English: "Precious opal ("opal pineapple") from Australia. (CMNH 10146, Cleveland Museum of Natural History, Cleveland, Ohio, USA)

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 4900 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 - www.uwgb.edu/dutchs/acstalks/acscolor/OPALSPHR.jpg) 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 precious opal specimen shown above is a very rare and valuable "opal pineapple" from Australia's White Cliffs Opal Field. The crystal structure of this sample is unusual and unexpected, as opal does not crystallize. This is a pseudomorph ("false form") - opal has replaced a previous mineral - in this case, one that had a radiating crystal cluster form. The morphology is consistent with glendonite (see: www.flickr.com/photos/jsjgeology/14937000890), which is itself a pseudomorph. Glendonite is calcite after ikaite (= CaCO3 after CaCO3·6H2O) (= calcium carbonate after hydrous calcium carbonate). Double pseudomorphs are rare - this is opal after glendonite after ikaite (see Frazier & Frazier, 2007).

Opal in the White Cliffs Opal Field is hosted in Cretaceous sedimentary rocks consisting of interbedded sandstones and arenaceous to silty claystones. Opalization postdates deposition of the sediments and occurred during the latest Cretaceous to Early Eocene (see Burton & Mason, 1998).

Host rocks: Doncaster Member, Wallumbilla Formation, Aptian Stage, upper Lower Cretaceous

Locality: White Cliffs Opal Field, western New South Wales, southeastern Australia

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Photo gallery of opal:

www.mindat.org/gallery.php?min=3004

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References 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.

Burton & Mason (1998) - Controls on opal localisation in the White Cliffs area. Quarterly Notes, Geological Survey of New South Wales 107: 1-11.

Frazier & Frazier (2007) - White Cliffs in New South Wales: opals and pineapples. in: Opal, the phenomenal gemstone. ExtraLapis [English edition] 10: 30-32." -James St John