Amethyst

General

Category: Silicate mineral

Formula: SiO2 + (Traces of Fe3)

Crystal system: Trigonal

Crystal class: Trapezohedral (32)

Space group: P3221 (no. 154)


Identification

Color: Purple, violet, dark purple

Crystal habit: 6-sided prism ending in 6-sided pyramid (typical)

Twinning: Dauphine law, Brazil law, and Japan law

Cleavage: None

Fracture: Conchoidal

Mohs scale hardness: 7–lower in impure varieties

Luster: Vitreous/glassy

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: Weak to moderate purple/reddish purple

Melting point: 1650±75 °C

Solubility: Insoluble in common solvents

Other characteristics: Piezoelectric

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