![]() 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 (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 5400 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 Dendritic opal (29816456982).jpgĭendritic opal (public display, Nebraska State Museum of Natural History, Lincoln, Nebraska, USA)Ī 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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