![]() This is a real geode, with real quartz crystals, but the purplish coloration is fake. ![]() The most common geode-lining mineral is quartz. They form when water enters a void in a host rock and precipitates crystals. Geodes are small to large, subspherical to irregularly-shaped, crystal-lined cavities in rocks. Quartz can be any color: clear, white, gray, black, brown, pink, red, purple, blue, green, orange, etc. It has a glassy, nonmetallic luster, is commonly clearish to whitish to grayish in color, has a white streak, is quite hard (H≡7), forms hexagonal crystals, has no cleavage, and has conchoidal fracture. It is composed of the two most abundant elements in the crust - oxygen and silicon. Quartz (silicon dioxide/silica - SiO2) is the most common mineral in the Earth's crust. Many silicates have a significant percentage of aluminum (the aluminosilicates). All other silicates have silica + impurities. The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). The resulting formula for silica is thus SiO2, not SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. 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. All silicates have silica as the basis for their chemistry. The silicates are the most abundant and chemically complex group of minerals. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. Mineral classification is based on anion chemistry. Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common. At its simplest, a mineral is a naturally-occurring solid chemical. Description Fake amethyst geode (Morocco) (31769323994).jpgĪ mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.
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