Lazurite in pyritic marble from the Precambrian of Afghanistan. (field of view ~2.5 centimeters across)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

Lazurite is one of several silicate minerals called "feldspathoids". Feldspathoids are chemically similar to the feldspars, but they have far less silica (SiO2) and end up enriched in sodium and/or potassium (typically). Lazurite is a rare feldspathoid that is famous for its blue coloration. It has the moderately complex chemical formula (Na,Ca)8(AlSiO4)6(SO4,S,Cl)2 - sodium calcium aluminosilicate-sulfate-sulfide-chloride. Lazurite has a nonmetallic luster, is blue to slightly purplish blue in color, and has a hardness of about 5 to 5.5. Lazurite tends to occur in contact metamorphic rocks. Sometimes, lazurite is a rock-forming mineral and forms "lapis lazuli" (see: <a href="https://www.flickr.com/photos/jsjgeology/albums/72157646941999842">www.flickr.com/photos/jsjgeology/albums/72157646941999842</a>).

The rock shown above is pyritic marble (whitish-gray = calcite; brassy gold = pyrite) with a lazurite crystal (blue). The actual specimen has a hint of purplish coloration not apparent in the photo. This rock is a contact metamorphic marble of Precambrian age in Afghanistan's Hindu Kush Mountains.

Geology: Sar-e-Sang Deposit (~1 to 8 meter thick & ~20 to 450 meter long, north-south trending veins, lenses, and layers of retrograde and/or metasomatic lazurite and lapis lazuli occurring irregularly in whitish, high-pressure, amphibolite- to granulite-grade marbles/calciphyres adjacent to alaskite granite & pegmatite & mafic dike intrusions), marble member of the Sakhi Formation (at least 400 meters thick), Sanglich Group, Sar-e-Sang Series, Neoarchean to lower Paleoproterozoic (?), metamorphism at 2.4 to 2.7 Ga (metamorphic date from Pamir, Tajikhistan)

Locality: Sar-e-Sang Mines, ~1500 feet above river water level, Sar-e-Sang Mining District, above the near-lowermost stretch of the Sar-e-Sang River (= west-flowing tributary of the Kokcha River), 5 kilometer stretch on the eastern side of the Kokcha River between the town of Koran-o-Munjan (to the south) & the village of Robat-e-Payan (to the north), ~100 kilometers south-southeast of Fayzabad, Kokcha River Valley, northern flanks of the western Hindu-Kush Mountains, northern Kuran Wa Munjan District, southern Badakhshan Province, northeastern Afghanistan (location of Sar-e-Sang Mine adits: approximately 36° 12.2’ to 36° 14.14’ North latitude & 70° 47.85’ to 70° 48.63’ East longitude)

Photo gallery of lazurite: