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| Amethyst | |
|---|---|
Amethyst cluster from Magaliesburg, South Africa.
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| General | |
| Category | Mineral variety |
| Formula (repeating unit) |
Silica (silicon dioxide, SiO2) |
| Identification | |
| Color | Purple, violet |
| Crystal habit | 6-sided prism ending in 6-sided pyramid (typical) |
| Crystal system | rhombohedral class 32 |
| Twinning | Dauphine law, Brazil law, and Japan law |
| Cleavage | None |
| Fracture | Conchoidal |
| Mohs scale hardness | 7–lower in impure varieties |
| Luster | Vitreous/glossy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.65 constant; variable in impure varieties |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.543–1.553 nε = 1.552–1.554 |
| Birefringence | +0.009 (B-G interval) |
| Pleochroism | None |
| Melting point | 1650±75 °C |
| Solubility | insoluble in common solvents |
| Other characteristics | Piezoele |
Malachite is a copper carbonate hydroxide mineral, with the formula Cu2CO3(OH)2. This opaque, green banded mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses, in fractures and spaces, deep underground, where the water table and hydrothermal fluids provide the means for chemical precipitation. Individual crystals are rare but do occur as slender to acicular prisms. Pseudomorphs after more tabular or blocky azurite crystals also occur.[3] Typical malachite is laminated[citation needed] and whether or not microbes intervene in its formation is unknown