and may well rank among the finest examples of the species yet found. Thousands of fine crystals and crystal clusters were recovered before mining progressed through the zone.


The Balmat mines are located in northern New York State, about 26 miles southeast of Ogdensburg on the Canadian border. Other well-known mineral localities in the area include DeKalb (diopside), Antwerp (millerite), Gouvernenr (tourmaline), Rossie (galena) and Pierrepont (black uvite).

At the time the magnetite specimens were collected (1991-1992), access to mining operations in the Fowler orebody was gained by a vertical shaft at the No. 4 mine on Sylvia Lake Road, just north of Sylvia Lake in the town of Fowler. The Zinc Corporation of America also operated the adjacent No. 3 mine at Balmat, the Hyatt mine at Talcville, and the Pierrepont mine on Crary Road in the town of Pierrepont, all in St. Lawrence County. All four mines exploit stratabound sphalerite deposits in Precambrian metasediments.


Zinc mining in northern New York began in 1915 when the Northern Ore Company began production at the Edwards mine at the northeastern end of the Balmat-Edwards marble belt (see Johnson, 1998, for a historical review). After a brief period of ownership by the New York Zinc Company, the property was bought by the St. Joe Lead Company in 1926. St. Joe continued the Edwards operation and began exploration in Balmat about 13 kilometers to the southwest on an outcropping of zinc ore that had been described as early as 1838 by Ebenezer Emmons in a New York Geological Survey report. This effort led to the discovery of a very large deposit consisting of the Main, Hanging Wall, and Streeter orebodies, all of which were accessed by the No. 2 mine shaft.

Subsequent exploration met with continuing success: the Loomis and Gleason orebodies were discovered in 1948 and the Fowler and Upper Fowler orebodies in 1966. Additional smaller satellite bodies were also encountered during this time. Exploration of the Pierrepont marble belt, 45 kilometers northeast of Balmat, led to the discovery of the very high-grade Grange and Crary orebodies at the Pierrepont mine in 1979. Ownership was transferred to the Fluor Corporation in 1981 and finally, in 1987, to the present owner, the Zinc Corporation of America, a subsidiary of Horsehead Industries. The Balmat, Edwards, Pierrepont, and small Hyatt mines have produced over 36 million tons of zinc ore at a grade of 9.5 percent zinc--truly a world-class district.

The Magnetite Zone

A small but crystallographically unique occurrence of magnetite crystals was encountered in the Fowler orebody in 1991. The occurrence was in an alteration zone that had affected the primary sulfide minerals, sphalerite and pyrite, to produce an unusual suite of mineral associations.


The magnetite and associated minerals occur in a zone of hydro-thermal alteration in the F-24 stope in the Fowler orebody, just below the 2,500-foot top mining sublevel. The coordinate location is 44[degrees] 16' 21" N, 75[degrees] 24' 11" W and 688 meters below the surface. Several years of mining in this stope had given little indication that there was a significant zone of unusual mineralization to be encountered.


The initial entrance into the alteration zone was a drift through massive sphalerite with disseminated pyrite. The first sign of anything unusual was the partial alteration of the pyrite grains to magnetite. Farther along, layers with heavily disseminated magnetite were interlayered with typical sphalerite. Still farther along, this was supplanted by intense alteration and a change in the ore texture: most of the original sphalerite and pyrite were replaced with ore having a "birds-eye" texture made up of small cores of magnetite and sphalerite ringed by concentric bands of fleshy pink secondary sphalerite, hematite and magnetite.

Late in the summer of 1991, veins filled with very coarse-grained (up to several centimeters) halite, anhydrite and lesser amounts of calcite and lined with small magnetite and sphalerite crystals were encountered. In the first pass, drifting about 35 meters along strike exposed the bottom of the alteration zone, and small magnetite crystals were recovered. Subsequent shrink mining exposed the top of the alteration zone where the most abundant and largest magnetite crystals were recovered beginning on May 29, 1992.

Because the ore was only slowly mucked, collecting continued throughout the summer and into the fall. Some of us visited the site together on October 2. Shortly thereafter, mining in the drift resumed, and all traces of the occurrence were removed. A second, smaller alteration zone was encountered nearby at a lower elevation in 1994 and a few more specimens were recovered.

Many people helped recover and preserve specimens during the year-long period that the unusual mineralization was exposed, including Charlie Bowman, Chuck Bowman, Dave Bowman, Bill deLorraine, John Johnson, Gary Stacey, and David Nace. Through the efforts of these people, thousands of specimens were collected, thus insuring the status of this occurrence as an important locality for magnetite.


Regional Geology

The zinc deposits of the Balmat-Edwards district are hosted by a sequence of Precambrian metasedimentary rocks of the Grenville Series. The geology of the Balmat district is complex and involves multiple episodes of structural deformation. For a more detailed discussion see Brown and Engel (1956) and deLorraine (1979).

Locally the geologic column is characterized by mostly clean, siliceous dolomitic marbles, but with major occurrences of anhydrite and talc-tremolite schist. The sphalerite-pyrite ore is thought to have accumulated as massive sheets and lenses in a sedimentary environment as sulfides precipitated out of metalliferous thermal brines which vented to the sea floor via fault channels. These thermal vents may have resembled the so-called "black smokers" now found along the mid-oceanic ridges, except that they formed in a shallow, carbonate shelf environment. The stratigraphic sequence hosting the sulfide ores was subsequently buried and subjected to high-grade metamorphism approximately 1.1 billion years ago. Over time the ore and host rocks were complexly folded and sheared.

After the region had been uplifted to nearly its present elevation, post-metamorphic hydrothermal fluids permeated small portions of several of the orebodies along fractures, causing alteration in some areas. In 1936, J. S. Brown, a geologist for the St. Joe Lead Company, described "supergene sphalerite, galena, and willemite" in the No. 2 mine at Balmat. This alteration zone was composed of a pervasive red earthy hematite, pyrite grains rimmed with magnetite, and secondary replacement minerals along with high-grade primary sphalerite. The term "birds-eye" ore was used to describe the textural characteristics of the altered ore. Brown described a zone extending from the surface to the 700-foot level. Other geologists were later able to trace the zone down to a depth of 2,000 feet.


Generally such oxidation penetrates only to shallow depths, the degree of alteration decreasing with depth. Thus, it was a complete surprise when mining operations encountered a small (approximately 30 by 100 meters) alteration zone deep in the Fowler orebody nearly 700 meters (2,275 feet) beneath the surface. There had been no other evidence of alteration encountered either above or below the 1,450-meter plunge-length of the Fowler orebody.

Fowler Orebody

The Fowler orebody is composed of massive sphalerite and pyrite of varying proportions, and minor galena. The orebody extends from the 1,300-foot level, plunging northward down to the 3,100-foot level of the No. 4 mine. Like all the orebodies in the Balmat-Edwards district, it has been affected by multiple phases of folding and shearing at a high metamorphic grade. Under these conditions, sulfides were very mobile and were forced into gash veins, extension fractures, and other structural sites in the hinges of major folds. The orebody occurs in the axial region of an isoclinal fold, the limbs of which are separated by a thrust fault. Conformable ore in the upper and lower limbs of the fold was remobilized into and along the shear zone by plastic flow during the metamorphic event.

The alteration zone hosting the magnetite crystals is located in the "cross-cutting" ore horizon of the Fowler orebody that occurs along a ductile fault. Here the ore was exposed to upper amphibolite-facies metamorphism reaching temperatures of 650[degrees] to 700[degrees]C. Most of the sulfides in the cross-cutting ore zone were further subjected to recurrent deformation and strain partitioning resulting in an ore that is locally layered with alternating pyrite-rich and sphalerite-rich bands. As much as 85% of the cross-cutting ore has durchbewegung texture from the incorporation of numerous rounded, lenticular inclusions of wall rock. By contrast, as much as 15% of the cross-cutting ore consists of enclaves of pure sulfide that had developed a much coarser texture during recrystallization. The cubic magnetite occurrence appears to have been in an alteration zone affecting one of these massive sulfide enclaves.

Along the periphery of the altered area there was partial to complete replacement of the pyrite in the ore by magnetite. Nearer the center there was an intensely altered area where the original ore appears to have been replaced by birds-eye ore consisting of very fine-grained pink sphalerite and hematite interlayered with magnetite and surrounding dark cores of magnetite and sphalerite. Cubic magnetite crystals were found lining halite-filled veins, sometimes mixed with sphalerite crystals and frozen in a greenish gray matrix of fine-grained talc and halite. The entire main alteration zone occupied an area of approximately 10 X 16 X 16 meters at the structural top of the ore shoot. Within this zone, secondary sphalerite was relatively depleted in iron compared to the typical sphalerite outside the alteration zone and pyrite was virtually absent. Some relict banding similar to that of the original sulfide ore was observed, particularly in the more magnetite-rich layers which preferentially replaced the original pyrite-rich layers. Grains, lenses, and masses of chalcopyrite up to a meter across were encountered in the center of the intensely altered zone. Some of these displayed spectacular bornite rims and contained minor amounts of galena as well as betekhtinite and other copper sulfides. Breccias along the east side of the alteration zone contained subangular, partially altered fragments of sphalerite in a matrix of earthy hematite and talc.

Except for a smaller alteration zone with more cubic magnetite encountered nearby but at lower depth, no other similar alteration zones have been encountered in the district. Birds-eye ore was encountered in the No. 2 mine, but large chalcopyrite pods with other sulfides are thus far unique to this one alteration zone in the orebodies.



Cubic crystals of magnetite and the associated minerals listed in Table 1 were found in three different environments: (1) in halite-filled veins with associated talc-halite; (2) in the sulfide pods; and (3) in assemblages that appear to have formed after mining while the broken ore sat in a sodium chloride brine formed by washing the ore pile with tap water. All species reported have been confirmed by X-ray diffraction, electron microprobe analysis or both. Magnetite crystals occur frozen in a matrix of microscopically intergrown halite and talc, lining the walls of halite-filled veins, and as isolated crystals in the halite core of halite-filled veins.
Table 1. Minerals associated with cubic magnetite at Balmat, New York

Anhydrite      [CaSO.sub.4]
Arsenopyrite   FeAsS
Atacamite      [Cu.sub.2][Cl(OH).sub.3]
Betekhtinite   [Cu.sub.10](Pb,Fe)[S.sub.6]
Bornite        [Cu.sub.5][FeS.sub.4]
Bottallackite  [Cu.sub.2]Cl[(OH).sub.3]
Calcite        [CaCO.sub.3]
Celestine      [SrSO.sub.4]
Chalcocite     [Cu.sub.2]S
Chalcopyrite   [CuFeS.sub.2]
Digenite       [Cu.sub.9][S.sub.5]
Galena         PbS
Gordaite       [NaZn.sub.4]([SO.sub.4])[(OH).sub.6]Cl*6[H.sub.2]O
Halite         NaCl
Hematite       [Fe.sub.2][O.sub.3]
Malachite      [Cu.sub.2]([CO.sub.3])[(OH).sub.2]
Nantokite      CuCl
Namuwite       [(Zn,Cu).sub.4][SO.sub.4][(OH).sub.6]*4[H.sub.2]O
Paratacamite   [Cu.sub.2]Cl[(OH).sub.3]
Pyrite         [FeS.sub.2]
Silver         Ag
Sphalerite     ZnS
Talc           [Mg.sub.3][Si.sub.4][O.sub.10][(OH).sub.2]

Crystal Sizes

The largest single magnetite crystal we have observed is a tet-rahexahedron with minor cube faces measuring 3.3 cm on edge. Several other crystals we measured reach 2.7 cm on edge. Crystals to 1 or 2 cm on edge were common in the occurrence.

Specimens include single crystals, both as "floaters" and in matrix, miniature and cabinet-sized groups with and without matrix, double-sided slabs of crystals, and sections of halite-filled vein wall covered with crystals. Hollmann (1992) reported the recovery of slabs of crystals up to 15 by 20 cm with crystals on both sides.


Three crystal forms in several combinations have been identified: the cube {100}; the octahedron {111}; and the tetrahexahedron {015}. Eight distinctive habits have been recognized: six of these are equant crystals, one is tabular, and one is pseudoprismatic. The two most common habits are (1) cubes with minor octahedral faces and (2) the combination of cube and tetrahexahedron in nearly equal development. Tetrahexahedrons with minor cube faces and tetrahexahedrons with both minor cube and octahedron faces are less common. The combination of nearly equal development of the tetrahexahedron and the octahedron is also uncommon, and some of these crystals show a peculiar growth distortion wherein one octahedron face is much larger than the other seven. Tabular crystals with both cube and tetrahexahedron faces are rare. Elongated crystals with cube feces and minor octahedron modifications have been observed only in microscopic crystals recovered by dissolving halite from the cores of halite-filled veins. In hand specimens, the cube faces are typically splendent and the octahedral faces are slightly less so. Tetrahexahedron faces can be splendent, striated, pitted, or velvety. Magnetite crystals that formed isolated in the halite in the cores of halite-filled veins typically have splendent luster on the faces of all three crystal forms. Some floater crystals from the halite-filled veins consist of larger cubes covered with many smaller cubes.



Some magnetite crystals with prominent cube faces show growth distortion which gives them a diamond-shaped rather than a square outline. Other cube faces show bowed edges made up of multiple linear segments.

Because magnetite crystals of cubic and tetrahexahedral habit are so rare worldwide, we investigated their crystal habits at this locality in more detail (Chamberlain and Robinson 1993; Morgan, et al. 2006). One such study was aimed at characterizing the distribution of crystal habits. The eight crystal drawings shown comprise the set of observed magnetite crystal habits, made up of varying degrees of the development of just three forms: the cube, the tetrahexahedron, and the octahedron. We examined and characterized all the crystallized magnetite specimens in the collection of the New York State Museum (NYSM), which includes the recently acquired Kenneth Hollmann collection (329 specimens) and the Steven C. Chamberlain collection (164 specimens). We also examined 225 microscopic single crystals that Hollmann had found frozen in halite; these are preserved in a number of vials now in the New York State Museum. Cubes and cubes with tetrahexahedron modifications are by far the two most common habits. All three samples no doubt emphasized the rarer habits because they were preferentially sought.

We did find one crystal habit in microscopic crystals that was not present in hand specimens. Rarely, microscopic magnetite crystals were found to be elongated cubes of pseudo-prismatic rather than equant habit. No large specimens of such elongated magnetite crystals are known from this occurrence.

SEM Examination

Another study involved the careful examination of microscopic magnetite crystals with scanning electron microscopy. At the outset, we thought perhaps that microscopic crystals might show crystal forms not present in larger crystals because they disappeared as the crystals grew larger. However, scanning electron micrographs revealed only the same three forms already found on large specimens. We also carefully examined the surfaces of the three forms and found that many of them are smooth and planar even when viewed with the scanning electron microscope. Octahedron faces occasionally show triangular growth hillocks. Tetrahexahedron faces, although sometimes smooth, more often show a variety of very interesting microscopic textures that lead to striations and frosted surfaces when such specimens are viewed with the naked eye. Cube faces sometimes show four-sided growth pits with edges parallel to the cube-octahedron edge and with inward-sloping octahedron faces. The edges of cubes are sometimes not straight lines but are composed of a number of off-set straight-line segments which, taken together, present a bowed appearance. Such growth distortion is more rarely seen in macroscopic crystals.


Unexpected, and not particularly rare, are microscopic cube crystals with a myriad of odd growth features on the cube faces. Large, four-sided pits have hemispherical voids on their bottom surfaces, where cube faces should be. Grooves are sometimes at crystallographic orientations, with octahedron faces forming their walls; others are curved with walls that look more like erosional surfaces. The bottoms of these valleys also have hemispherical voids. The cube edges have an eroded appearance with semi-spherical voids. The impression is that the crystal is made of styrofoam inside with six solid plates forming the cube faces that don't quite meet at the edges and are partially eroded away.



Crystallography of magnetite

Describing (he habit of magnetite. Hintze (1933) noted that "usually the octahedron prevails, more rarely the rhombic dodecahedron predominates ... still more rarely {100} is found alone or with prevailing development" (translation of p. 34). Perusal of Goldschmidt's Atlas der Krystallformen (1918) shows few localities for the cubic habit: Gulsen. Steyermark, Austria; Arendal, Norway; the O'Neil mine in Orange County. New York; Eisenach. Germany; Mossgrufva, Nordmark, Sweden; and Split Rock, Essex County, New York. There are a lew other localities such as the Sunset Crater volcano in northern Arizona (Hanson et al., 2008) but only at Balmat does magnetite occur as cubes in such size and profusion. Interestingly, one of the most common occurrences of cubic magnetite is biogenic (Violante et al., 2003; Vali et al., 2004). The presence of cubic magnetite as evidence of life became a hot topic during the analysis and interpretation of the Martian meteorite (Thomas-Keprta et al., 2000).

Magnetite crystals with tetrahexahedron faces are even rarer. Again, a perusal of Goldschmidt (1918) shows only three localities where the crystals show minor tetrahexahedraon faces: Nordmark, Sweden; Wildkreuzjoch, Pfitschtal, Italy; and Rotenkopf, Zillertal, Austria. We are aware of no other localities for magnetite crystals with a predominantly tetrahexahedral habit. In that regard the crystals from Balmat, New York, appear to be unique.


Analysis of the chemistry and mineral phases in the Balmat cubic magnetite was published by Clark and Evans (1997). They found that, unlike octahedral habit magnetite from other localities that may typically contain Ca, Cr and Ti as impurities, Balmat cubic magnetite crystals contain Zn in the range 0.05 to 0.09 apfu (atoms per formula unit) in the formula [Zn.sub.x][Fe.sub.[3-x]][O.sub.4]. Additional analysis using Mossbauer spectroscopy showed that some of this zinc is contained in a separate zinc ferrite ([ZnFe.sub.2][O.sub.4]) phase, so that the actual amount of zinc in the magnetite phase was between 0.02 and 0.03 apfu in the preceding formula. They suggested that the presence of an impurity such as Zn, which favors the tetrahedral site in the spinel structure (occupied by [Fe.sup.2+] in magnetite), leads to the suppression of the predominantly octahedral habit typical of most magnetite crystals and the expression of the cube instead. These same authors note that Mn seems to play a similar role in other cubic magnetites.

Our SEM examination of the tetrahexahedron faces of Balmat magnetite, which are even less commonly found than cube faces, further suggests that even under circumstances that permit their development, there is serious competition from other forms leading to the rich variety of textures on the surface of tetrahexahedron faces. It's not easy to be a tetrahexahedron in a magnetite crystal.


The discovery of a zinc ferrite phase distinct from a zinc-rich magnetite phase in the magnetite crystals from this deposit places some restrictions on the temperature of formation. Mason's original conclusions that there was complete miscibility in the [Fe.sub.3][O.sub.4]-[ZnFe.sub.2][O.sub.4] system (Mason, 1947) were subsequently revised by the analysis of magnetite-franklinite-pyrophanite intergrowths from Sterling Hill, New Jersey (Valentino et al., 1990). These authors concluded that a miscibility gap exists below 500[degrees]C. The clear presence of a zinc ferrite ([ZnFe.sub.2][O.sub.4]) phase thereby establishes an upper limit on the temperature of formation of 500[degrees]C. This eliminates the possibility that the magnetite, halite, and associated minerals formed from a halite melt (since halite only liquefies above 800[degrees]C) and favors the conclusion that crystallization occurred from a sodium chloride brine. This assumption is further endorsed by Hill and Darling (1997), who analyzed the fluid inclusions in sphalerite taken from vugs in the Balmat-Edwards district and found that the mineralizing fluids were possibly brines with approximately 20 weight percent [CaCl.sub.2] and 10 weight percent NaCl; these brines reached elevated temperatures and had the characteristics of the "shield-type" brines described from elsewhere in the Canadian Shield. Examination of fluid inclusions during the present study further substantiates this theory. Daughter crystals of halite were found in fluid inclusions of colorless sphalerite associated with the cubic magnetite. Anhydrite, talc, and sphalerite daughter crystals were observed in fluid inclusions in the cubic magnetite itself. This upper limit of 500[degrees]C is also consistent with magnetite formation after the maximum regional metamorphism at 650[degrees] to 700[degrees]C.

Petrographic examination of the sulfide pods shows that the chalcopyrite formed by replacement of sheared pyrite, but similar pods of pyrite in the Balmat orebodies that might represent precursors are absent. The chalcopyrite pods are usually, but not always, enveloped by a rind of bornite that clearly replaces the chalcopyrite, and the bornite is commonly replaced by chalcocite in both monoclinic and hexagonal polymorphs, the latter of which suggests a minimum temperature of formation of 105[degrees]C. The successive alteration of chalcopyrite to bornite to chalcocite also indicates a progressive leaching of iron from the pods.

It is clear that the copper chlorides and sulfates formed as post-mining secondary minerals. Water from mining operations promoted reactions between halite and chalcocite in the muck pile, resulting in the formation of the observed chlorides and other post-mining minerals. The nonmetallic minerals in the alteration zone are present in the adjacent metasediments from which they were very likely derived. These include halite, calcite, talc, and anhydrite.


The magnetite occurrence at Balmat, New York, is noteworthy because it produced crystallized magnetite specimens of exceptional quality and beauty; because the association of magnetite and halite is unusual, perhaps unique; and because of the unusual geochemistry of the environment which produced a magnetite with zinc as an impurity that may have been influential in producing the unusual cubic and tetrahexahedral morphology. A unique set of geological circumstances led to a unique mineral assemblage.


We thank the Zinc Corporation of America for permitting access to their property and making specimens available for research, and Dr. Jeremy Gilbert and Gregory Rommel for their assistance with the scanning electron microscopy. We appreciate the assistance of Michael Hawkins in making the collections of the New York State Museum available for detailed study. We acknowledge the late Kenneth Hollmann for having assembled an outstandingly comprehensive collection of specimens from this occurrence. A longer version of this paper was published in Rocks and Minerals (2008, pages 224-239). All mineral specimens shown are now in the collection of the New York State Museum.


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CHAMBERLAIN, S. C., and ROBINSON, G. W. (1993) Unusual occurrence of magnetite crystals from the Balmat District, St. Lawrence County, New York. Rocks & Minerals, 68, 122-123.

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MASON, B. (1947) Mineralogical aspects of the system [Fe.sub.3][O.sub.4]-[Mn.sub.3][O.sub.4]-[ZnMn.sub.2][O.sub.4]-[ZnFe.sub.2][O.sub.4]. American Mineralogist, 32, 426-441.

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VALI, H., WEISS, B., LI, Y-L, SEARS, S. K., KIM, S. S., KIRSCHVINK, J. L., and ZHANG, C. (2004) Formation of tabular single-domain magnetite induced by Geobacter metallireducens GS-15. Proceedings of the National Academy of Sciences (USA), 101, 16121-16126,

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Steven C. Chamberlain (1)

George W. Robinson (2)

Marian Lupulescu (3)

Timothy C. Morgan (4)

John T. Johnson (5)

William M. deLorraine (6)

(1) Center for Mineralogy, New York State Museum, 3140 CEC, Albany, New York 12230; email:

(2) A. E. Seaman Mineral Museum, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931; email:

(3) Center for Mineralogy. Research & Collections, New York State Museum, 3140 CEC, Albany, New York 12230; email:

(4) Johns Hopkins University, Division of Brain Injury Outcomes, 1550 Orleans St., 3M South, Baltimore, Maryland 21231

(5) 266 Weldon Road, Gouverneur, New York 13642

(6) 1A Pumphouse Road, Gouverneur, New York 13642
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Author:Chamberlain, Steven C.; Robinson, George W.; Lupulescu, Marian; Morgan, Timothy C.; Johnson, John T.
Publication:The Mineralogical Record
Geographic Code:1U2NY
Date:Nov 1, 2010
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