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Opal-AN

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Formula:
SiO2 · nH2O
Colour:
colourless, white, creme-coloured
Lustre:
Vitreous
Hardness:
5½ - 6½
Specific Gravity:
2.0 - 2.2
A variety of Opal

Opal-AN is a term for amorphous opal with a glass-like structure (Graetsch, 1994). The "A" in the name stands for amorphous; the subscript "N" is to imply its structure is network-like similar to silica glass; however, it still contains about 3-8% water, both as molecular water and bound in silanole (SiOH) groups.

This form of opal occurs as hyalite in volcanic and pegmatite environments where the silica deposits from the gas phase (Flörke et al, 1973) and forms globular and irregular crusts. Because of its homogeneous internal structure, it does not show opalescence and is transparent if pure. The refractive index varies slightly with water content and lies between 1.44 and 1.46 (Graetsch, 1994).



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Physical Properties of Opal-ANHide

Vitreous
Transparency:
Transparent, Translucent
Colour:
colourless, white, creme-coloured
Comment:
no opalescence (play of colour)
Streak:
white
Hardness:
5½ - 6½ on Mohs scale
Tenacity:
Brittle
Cleavage:
None Observed
Fracture:
Conchoidal
Density:
2.0 - 2.2 g/cm3 (Measured)    
Comment:
varies with water content

Chemical Properties of Opal-ANHide

Formula:
SiO2 · nH2O

Synonyms of Opal-ANHide

Other Language Names for Opal-ANHide

Common AssociatesHide

Associated Minerals Based on Photo Data:
Quartz132 photos of Opal-AN associated with Quartz on mindat.org.
Fluorite131 photos of Opal-AN associated with Fluorite on mindat.org.
Schorl130 photos of Opal-AN associated with Schorl on mindat.org.
Muscovite79 photos of Opal-AN associated with Muscovite on mindat.org.
Feldspar Group78 photos of Opal-AN associated with Feldspar Group on mindat.org.
Smoky Quartz78 photos of Opal-AN associated with Smoky Quartz on mindat.org.
Carbonate-rich Fluorapatite63 photos of Opal-AN associated with Carbonate-rich Fluorapatite on mindat.org.
Aquamarine47 photos of Opal-AN associated with Aquamarine on mindat.org.
Orthoclase46 photos of Opal-AN associated with Orthoclase on mindat.org.
Topaz44 photos of Opal-AN associated with Topaz on mindat.org.

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-ANHide

Reference List:
Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Flörke, O.W., Jones, J.B., Segnit, E.R. (1973) The genesis of hyalite. Neues Jahrbuch der Mineralogie, Monatshefte, H.2, 82-89.
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-ANHide

Localities for Opal-ANHide

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 ListShow

Mineral and/or Locality  
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