Alacranite
| Alacranite | |
|---|---|
| General | |
| Category | Sulfide mineral |
|
Formula (repeating unit) |
As8S9 |
| Strunz classification | 2.FA.20 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | P2/c |
| Unit cell | a = 9.942, b = 9.601 c = 9.178 [Å]; β = 101.94°; Z = 2 |
| Identification | |
| Color | Orange to pale gray, rose-yellow internal reflections |
| Crystal habit | Equant grains |
| Cleavage | Imperfect on {100} |
| Tenacity | Brittle |
| Mohs scale hardness | 1.5 |
| Luster | Vitreous, resinous, greasy |
| Streak | Yellow-orange |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.4 - 3.46 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 2.390(1) nγ = 2.520(2) |
| Birefringence | 0.1300 |
| References | |
Alacránite (As8S9) is an arsenic sulfide mineral first discovered in the Uzon caldera, Kamchatka, Russia. It was named for its occurrence in the Alacrán silver, arsenic, antimony mine, Pampa Larga, Chile. It is generally more rare than realgar and orpiment. Its origin is hydrothermal. It occurs as subhedral to euhedral tabular orange to pale gray crystals that are transparent to translucent. It has a yellow-orange streak with a hardness of 1.5. It crystallizes in the monoclinic crystal system. It occurs with realgar and uzonite as flattened and prismatic grains up to 0.5 mm across.
When alacranite was first discovered in 1970 by Clark, it was associated with barite and calcite veins in the Alacran silver mine. They also assumed that the mineral was identical to the species occurring in the Ag-As-Sb vein deposit at Alacran due to the similarity of its X-ray diffraction powder pattern. They discovered that alacranite was similar to the high-temperature α polymorph of As4S4 in X-ray characteristics. In addition, alacranite was considered as realgar-like mineral. After that, they reported the composition of alacranite as As8S9 when they noticed another occurrence of Alacranite in Uzon Caldera associated with realgar and uzonite as cement in sandy gravels. They reported the composition of alacranite to be As8S9 regarding to the electron-microprobe analyses. When they analyzed a mineral during seafloor sampling consisting of red and orange arsenic sulfides by X-ray diffraction, a mixture of realgar and alacranite was resulted and it was noticed that the mineral was identical to synthetic β-As4S4 supposed that the original chemical formula of alacranite (As8S9) was incorrect. They argued that it is identical to alacranite because of the similarity of the physical properties and the unit-cell dimensions but further studies argue that it is different with respect to chemical formula and unit-cell volume. The chemistry of the average of four analyses in Uzon caldera, Russia corresponding to alacranite results in 67.35% arsenic and 32.61% sulfur resulting in a total of 99.96%, yielding the formula As7.98S9.02, ideally As8S9.
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