Image: Dumortierite-quartz (Brazil) 10

Description: 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 5700 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. Dumortierite is a scarce silicate mineral having the formula (Al,Fe)7(SiO4)3(BO3)O3 - aluminum iron borosilicate. It is often bluish, but can be other colors (greenish, purplish, pinkish, reddish). Dumortierite is fairly hard (H=7 to 8 on the Mohs Hardness Scale). Seen here is a dumortierite-quartz specimen from the Precambrian of Brazil. It comes from an area having dumortierite-bearing quartzites (= metamorphosed sandstones) that are quarried, cut, and polished as decorative stones ("Azul Macaubas"). Locality: unrecorded/undisclosed site in Brazil (but probably from or near the Vaca Morta Quarry, southwest of Boquira, Bahia State, eastern Brazil)

Photo gallery of dumortierite:

www.mindat.org/gallery.php?min=1329
Author: James St. John
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