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© Antonio Borrelli
|Colour:||Colorless, white, ...||Hardness:||5½ - 6½|
|Name:||From the Sanskrit "upala", meaning "stone" or "precious stone".|
Although it is still (2007) regarded as a valid mineral species for historical reasons, Opal is not a true mineral in the accepted sense of the word as it is either composed of Cristobalite and/or Tridymite or composed of amorphous silica.
Opal is broken down into four types:
Opal-AG Amorphous -Gel (closely packed amorphous silica spheres form a diffraction grating to create Precious Opal).
Opal-AN Amorphous-Network (Hyalite)
Transitions between opal-AG, opal-CT and opal-C are common.
Visit gemdat.org for gemological information about Opal. Currently in public beta-test.
Classification of Opal
|IMA status:||Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"|
|Strunz 8th edition ID:||4/D.01-80|
|Nickel-Strunz 10th (pending) edition ID:||4.DA.10|
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
D : Metal: Oxygen = 1:2 and similar
A : With small cations: Silica family
|Dana 8th edition ID:||184.108.40.206|
75 : TECTOSILICATES Si Tetrahedral Frameworks
2 : Si Tetrahedral Frameworks - SiO2 with H2O and organics
|Hey's CIM Ref.:||7.8.8|
7 : Oxides and Hydroxides
8 : Oxides of Si
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Occurrences of Opal
|Geological Setting:||Altering volcanic tuffs, basalts. Silicous deep-water marine sediments. Opal-C, opal-CT and opal-AG formation is restricted to low pressure and low temperature environments. |
Physical Properties of Opal
|Lustre:||Vitreous, Waxy, Greasy, Dull|
|Diaphaneity (Transparency):||Transparent, Opaque|
|Colour:||Colorless, white, yellow, red, orange, green, brown, black, blue|
|Hardness (Mohs):||5½ - 6½|
|Fracture:||Irregular/Uneven, Splintery, Conchoidal|
|Density (measured):||1.9 - 2.3 g/cm3|
Chemical Properties of Opal
|Essential elements:||H, O, Si|
|All elements listed in formula:||H, O, Si|
Relationship of Opal to other Species
|Related Minerals - Nickel-Strunz Grouping):|
|Related Minerals - Hey's Index Grouping:|
Other Names for Opal
|Fluorescence in UV light:||Yellow to green (uranyl)|
|Health Warning:||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
Jones, J.B. and E.R. Segnit (1971) The nature of opal. I Nomenclature and constituent phases. Journal of the Geological Society of Australia: 18: 57-68.
Anal. Chem. Acta (1994): 286: 107.
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.
Li, D., Bancroft, G.M., Kasrai, M., Fleet, M.E., Secco, R.A., Feng,X.H., Tan, K.H., and Yang, B.X. (1994) X-ray absorption spectroscopy of silicon dioxide (SiO2) polymorphs: the structural characterization of opal. American Mineralogist: 79: 622-632.
Lapis Extra No. 10, Opal (1996).
Adamo, I., Ghisoli, C. & Caucia, F. (2010) A contribution to the study of FTIR spectra of opals. Neues Jahrbuch für Mineralogie - Abhandlungen 187, 63-68.
Caucia, Franca; Ghisoli, Christian; Marinoni, Luigi; Bordoni, Valentina (2013): Opal, a beautiful gem between myth and reality. Neues Jahrbuch für Mineralogie - Abhandlungen, 190, 1-9.
Mindat.org articles about Opal
Internet Links for Opal
|Specimens:||The following Opal specimens are currently listed for sale on minfind.com.|
Localities for Opal
The 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.