Lazurite

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Lazurite
Lazurite.jpg
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 symmetry Cubic hexoctahedral 43m
Unit cell a = 9.09 Å; Z = 1
Identification
Color Deep blue, azure, violet-blue, greenish blue
Crystal habit Crystals occur as dodecahedra, or rarely cubes; granular, disseminated, or massive
Crystal system Isometric
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 [1][2][3][4][5]

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.[1]

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.[4]

Lazurite was first described in 1890 for an occurrence in the Sar-e-Sang District, Koksha Valley, Badakhshan Province, Afghanistan.[2] It has been mined for over 6,000 years in the lapis lazuli district of Badakhshan, Afghanistan. It has been used as a pigment in painting and cloth dyeing since at least the sixth or seventh century CE.[6] It is also mined at Lake Baikal in Siberia; Mount Vesuvius; Burma; Canada; and the United States.[6] The name is from the Persian lazward for blue.[3]

See also[edit]

References[edit]

  1. ^ a b Handbook of Mineralogy
  2. ^ a b Mindat with location data
  3. ^ a b Webmineral data
  4. ^ a b Hurlbut, Cornelius S. and Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, p. 459 ISBN 0-471-80580-7
  5. ^ Mineralogical Society of America
  6. ^ a b Eastaugh, Nicholas et al., 2004, The pigment compendium : optical microscopy of historical pigments Oxford: Elsevier Butterworth-Heinemann, p. 219, ISBN 0-7506-4553-9