![]() ![]() Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture. Common opal has a wax-like luster & is often milky whitish with no visible color play at all. Not all opals have the famous play of colors, however. Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979). If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced. Different opalescent colors are produced by colloids of differing sizes. This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids. Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence). Opal is made up of extremely tiny spheres called colloids that can be seen with a scanning electron microscope (SEM). Opal is supposed to be called a mineraloid. ![]() Technically, opal is not a mineral because it lacks a crystalline structure. 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 5600 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. (CMNH 10146, Cleveland Museum of Natural History, Cleveland, Ohio, USA)Ī mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. Precious opal ("opal pineapple") from Australia. Precious opal after glendonite (White Cliffs Opal Field, New South Wales, Australia) ![]()
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