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Opal-AG

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About Opal-AGHide

Formula:
SiO2 · nH2O
Lustre:
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" 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

Pronounciation:
PlayRecorded byCountry
Jolyon & Katya RalphUnited Kingdom

Physical Properties of Opal-AGHide

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

Chemical Properties of Opal-AGHide

Formula:
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.
Australia
 
  • New South Wales
    • Finch Co.
https://www.mindat.org/gallery.php?frm_id=pager&cform_is_valid=1&min=&loc=93&u=&potd=&pco=1&d=&showtype=1&phototype=0&checkall=&filtmin=0&loctxt=&keywords=potch&orderxby=&submit_pager=Filter+Search
Belgium
 
  • Hainaut Province (Henegouwen; Hennegau)
    • Strépy-Bracquegnies
Les minéraux de Belgique - Page 267 books.google.be/books?id=PncLAQAAIAAJ Joseph Mélon, ‎Pol Bourguignon, ‎André Mathieu Fransolet - 1976 - ‎Extraits -
France
 
  • Auvergne-Rhône-Alpes
    • Puy-de-Dôme
Japan
 
  • Kyushu Region
    • Nansei Archipelago
      • Kagoshima Prefecture
        • Mishima village
          • Satsuma-Ioujima (Satsuma-Iwojima; Iou island)
            • Iodake (Iwodake; Ioudake)
PXRD by Alfredo Petrov, 15-10-2017.
Mineral and/or Locality  
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