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© Antonio Borrelli
|Colour:||Colorless, white, ...||Hardness:||5½ - 6½|
|Name:||The origin of the name is uncertain. It may be from the Sanskrit "upala", meaning "stone" or "precious stone" or from opalus, the ancient Latin name for the gem (Pliny the elder, 75-79). Pliny may have also referred to the gem as paederos, but modern commentary by Kostov (2008) question if that name was actually applied to the opal of modern sense.|
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 classified into four types:
Opal-AG Amorphous-Gel (closely packed amorphous silica spheres form a diffraction grating to create Precious Opal).
Opal-AN Amorphous-Network (found as 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:||220.127.116.11|
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, Translucent|
|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|
Optical Data of Opal
|RI values:||nα = 1.400 - 1.460|
|Birefringence:||Opal-AG and Opal-AG are optically isotropic, but may show anomalous birefringence due to strain. The microcrystalline varieties show birefringence: opal-C is length-fast, opal-CT is length-slow, but almost isotropic.|
|Maximum Birefringence:||δ = 1.400 - Isotropic minerals have no birefringence|
Chemical Properties of Opal
|Simplified for copy/paste:||SiO2·nH2O|
|Essential elements:||H, O, Si|
|All elements listed in formula:||H, O, Si|
|Analytical Data:||Contains 2 - 10% water.|
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.
Elzea, J.M., Odom, I.E., Miles, W.J. (1994) Distinguishing well ordered opal-CT and opal-C from high temperature cristobalite by X-ray diffraction. Analytica Chimica Acta 286, 107-116.
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).
Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., Ostroumov, M. (2008) Common gem opal: An investigation of micro- to nano-structure. American Mineralogist, 93, 1865-1873.
Kostov, Rusian I. (2008) Orphic Lithica As A Source Of Late Antiquity Mineralogical Knowledge, Annual Of The University Of Mining And Geology “ST. Ivan Rilski”, Vol. 51, Part I, Geology And Geophysics, p. 109-115.
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.
(TL) indicates type locality. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.