English: Precious opal from the Tertiary of Ethiopia.

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 6000 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.

Opal is hydrous silica (SiO2·nH2O). Technically, opal is not a mineral because it lacks a crystalline structure. Opal is supposed to be called a mineraloid. Opal is made up of extremely tiny spheres (colloids) that can be seen with a scanning electron microscope (SEM).

Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence). 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. Different opalescent colors are produced by colloids of differing sizes. If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced. Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979).

Not all opals have the famous play of colors, however. Common opal has a wax-like luster & is often milky whitish with no visible color play at all. Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.

Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians. Some organisms incorporate opal into their tissues, for example horsetails/scouring rushes and sawgrass. Sometimes, fossils are preserved in opal or precious opal.

The specimen seen here is opal-CT, which consists of extremely tiny cristobalite-tridymite aggregates called leptospheres. This is in contrast to Australian precious opal, which is opal-A (= consists of amorphous, hydrous silica colloids). Ethiopian precious opal occurs as nodules in Tertiary-aged volcanic tuffs.

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Photo gallery of opal: www.mindat.org/gallery.php?min=3004

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Reference cited:

Carr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program. Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51. 68 pp.