Lazurite is a member of sodalite group in feldspathoid mineral also it is tectosilicate with formula is (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]. A sodium calcium aluminosilicate, lazurite is the main component of the gemstone lapis lazuli and accounts for the stone’s intense blue color, although lapis lazuli also typically contains pyrite, calcite, sodalite, and haüyne. Lazurite specimens are always deep or vibrant blue.
Distinct crystals were thought to be rare until large numbers were brought out of mines in Badakhshan, Afghanistan, in the 1990s. These are usually dodecahedral and are much sought after. Most lazurite is either massive or occurs in disseminated grains. Lapis lazuli is relatively rare. It forms in crystalline limestones as a product of contact metamorphism. The best quality lapis lazuli is dark blue with minor patches of calcite and pyrite. In addition to its use as a gemstone, lapis lazuli was used as one of the first eye shadows. (Bonewitz, 2012)
Lazurite crystallizes in the isometric system, although it is well-formed crystals. Generally, massive and precious stone forms the mass of lapis lazuli.
Name: For its color resemblance to azurite, named from the Persian lazhward, for blue
Association: Calcite, pyrite, diopside, humite, forsterite, hauyne, muscovite
Polymorphism & Series: Triclinic and monoclinic polymorphs are known
Mineral Group: Sodalite group
Diagnostic Features: Characterized by its blue color and the presence of associated pyrite.
Lazurite Composition: Na4-5Al3Si3012S, with small- amounts of the sodalite and hauynite molecules in isomorphous replacement.
Crystallography: Isometric. Crystals rare, usually dodecahedral. Commonly massive, compact.
Contents
Chemical Properties
| Chemical Classification | Tectosilicate |
| Formula | (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)] |
| Idealised Formula | Na3Ca(Si3Al3)O12S |
| Common Impurities | Fe,Mg,K,H2O |
Lazurite Physical Properties
| Color | Ultramarine, midnight blue, bluish green, green |
| Streak | Bright blue for Lazurite, White for the small cell Hauynes |
| Luster | Resinous |
| Cleavage | Imperfect/Fair Imperfect on {110} |
| Diaphaneity | Opaque |
| Mohs Hardness | 5 – 5,5 |
| Crystal System | Isometric |
| Tenacity | Brittle |
| Density | 2.38 – 2.45 g/cm3 (Measured) 2.4(1) g/cm3 (Calculated) |
| Fracture | Sub-Conchoidal |
Lazurite Optical Properties
| Type | Isotropic |
| Crystal habit | Crystals occur as dodecahedra, or rarely cubes; granular, disseminated, or massive |
| RI values: | n = 1.502 – 1.522 |
| Birefringence | Isotropic minerals have no birefringence |
| Relief | Low |
| Twinning | None observed |
Occurrence
Lazurite is a rare mineral, occurring usually in crystalline limestones as a product of contact metamorphism. Lapis lazuli is usually a mixture of lazurite with small amounts of calcite, pyroxene, and other silicates, and commonly contains small disseminated particles of pyrite. The best quality of lapis lazuli comes from northeastern Afghanistan. Also found at Lake Baikal, Siberia; and in Chile.
Uses Area
Lapis lazuli is highly prized as an ornamental stone, for carvings, etc. As a powder it was formerly used as the paint pigment ultra-marine. Now ultramarine is produced artificially.
Distribution
- Exceptional crystals from Sar-e-Sang, Badakhshan Province, Afghanistan.
- Well-crystallized material from the basins of the Slyudyanka and Bystraya Rivers, Sayan Mountains, near Lake Baikal, Siberia, Russia.
- At Lyadzhuar-Darinsk, near Ishkashima, Pamir Mountains, Tajikistan.
- From Monte Somma, Campania, and in the Alban Hills, Lazio, Italy.
- In the USA, at Ontario Peak and Cascade Canyon, San Bernardino Co., California, and on North Italian Mountain, Gunnison Co., Colorado.
- In Canada, about 15 km north of Lake Harbour, Ba±n Island, Northwest Territory.
- At Thabapin, near Mogok, Myanmar (Burma).
- From along the Cazadero River, near Ovalle, Chile.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Dana, J. D. (1864). Manual of Mineralogy… Wiley.
- Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
- Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
