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Pseudomorphic melanophlogites from California.

Melanophlogite, the tetragonal pseudocubic polymorph of Si[O.sub.2], is known from at least five localities within the Franciscan Formation of the Coast Ranges, California. One of these localities (Mount Hamilton) has provided the best crystals and the only modified crystals ever discovered.


Melanophlogite (Si[O.sub.2] plus organic compounds) and its chalcedony (quartz) pseudomorphs, have been identified from at least five localities in California, notably within the geologically diverse Franciscan formation of the Mt. Diablo and Coast Ranges. This rare pseudocubic polymorph of Si[O.sub.2] has been the object of considerable study since its description by Von Lasaulx in 1876 (Skinner and Appleman, 1963).

Although melanophlogite is considered to have a cubic structure, studies of material from two localities (Chvaletice, E. Bohemia, Czech Republic and Mount Hamilton, California) have shown the presence of a tetragonal superstructure (Zak, 1975). This supercell is thought to be the result of structure-stabilizing guest molecules trapped within the open clathrate structure during crystallization (Kamb, 1965). When the structure-stabilizing effect of these guest molecules is removed by a natural weathering mechanism, the melanophlogite structure collapses and inverts to a higher density form of silica, namely chalcedony (quartz). This naturally occurring phenomenon has been the case for melanophlogite from several of the California localities.


Skinner and Appleman (1963) gave an extensive historical account of melanophlogite research from 1876 through 1891. The name melanophlogite is derived from the fact that the mineral turns black after heating. This color change is due to the thermal breakdown of organic guest molecules within the open structure. Classical localities for melanophlogite are the old Sicilian sulfur deposits, in particular those at Racalmuto in Agrigento Province and at Lercara in Palermo Province, Italy.

Zak (1967, 1972) has studied the crystal chemistry of melanophlogite from Chvaletice (E. Bohemia), Czech Republic. This locality provided material which has substantially contributed to the understanding of the crystallography and transformation twinning of melanophlogite (Zak, 1972, 1973, 1975). Cooper and Dunning (1972) described the first United States (California) occurrence of exceptional melanophlogite crystals; these show modification of the cube edges and a visible internal hopper-shaped growth pattern. Since 1972, several other California localities for melanophlogite have been discovered; their descriptions are presented here.



Zak (1972) has shown by optical and single-crystal X-ray studies that cubes of the Chvaletice melanophiogite are composed of six pyramids, whose apices meet at the center of the cube; the pyramid bases constitute the cube faces. These pyramids are composed of fine tetragonal tablets, growing parallel with their bases together. This unusual growth habit can be observed as zoning by directing a light beam horizontally into the crystals at a critical angle or by examining them under polarized light. The vertical axis of each pyramid is common and perpendicular to the corresponding cube face. Identical optical effects were observed in melanophlogite from Sicily by Bertrand (1880), Friedel (1890) and Skinner and Appleman (1963). Zak (1972) suggested that the adjoining pyramids and the lamellae in them probably represent a form of twinning.

Zak (1973) defined the transformation twinning of the Chvaletice melanophlogite by a rational fourfold twin axis [001] of the tetragonal cell. He found that this twinning is due to a tetragonal distortion of a cubic cell, which can be explained in terms of a 1800 rotation about [201] of the tetragonal supercell. This twin law, however, failed to explain lamellae parallel to {201} of the tetragonal supercell previously observed by Zak (1972). These lamellae can now be successfully explained by another twin law with {201} twin planes, and are the direct result of the distortion of a pseudocubic tetragonal cell. Zak (1975) found a tetragonal superstructure, identical to that observed in the Chvaletice melanophlogite, in crystals from Mount Hamilton, California.


Studies by Appleman (1965) and Kamb (1965) have shown that melanophlogite possesses a clathrate-type structure. This structure consists of Si[O.sub.4] tetrahedra sharing corners to form five-membered and six-membered rings, which define large polyhedral cages in the shapes of pentagonal dodecahedra and tetrakaidecahedra. Guest molecules, in the form of aliphatic hydrocarbons, [H.sub.2]O, [CO.sub.2] and S, may occupy these cavities in the clathrate-type framework structure. The number and type of guest molecules trapped in the structure during growth appears to be directly related to the mineral's stability in nature. Upon exposure to ultraviolet light or to heating, the structure of the tetragonal pseudocubic melanophlogite first transforms to the cubic structure and then collapses and inverts to silica phases higher in specific gravity, such as cristobalite or chalcedony.

The several California occurrences of chalcedony pseudomorphs after melanophlogite (described here) would strongly suggest that the number and type of stabilizing guest molecules were critical to the mineral's structural stability. After crystallizing, the metastable structure begins to lose its stabilizing molecules and inverts to a quartz-type phase upon exposure to a weathering mechanism over an unknown period of time. The exact mechanism which triggers this inversion is not fully known but may be related to ultraviolet radiation exposure. It is interesting to note that the California melanophlogites contain very little, if any, sulfur in their structure. Chemical analyses show that the melanophlogites from Italy, however, contain appreciable sulfur (Skinner and Appleman, 1963; Zak, 1972).


The crystal chemistry of melanophlogite has been under study since the mineral was first described in 1876. Von Lasaulx (1876) and succeeding workers established the composition of melanophlogite essentially as Si[O.sub.2]. However, the presence of organic matter, S and [H.sub.2]O was puzzling. It was suggested at an early date that the organic matter was a mechanically incorporated pigmenting agent which caused the color zones in the crystals and gave the mineral its peculiar thermal properties (Skinner and Appleman, 1963). Between the years 1880 and 1918, considerable work was done on both the chemistry and optical properties of melanophlogite.

The first detailed chemical examination of melanophlogite was reported by Skinner and Appleman (1963) on material from the Sicilian sulfur deposits. They also concluded that melanophlogite is essentially Si[O.sub.2]. Infrared-absorption studies confirmed the presence of abundant hydrocarbon compounds in the mineral, but specific compound identification proved impossible. The presence of [H.sub.2]O and [CO.sub.2] was also confirmed. Sulfur was reported as total S[O.sub.3], but the specific molecule in the structure was not determined.

The solving of the crystal structure of melanophlogite by Kamb (1965) and Appleman (1965) gave fresh insights into the role of the organic chemicals present in the melanophlogite analyses reported in the literature.

Zak (1972) made an important contribution to the crystal chemistry of melanophlogite from Chvaletice, Czech Republic. Sulfur, silicon and oxygen were determined by neutron activation analysis, carbon and hydrogen by microchemical methods from powdered samples, and silicon and carbon by electron microprobe analysis. A formula for the Chvaletice melanophlogite, based on 46 Si[O.sub.2] per unit cell (Kamb, 1965), was found to be ~ 46 Si[O.sub.2] [C.sub.2][H.sub.17][O.sub.5][S.sub.0.9]. Zak (1972) suggested that the following element combinations can be present in the guest molecules: C + H [+ or -] S [+ or -] O, H + O, C + O, and S [+ or -] O [+ or -] H.

The chemical analysis reported by Skinner and Appleman (1963) for the Racalmuto melanophlogite can be represented by the general formula 46 Si[O.sub.2][C.sub.2-3][H.sub.23][O.sub.5][S.sub.2].


All of the California melanophlogite localities occur along the Coast Ranges (Franciscan Formation), which are composed of late Jurassic, Cretaceous, Tertiary and Quaternary sediments, volcanics and shallow intrusive bodies (Bailey et al., 1964). These sediments and volcanics are highly fractured and folded, often showing local intrusion by serpentinite masses. Local fluid metamorphism has altered portions of the serpentinite to small bodies of silica-carbonate rocks. Low temperature silica-rich fluids, containing organic and other molecules, formed stable melanophlogite at temperatures below I 112[degrees]C, the melting point of sulfur. Although not a condition of formation, melanophlogite may occur associated with mercury mineralization. This mercury mineralization is generally confined to silica-carbonate rocks within the Franciscan Formation that have experienced multiple hydrothermal events, one of which formed melanophlogite. Subsequent fluids have deposited carbonates such as calcite and dolomite over melanophlogite. In rare instances, minute quartz crystals have formed over the melanophlogite in addition to thin coatings of opal or chalcedony.


Mount Hamilton, Santa Clara County

The Mount Hamilton locality, previously described by Cooper and Dunning (1972), hosts some of the best melanophlogite crystals known for the species. Not only are these crystals of exceptional size, but so far it is the only locality where visible crystal forms other than the cubic * faces have been observed. The water-clear crystals, measuring up to 5 mm along an edge, are generally simple or multiple intergrowths. Modification of the cube edges by the tetrahexahedron form (0121 is well-developed on about 10% of the crystals. In addition, another modification was observed during a recent SEM examination of the crystals. A single crystal was found showing two {hkl} corner modifications, which were identified as the faces (112) and (121) of the trapezohedron form (112).

An unusual internal hollow "hopper-shaped" structure characterizes the growth sequence of the mineral. The cube face above this hollow void is very thin and easily broken. Off-center variants are common, even within intergrown crystals. Also, the six-fold pyramidal growth pattern may be observed when a light source is focused through the crystals.

Zak (1975) has determined a tetragonal supercell with a = 26.82 A and c = 13.37 A (space group [P4.sub.2]/nbc) for the Mount Hamilton crystals. Upon heating to 1050[degrees]C, this structure reverts to a cubic cell with a = 13.4 A (space group [P4.sub.2]32). He suggested that formation and stabilization of the tetragonal superstructure is directly related to the type, quantity and position of guest molecules present in the open clathrate structure during crystallization.

Only a small percentage of the Mount Hamilton melanophlogite crystals has shown inversion to quartz. Crystals with a cloudy rim yield a quartz X-ray pattern. These particular crystals were recovered in an area routinely exposed to direct sunlight along the roadcut. The majority of the crystals, however, were recovered along a vertical vein covered with soil rich in organic matter.

Recently Nakagawa et al. (2001) presented data on the crystal structure of Mount Hamilton melanophlogite. In their description of the structure they established the position of the stabilizing constituents within this very open-structured mineral. A new formula which includes the stabilizing molecules is given as 46Si[O.sub2] [6M.sup.14] [2M.sup.12], where [M.sup.14] = [N.sub.2], [CO.sub.2], and [M.sup.12] = [CH.sub.4], [N.sub.2]. The Mount Hamilton melanophlogite transforms at 65[degrees] C to the cubic form, [beta]-melanophlogite. However, this transformation temperature varies with the locality.

Borges Quarry, Napa County

An unusual occurrence of what appeared to be a clear cubic mineral associated with chalcedony and dolomite was discovered in 1981 by Mr. Clive Matson of Oakland, California. This find was made at the inactive Borges quarry, on American Canyon Road near Vallejo, Napa County. The cubes were examined optically by the late Dr. Francis Jones, who determined their refractive index to be 1.535, which is within the range for chalcedony (Jones, 1987). The outward appearance of these cubes resembles the usual stepped growth habit of melanophlogite from other localities. An X-ray powder-diffraction analysis of the mineral was consistent with quartz. These crystals may be considered quartz (chalcedony) pseudomorphs after melanophiogite.

Clear Creek mine, San Benito County

During field work at the Clear Creek mercury mine, San Benito County, several samples of what appeared to be small unmodified cubes covered with an opaline material were discovered in rocks at the open pit. These crystals were further investigated by optical tests and found to be a mixture of melanophlogite and a fibrous form of chalcedony. The host rock is a low-grade silicified serpentinite containing cinnabar and native mercury.

Vaughn mine, north of Hollister, San Benito County

The Vaughn mine is located about 5 km north of the Stayton district, Hollister, California. This mine, once worked for cinnabar in the late 1800's, was developed by a shallow inclined shaft about 10 meters deep which explored a silicified, mercury-bearing, hydrothermally-altered, pale tan-colored serpentinite. Scattered around the mouth of the inclined shaft and down the slope are many small rocks covered by what appeared to be very fine drusy quartz. Upon further examination using the SEM, this "drusy quartz" was found to consist of abundant, complex interpenetrating cubes. A subsequent X-ray powder pattern identified the cubes as quartz. These "cubes" may best be described as quartz pseudomorphs after melanophlogite.

Pescadero Beach, Pescadero, San Mateo County

In 1960, Mr. George Shokal of San Carlos, California, collected a float sample of what appeared to be a type of basalt from the beach at Pescadero. This sample, which washed ashore from an underwater formation, was identified as a vesicular black metavolcanic rock containing calcite and chalcedony. The groundmass is composed essentially of hornblende. When etched with formic acid to remove the calcite, both rounded masses and distinct cubes were observed within several of the vesicles. At high magnification, the surface of the rounded masses is seen to consist of abundant intergrown cubes. Optical tests indicate an index of refraction near chalcedony for both types. The cubes, which average 0.2 to 0.3 mm on an edge, show fine intergrowth features typical of melanophlogite. This mineral may best be termed a chalcedony (quartz) pseudomorph after melanophlogite. This is the first observation of pseudomorphic melanophlogite from a metavolcanic rock. Jakes and White (1972) give a thorough study of hornblendes from cale-alkaline volcanic rocks of island arcs and continental margins.


We thank the following individuals for their contributions to this paper: Mr. Clive Matson, Oakland, California, who graciously conducted a collecting trip to the Borges quarry to obtain representative samples of melanophlogite; Mr. George Shokal, San Carlos, California, who kindly provided samples of melanophlogite from Pescadero Beach for study, and the late Dr. Francis Jones, who provided many stimulating discussions on melanophlogite and other forms of silica.

* The morphology is described here in terms of a cubic cell, and the forms identified on that basis by simple inspection, with the understanding that the species is actually pseudocubic.


APPLEMAN, D. E. (1965) The crystal structure of melanophlogite, a cubic polymorph of Si[O.sub.2]. (Abstr.) American Crystallographic Association and Mineralogical Society of America Joint Meeting, Gatlingburg, TN, p. 80.

BAILEY, E. H., IRWIN, W. P., and JONES, D. L. (1964) Franciscan and related rocks and their significance in the geology of western California. California Division of Mines Bulletin 183, 177 p.

BERTRAND, E. (1880) Sur la thaumasite et la melanophlogite. Bulletin Societe Francaise Mineralogie, 3, 159-160.

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FRIEDEL, G. (1890) Sur la melanophlogite. Bulletin Societe Francaise Mineralogie, 13, 356-372.

JAKES, P., and WHITE, A. J. R. (1972) Hornblendes from cale-alkaline volcanic rocks of island arcs and continental margins. American Mineralogist, 57, 887-902.

JONES, F. T. (1987) A mineral mystery--chalcedony after melanophlogite? California Geology, 40, 127-129.

KAMB, B. (1965) A clathrate crystalline form of silica. Science, 148, 232-234.

LASAULX, A. VON (1876) Mineralogisch-kristallographische Notizen. VII. Melanophogit, ein neues Mineral. Nenes Jahrbuch fur Mineralogie, Geologie und Paleontologie, 1876, 250-257.

NAKAGAWA, T., KIHARA, K., and HARADA, K. (2001) The crystal structure of low-melanophlogite. American Mineralogist, 86, 1506-1512.

SKINNER, B. J., and APPLEMAN, D. E. (1963) Melanophlogite, a cubic polymorph of silica. American Mineralogist, 48, 854-867.

ZAK, L. (1967) Find of pyrophanite and melanophlogite in Chvaletice (E. Bohemia). Casopis pro Mineralogii a Geologii, 12, 451-452.

ZAK, L. (1972) A contribution to the crystal chemistry of melanophlogite. American Mineralogist, 57, 779-796.

ZAK, L. (1973) The transformation twinning of melanophlogite. Neues Jahrbuch fur Mineralogie, Monatshefte, 183-189.

ZAK, L. (1975) New observations on melanophlogites from Chvaletice and Mt. Hamilton, Neues Jahrbuch fur Mineralogie, Monatshefte, 509-513.
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Author:Dunning, Gail E.; F. Cooper, Joseph, Jr.
Publication:The Mineralogical Record
Geographic Code:1U9CA
Date:May 1, 2002
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