Pearceite
| Pearcbuteite | |
|---|---|
Pearceite from Butte, Montana, US
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| General | |
| Category | Mineral |
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Formula (repeating unit) |
Cu(Ag,Cu)6Ag9As2S11 |
| Strunz classification | 2.GB.15 (10 ed) 2/E.05-20 (8 ed) |
| Dana classification | 3.1.8.1 |
| Crystal system | Monoclinic or trigonal |
| Identification | |
| Formula mass | 2,096.80 g/mol |
| Color | Black |
| Crystal habit | Pseudohexagonal prisms |
| Cleavage | {001} Poor |
| Fracture | Conchoidal to irregular |
| Tenacity | Brittle |
| Mohs scale hardness | 3 |
| Luster | Metallic |
| Streak | Black |
| Diaphaneity | Opaque |
| Specific gravity | 6.15 |
| Optical properties | Biaxial |
| Refractive index | 2.7 |
| Birefringence | 2.7 |
| Pleochroism | RL Pleochroism (in reflected plane polarised light): Very weak in air, fair in oil |
| Other characteristics | Non-fluorescent, nonmagnetic, not radioactive |
| References | |
Pearceite is one of the four so-called "ruby silvers", pearceite Cu(Ag,Cu)6Ag9As2S11, pyrargyrite Ag3SbS3, proustite Ag3AsS3 and miargyrite AgSbS2. It was discovered in 1896 and named after Dr Richard Pearce (1837–1927), a Cornish–American chemist and metallurgist from Denver, Colorado.
Pearceite and polybasite are closely related minerals that form the pearceite-polybasite series. Originally pearceite was thought to be an arsenic analogue of polybasite Cu(Ag,Cu)6Ag9Sb2S11, and was called arsenpolybasite, and one polytype of polybasite was called antimonpearceite. Arsenpolybasite was found to represent two different polytypes, arsenpolybasite-221 and arsenpolybasite-222. In modern usage the old name pearceite is replaced by the polytype name pearceite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c and antimonpearcite by polybasite-Tac. Pearceite-Tac forms a series with polybasite-Tac.
Two structural varieties, trigonal and monoclinic, are known. The trigonal variety crystallizes in the hexagonal scalenohedral class 3m (3 2/m), space group P3m1 (P3 2/m 1). The monoclinic variety crystallises in the prismatic 2/m class, space group C2/m.
Unit cell parameters
The crystal structure consists of sheets stacked along the c axis. The arsenic atoms form isolated (As,Sb)S3 pyramids, copper cations link two sulfur atoms and the silver cations are found in various sites with low coordination numbers, 2,3 and 4, as is usually the case with silver.
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Wikipedia