Black smoky quartz

General

 

Category: Silicate mineral

 

Chemical formula: SiO2 + (Traces of Al13)

 

Strunz classification: 04.DA.05

 

Dana classification: 75.01.03.01

 

Crystal system: α-quartz: trigonal trapezohedral class 3 2; β-quartz: hexagonal 622

 

Space group: Trigonal 32

 

Unit cella = 4.9133 Å, c = 5.4053 Å; Z=3

 

 

Identification

 

Colour: Brown to grey, opaque

 

Crystal habit: 6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive

 

Twinning: Common Dauphine law, Brazil law and Japan law

 

Cleavage: {0110} Indistinct

 

Fracture: Conchoidal

 

Tenacity: Brittle

 

Mohs scale hardness: 7 – lower in impure varieties (defining mineral)

 

Lustre: Vitreous – waxy to dull when massive

 

Streak: White

 

Diaphaneity: Transparent to nearly opaque

 

Specific gravity: 2.65; variable 2.59–2.63 in impure varieties

 

Optical properties: Uniaxial (+)

 

Refractive index: nω = 1.543–1.545

nε = 1.552–1.554

 

Birefringence: +0.009 (B-G interval)

 

Pleochroism: weak, from red-brown to green-brown

 

Melting point: 1670 °C (β tridymite) 1713 °C (β cristobalite)

 

Solubility: Insoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2

 

Other characteristics: lattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)

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