Lazurite

Lazurite
Lazurite, Ladjuar Medam (Lajur Madan; Lapis-lazuli Mine), Sar-e-Sang District, Koksha Valley (Kokscha; Kokcha), Badakhshan (Badakshan; Badahsan) Province, Afghanistan
General
Category	Tectosilicate
Formula (repeating unit)	(Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]
Strunz classification	9.FB.10
Crystal system	Isometric
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	P43m
Unit cell	a = 9.09 Å; Z = 2
Identification
Color	Deep blue, azure, violet-blue, greenish blue
Crystal habit	Crystals occur as dodecahedra, or rarely cubes; granular, disseminated, or massive
Cleavage	Imperfect on {110}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5–5.5
Luster	Vitreous
Diaphaneity	Translucent to opaque
Specific gravity	2.38–2.45
Optical properties	Isotropic; anomalously anisotropic
Refractive index	1.502–1.522
Fusibility	3.5
Solubility	Soluble in HCl
References

Lazurite is a tectosilicate mineral with sulfate, sulfur and chloride with formula: (Na,Ca)₈[(S,Cl,SO₄,OH)₂|(Al₆Si₆O₂₄)]. It is a feldspathoid and a member of the sodalite group. Lazurite crystallizes in the isometric system although well formed crystals are rare. It is usually massive and forms the bulk of the gemstone lapis lazuli.

Lazurite is a deep blue to greenish blue. The colour is due to the presence of S−
3 anions. It has a Mohs hardness of 5.0 to 5.5 and a specific gravity of 2.4. It is translucent with a refractive index of 1.50. It is fusible at 3.5 and soluble in HCl. It commonly contains or is associated with grains of pyrite.

Lazurite is a product of contact metamorphism of limestone and typically is associated with calcite, pyrite, diopside, humite, forsterite, hauyne and muscovite.

Other blue minerals such as the carbonate azurite and the phosphate lazulite may be confused with lazurite, but are easily distinguished with careful examination. Lazurite at one time was used as a synonym for azurite.