Lazurite

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
Lazurite, Ladjuar Medam (Lajur Madan; Lapis-lazuli Mine), Sar-e-Sang District, Koksha Valley (Kokscha; Kokcha), Badakhshan (Badakshan; Badahsan) Province, Afghanistan
General
CategoryTectosilicate
Formula
(repeating unit)
(Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]
Strunz classification9.FB.10
Crystal systemIsometric
Crystal classHextetrahedral (43m)
H-M symbol: (4 3m)
Space groupP43n
Unit cella = 9.09 Å; Z = 2
Identification
ColorDeep blue, azure, violet-blue, greenish blue
Crystal habitCrystals occur as dodecahedra, or rarely cubes; granular, disseminated, or massive
CleavageImperfect on {110}
FractureUneven
TenacityBrittle
Mohs scale hardness5–5.5
LusterVitreous
DiaphaneityTranslucent to opaque
Specific gravity2.38–2.45
Optical propertiesIsotropic; anomalously anisotropic
Refractive index1.502–1.522
Fusibility3.5
SolubilitySoluble in HCl
References[1][2][3][4][5]

Lazurite is a deep‐blue to greenish‐blue. The colour is due to the presence of S
3
anions.[6] 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 on Wolfgang Franz von Kobell's fusibility scale, and soluble in HCl. It commonly contains or is associated with grains of pyrite.

Lazurite is a product of contact metamorphism of limestone and is typically 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. At one time, lazurite was 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 more than 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.[7] It is also mined at Lake Baikal in Siberia; Mount Vesuvius; Burma; Canada; and the United States.[7] The name is from the Persian lazward for blue.[3]

Structure

Lazurite and hauyne seem to have the same structure and both are sulfate-dominant minerals.[8] Lazurite is a pigment (so opalescent) and has a bright blue streak (especially as a component of the semi-precious stone lapis lazuli). Many hauynes have a white/ pale blue streak and are translucent. The difference might be a consequence of the redox state (sulfate/ sulfide ratio).[6][9]

See also

  • Lapis lazuli  A contact metamorphic rock containing lazurite, pyrite and calcite
  • Ultramarine  A deep blue color pigment which was originally made with ground lapis lazuli

References

  1. Handbook of Mineralogy
  2. Mindat with location data
  3. Webmineral data
  4. 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. Tauson V L, Sapozhnikov A N (2003). "On the nature of lazurite coloring" (PDF). Zapiski Vserossijskogo Mineralogicheskogo Obshchestva (in Russian). 132 (5): 102–107.
  7. Eastaugh, Nicholas et al., 2004, The pigment compendium : optical microscopy of historical pigments Oxford: Elsevier Butterworth-Heinemann, p. 219, ISBN 0-7506-4553-9
  8. Moore, T.P., Woodside, R. W. M. (2014). "The Sar-e-Sang Lapis Mines". Mineralogical Record. 45 (3): 281–336.CS1 maint: uses authors parameter (link)
  9. Hettmann K, Wenzel T, Marks M, Markl G (2012). "The sulfur speciation in S-bearing minerals: New constraints by a combination of electron microprobe analysis and DFT calculations with special reference to sodalite-group minerals". American Mineralogist. 97: 1653–1661.CS1 maint: uses authors parameter (link)
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