File:Gastropod in menilite opal nodule (Upper Miocene; Pantano Camarillas, Agramón, Spain).jpg

English: Gastropod fossil in menilite opal nodule from the Miocene of Spain. (width of specimen through the ~middle of the photo is ~1.8 centimeters)

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 5600 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 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 and blue and 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 - common opal has a waxy luster and is often milky whitish. Opal is moderately hard (H = 5 to 6), has a white streak, and breaks with 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 Spanish specimen seen here is known as "menilite" opal - it is a composite nodule (two have fused together) with a dull luster. The opal composition is due to its young age. Geologically juvenile opal nodules will eventually be altered to chert / flint. The subcircular pit near the center is part of a fossil snail shell.

Geologic context: lacustrine diatomites, Upper Miocene (Tortonian to Messinian Stages); Camarillas Basin, Prebetic Zone

Locality: Pantano Camarillas, near Agramón, southeastern Spain

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

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Photo gallery of menilite opal: www.mindat.org/gallery.php?min= 9796

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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.