Lazurite


Lazurite is a tectosilicate mineral with sulfate, sulfur and chloride with formula 8. 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 Trisulfur#Radical anion| 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 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.
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.
Lazurite was first described in 1890 for an occurrence in the Sar-e-Sang District, Koksha Valley, Badakhshan Province, Afghanistan. It has been mined for more than 6,000 years in the lapis lazuli district of Badakhshan. It has been used as a pigment in painting and cloth dyeing since at least the 6th or 7th century. It is also mined at Lake Baikal in Siberia; Mount Vesuvius; Burma; Canada; and the United States. The name is from the Persian lajvard for blue.

Structure

Lazurite and hauyne seem are sulfate-dominant minerals. Lazurite is a pigment and has a bright blue streak. Many hauynes have a white or pale blue streak and are translucent. The difference might be a consequence of the redox state.