Acanthite
Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below 173 °C (343 °F). Argentite is the stable form above that temperature. As argentite cools below that temperature its cubic form is distorted to the monoclinic form of acanthite. Below 173 °C acanthite forms directly. Acanthite is the only stable form in normal air temperature.
| Acanthite | |
|---|---|
Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco | |
| General | |
| Category | Sulfide mineral |
| Formula (repeating unit) | Ag2S |
| IMA symbol | Aca |
| Strunz classification | 2.BA.30a |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | P21/n |
| Unit cell | a = 4.229 Å, b = 6.931 Å c = 7.862 Å; β = 99.61°; Z = 4 |
| Identification | |
| Color | Iron-black |
| Crystal habit | Primary crystals rare, prismatic to long prismatic, elongated along [001], may be tubular; massive. Commonly paramorphic after the cubic high-temperature phase ("argentite"), of original cubic or octahedral habit |
| Twinning | Polysynthetic on {111}, may be very complex due to inversion; contact on {101} |
| Cleavage | Indistinct |
| Fracture | Uneven |
| Tenacity | Sectile |
| Mohs scale hardness | 2.0–2.5 |
| Luster | Metallic |
| Streak | Black |
| Diaphaneity | Opaque |
| Specific gravity | 7.20–7.22 |
| References | |
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