Pararealgar
Pararealgar is an arsenic sulfide mineral with the chemical formula As4S4, also represented as AsS. It forms gradually from realgar under exposure to light. Its name derives from the fact that its elemental composition is identical to realgar, As4S4. It is soft with a Mohs hardness of 1 - 1.5, is yellow orange in colour, and its monoclinic prismatic crystals are very brittle, easily crumbling to powder.
| Pararealgar | |
|---|---|
| General | |
| Category | Sulfide mineral |
| Formula (repeating unit) | As4S4 |
| IMA symbol | Prlg |
| Strunz classification | 2.FA.15b |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | P21/c |
| Unit cell | a = 9.909 Å, b = 9.655 Å, c = 8.502 Å; β = 97.20°; Z = 16 |
| Identification | |
| Colour | Bright yellow when powdery, to yellow-orange and orange-brown when granular |
| Crystal habit | Fine powder to granular |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 1 – 1.5 |
| Lustre | Vitreous to resinous |
| Streak | Bright yellow |
| Diaphaneity | Translucent |
| Specific gravity | 3.52 |
| Optical properties | Biaxial (?) |
| Birefringence | 2.02 |
| Pleochroism | High: x = orange yellow, y = bright yellow, z = orange red |
| References | |
It is one of the sulfides of arsenic and is one of two isomers of As4S4. It forms upon exposure of the symmetrical isomer to light. Its name derives from the fact that its elemental composition is identical to realgar, As4S4.
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