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A variety ofOpal-A
A type ofOpal consisting of aggregated spheres of amorphous silica, with water filling the gaps in between.Precious Opal andPotch Opal consist of this kind of structure - the difference being in the regularity of the sizes of the spheres and packing.
"Precious Opal" shows a play of colours ("opalescence") in white light that is due to diffraction from the regular packing of silica spheres of roughly equal size (Jones et al., 1964; Sanders, 1964). Although the packing of the spheres may be regular, there is neither short-range nor long-range order in this material, and the X-ray powder diffraction pattern is characterized by a distinct broad hump and a possible weak second hump indicative of material that is "X-ray amorphous" (Jones and Segnit, 1971). Since the packing of the spheres is similar to the structure of a gel, the subscript G ("gel-like") has been added (Flörke et al., 1991; Graetsch, 1994). Not all precious opal is opal-AG: The diffracting structure and the resulting opalescence may be preserved to some degree when opal-AG transforms to opal-CT by crystallization (Sanders, 1975).
Opal that shows no play of colours is also composed of the same spherical clusters, but they are non-uniform in size and do not pack in an orderly manner, thus cancel out any possible diffraction of the light (Jones et al., 1964; Sanders, 1964). This type of opal is termed collectively "potch opal" and includes the massive varieties. Porous varieties of inorganic origin such as geyserite and materials composed of tests of microorganisms such as diatomite and radiolarite are also recognized. The X-ray powder diffraction pattern is essentially identical for of all these types of opal, regardless of their origin.
A type ofOpal consisting of aggregated spheres of amorphous silica, with water filling the gaps in between.Precious Opal andPotch Opal consist of this kind of structure - the difference being in the regularity of the sizes of the spheres and packing.
Precious opal
Opal that shows no play of colours is also composed of the same spherical clusters, but they are non-uniform in size and do not pack in an orderly manner, thus cancel out any possible diffraction of the light (Jones et al., 1964; Sanders, 1964). This type of opal is termed collectively "potch opal" and includes the massive varieties. Porous varieties of inorganic origin such as geyserite and materials composed of tests of microorganisms such as diatomite and radiolarite are also recognized. The X-ray powder diffraction pattern is essentially identical for of all these types of opal, regardless of their origin.