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About Opal-AGHide

SiO2 · nH2O
Vitreous, Sub-Vitreous, Greasy
A variety of Opal

A type of Opal consisting of aggregated spheres of amorphous silica, with water filling the gaps in between. Precious Opal and Potch Opal consist of this kind of structure - the difference being in the regularity of the sizes of the spheres and packing.

Precious opal
"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 micro-organisms 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.

Pronounciation of Opal-AGHide

PlayRecorded byCountry
Jolyon & Katya RalphUnited Kingdom

Physical Properties of Opal-AGHide

Vitreous, Sub-Vitreous, Greasy
Transparent, Translucent
greasy to vitreous on freshly broken surfaces

Chemical Properties of Opal-AGHide

SiO2 · nH2O

Synonyms of Opal-AGHide

Other InformationHide

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Opal-AGHide

Reference List:
Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Sanders, S.V. (1964) Colour of precious opal. Nature, 204: 1151-1153.
Jones, J.B., Sanders, J.V., Segnit, E.R. (1964) Structure of opal. Nature, 204: 990-991.
Jones, J.B., Segnit, E.R. (1971) The nature of opal. I Nomenclature and constituent phases. Journal of the Geological Society of Australia: 18: 57-68.
Sanders, J.V. (1975) Microstructure and crystallinity of gem opals. American Mineralogist, 60, 749-757.
Flörke, O.W., Graetsch, H. Martin, B., Röller, K. (1991) Nomenclature of micro- and non-crystalline silica minerals based on structure and microstructure. Neues Jahrbuch der Mineralogie, Abhandlungen, 163: 19-42.
Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. In: Reviews in Mineralogy, Volume 29, Silica - Physical behavior, geochemistry and materials applications. Mineralogical Society of America, Washington, D.C.

Internet Links for Opal-AGHide

Localities for Opal-AGHide

This map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.

Locality ListHide

- This locality has map coordinates listed. - This locality has estimated coordinates. ⓘ - Click for further information on this occurrence. ? - Indicates mineral may be doubtful at this locality. - Good crystals or important locality for species. - World class for species or very significant. (TL) - Type Locality for a valid mineral species. (FRL) - First Recorded Locality for everything else (eg varieties). Struck out - Mineral was erroneously reported from this locality. Faded * - Never found at this locality but inferred to have existed at some point in the past (eg from pseudomorphs.)

All localities listed without proper references should be considered as questionable.
  • New South Wales
    • Finch Co.
  • Wallonia
    • Hainaut
      • Strépy-Bracquegnies
Les minéraux de Belgique - Page 267 Joseph Mélon, ‎Pol Bourguignon, ‎André Mathieu Fransolet - 1976 - ‎Extraits -
  • Auvergne-Rhône-Alpes
    • Puy-de-Dôme
  • Kagoshima
    • Mishima village
      • Satsuma-Ioujima (Satsuma-Iwojima; Iou island)
        • Iodake (Iwodake; Ioudake)
PXRD by Alfredo Petrov, 15-10-2017.
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