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Opal

This page kindly sponsored by Danny J. Sanchez
Formula:
SiO2 · nH2O
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-CT Cristobalite-Tridymite
Opal-C Cristobalite
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.

Classification of Opal

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
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
75.2.1.1

75 : TECTOSILICATES Si Tetrahedral Frameworks
2 : Si Tetrahedral Frameworks - SiO2 with H2O and organics
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

Vitreous, Waxy, Greasy, Dull
Diaphaneity (Transparency):
Transparent, Translucent
Colour:
Colorless, white, yellow, red, orange, green, brown, black, blue
Streak:
White
Hardness (Mohs):
5½ - 6½
Tenacity:
Brittle
Cleavage:
None Observed
Fracture:
Irregular/Uneven, Splintery, Conchoidal
Density:
1.9 - 2.3 g/cm3 (Measured)    

Optical Data of Opal

Type:
Isotropic
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.
Max Birefringence:
δ = 1.400 - Isotropic minerals have no birefringence
Surface Relief:
Moderate
Pleochroism:
Non-pleochroic

Chemical Properties of Opal

Formula:
SiO2 · nH2O
Essential elements:
All elements listed in formula:
Analytical Data:
Contains 2 - 10% water.

Relationship of Opal to other Species

4.DA.05QuartzSiO2
4.DA.10TridymiteSiO2
4.DA.15CristobaliteSiO2
4.DA.20MogániteSiO2
4.DA.25Melanophlogite46SiO2 · 6(N2,CO2) · 2(CH4,N2)
4.DA.30LechatelieriteSiO2
4.DA.35CoesiteSiO2
4.DA.40StishoviteSiO2
4.DA.45KeatiteSiO2
4.DA.50SeifertiteSiO2
7.8.1QuartzSiO2
7.8.2CoesiteSiO2
7.8.3TridymiteSiO2
7.8.4StishoviteSiO2
7.8.5CristobaliteSiO2
7.8.6LechatelieriteSiO2
7.8.7Silhydrite3SiO2 · H2O
7.8.9MogániteSiO2

Other Names for Opal

Name in Other Languages:
Basque:Opalo
Bosnian (Latin Script):Opal
Bulgarian:Опал
Czech:Opál
Dutch:Opaal
Esperanto:Opalo
Estonian:Opaal
Finnish:Opaali
French:Opale
Galician:Ópalo
Hungarian:Opál
Italian:Opale
Japanese:オパール
Lithuanian:Opalas
Norwegian (Bokmål):Opal
Polish:Opal
Portuguese:Opala
Romanian:Opal
Russian:Опал
Simplified Chinese:蛋白石
Slovak:Opál
Slovenian:Opal
Swedish:Opal
Traditional Chinese:蛋白石
Turkish:Opal
Ukrainian:Опал
Vietnamese:Đá opal

Other Information

Yellow to green (uranyl)
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Industrial Uses:
Gemstones

References for Opal

Reference List:
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.

Internet Links for Opal

Specimens:
The following Opal specimens are currently listed for sale on minfind.com.

Localities for Opal

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.
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