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Opal

This page kindly sponsored by Opal by Anderson-Beattie.com
Formula:
SiO2 · nH2O
View Commodity Page:
Colour:
Colourless, white, yellow, red, orange, green, brown, black, blue
Lustre:
Vitreous, Waxy, Greasy, Dull
Hardness:
5½ - 6½
Specific Gravity:
1.9 - 2.3
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 a modern commentary by Kostov (2008) questions 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.

Studies at low temperature show that water molecules way be organized into ice-like structure, which includes the cubic ice modification (Eckert et al., 2015).


Visit gemdat.org for gemological information about Opal.


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Classification of OpalHide

Approved, 'Grandfathered' (first described prior to 1959)
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

Physical Properties of OpalHide

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

Optical Data of OpalHide

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 OpalHide

Formula:
SiO2 · nH2O

Synonyms of OpalHide

Other Language Names for OpalHide

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

Varieties of OpalHide

AlumocalciteA variety of Opal with alumina and lime impurities.
AmatiteOpal formed of thick mounds deposited from hot silica-rich springs.
Amber OpalA yellow-brownish variety of Opal, resembling Amber.
Andean OpalA greenish-blue opal from Peru.
AndenopalGerman name for pink opal variety from Peru (Catamarca?).
Bandfire OpalA precious opal with bands showing a play of colours.
Black OpalThe most precious kind of opal, this variety of precious opal has a dark background colour.
Blue OpalTranslucent blue opal with no play of colours. Cause of colour can be light scattering effects or occasionally microscopic admixture of chrysocolla or other minerals. (See also "Andean Opal")
Bone OpalOpal replacing fossil bone.
Boulder OpalA variety of Precious Opal found in Queensland, Australia, as cracks or coatings in and around ironstone/sandstone boulders.
Cat's Eye OpalA rare chatoyant form of opal showing a thin line of fire.
Cherry OpalA variety of common or precious opal distinguished by its red-brown color.
ChrysopalGreen nickeloan variety of Opal transitive to Chrysoprase
Chrysopal (of Schumann)A translucent variety of common opal colored apple green by the presence of nickel.
Claro OpalTransparent mexican Precious Opal with an intense play of colours (red, green, blue and yellow).
Common OpalA type of opal that doesn't show any fire.
Contra Luz OpalA precious opal where the play of colours is only visible when held up to the light.
Crystal OpalA transparent to translucent variety of Precious Opal where the colour play is visible both on the surface and in the interior.
Fire OpalA red variety of Opal.
ForcheriteA yellow to orange variety of opal, owing its colour to microscopic inclusions of orpiment and realgar.
Originally reported from Holzbrücken mill, Ingering valley, Knittelfeld, Styria, Austria.
GirasolA bluish-white translucent opal with reddish reflections. Name dates back to at least 1837. Compare with Girasol Quartz which is named after this material.
Gold OpalA name for yellow or gold-coloured opal
Harlequin OpalA variety of Precious Opal in which the play of color is arranged in a vivid harlequin, diamond-shaped, or rectangular-shaped pattern.
Honey OpalAn unnecessary name for orange-brown opal.
Hungarian OpalAn old name applied to any opal from Europe. Now even more mistakenly used as a name for white opal.
HyaliteA colourless variety of Opal. Hyalite is opal-AN, an amorphous silica-glass containing about 3-8% water.
Because it has a different structure, it does not show the opalescence (play of color) that is found in precious opal.

Hyalite occurs as globular...
HydrophaneA variety of opal that turns translucent or transparent when immersed in water.
IsopyreAn impure dark-red Opal
Jasper OpalA brecciated Jasper cemented by opal.
Jelly OpalA transparent opal with a gelatinous appearance and a bluish sheen.
Lechosos OpalA variety with a milky-white background colour.
Lemon OpalAnother unnecessary name for a colour variety of Opal.
Levin OpalPrecious Opal with long thin lightning-like flashes.
Lluvisnando opalA pale yellow opal
MascareigniteA form of opaline silica, from a vegetable origin. Primarily siliceous remains of grasses and diatoms.

Originally reported from Réunion Island.
MeniliteGreyish-brown opal.
Originally reported from Ménilmontant, Paris, Ile-de-France, France.
Mexican Fire OpalA transparent red opal from Mexico.
Milk OpalA white Opal.
Moss OpalOpal with dendritic inclusions, usually green silicates.
Mother of OpalA sandstone or ironstone with Opal as a cement.
Mountain OpalUnnecessary name for Opal from an igneous origin.
Onyx OpalA banded Opal resembling Onyx
Oolitic OpalOolitic Opal is a variety of opal made up of interlinked spherical grains, or oolites, of opal.
Opal MatrixA term for a thin layer of opal on a host rock.
Opal-AGA 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.

...
Opal-ANOpal-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, ...
Opal-AgateA variety of Opal showing agate-like coloured bands.
Opal-CA type of opal consisting of disordered α-Cristobalite.
Opal-CTConsists of packed microscopic (150-300 nm) spheres made up of tiny microcrystalline blades of Cristobalite and/or Tridymite, with water content as high as 10 wt%.
Opal-JasperA variety of yellow jasper-coloured Opal.
OpalinePseudomorphs of Opal after Serpentine. A local term.

Originally described from Quicksilver region, Napa Co., California, USA.
Painter BoulderSandstone boulders with a thin coating of opal.
Pineapple OpalPseudomorph of Opal after Ikaite
Pinfire OpalWIth very small pinhead-sized colour flashes.
Pipe OpalOpal filling long cylindrical cavities.
Pitch OpalAn opal variety with a pitchy lustre
Prase OpalA green opal.
Precious Fire OpalFire Opal displaying a play of colour.
Precious OpalA valuable gem-variety of opal.
Most precious opal is opal-AG. It shows a play of colours ("opalescence") that is due to diffraction of light from the regular packing of submicroscopic silica spheres of roughly equal size (Jones et al, 1964; Sanders, 19...
PyrophaneOpal where colour play appears to wander about at random.
Quinzite OpalRose to pink Opal.
Originally described from Quincy-sur-Cher, Bourges, Cher, Centre, France.
Radiolite OpalA smoky-brown opal coloured by inclusions of radiolaria exoskeletons.
Red Flash OpalPrecious Opal with red colour flashes that appear and disappear as the stone is turned.
Resin OpalA yellow-brown common opal with a resinous luster.
Shell OpalOpal replacing fossil shells.
Slocum StoneA synthetically grown opal
Wash OpalAlluvial pebbles of Opal.
Wax OpalA yellow-to brown opal with a waxy lustre.
White OpalPrecious Opal with a white background.
Wood OpalOpal that has replaced fossil wood.
Yowah NutSmall rounded pebbles of sandstone impregnated with Precious Opal.
Originally reported from Yowah opal field, Queensland, Australia.

Common AssociatesHide

Associated Minerals Based on Photo Data:
Chalcedony30 photos of Opal associated with Chalcedony on mindat.org.
Quartz28 photos of Opal associated with Quartz on mindat.org.
Calcite21 photos of Opal associated with Calcite on mindat.org.
Topaz15 photos of Opal associated with Topaz on mindat.org.
Goethite13 photos of Opal associated with Goethite on mindat.org.
Cinnabar12 photos of Opal associated with Cinnabar on mindat.org.
Phillipsite11 photos of Opal associated with Phillipsite on mindat.org.
Magnesite11 photos of Opal associated with Magnesite on mindat.org.
Fluorite10 photos of Opal associated with Fluorite on mindat.org.
Ikaite10 photos of Opal associated with Ikaite on mindat.org.

Related Minerals - Nickel-Strunz GroupingHide

4.DA.Carbon Dioxide IceCO2
4.DA.05QuartzSiO2Trig. 3 2 : P31 2 1
4.DA.10TridymiteSiO2Tric. 1
4.DA.15CristobaliteSiO2Tet. 4 2 2 : P41 21 2
4.DA.20MogániteSiO2Mon.
4.DA.25Melanophlogite46SiO2 · 6(N2,CO2) · 2(CH4,N2)Tet.
4.DA.30LechatelieriteSiO2
4.DA.35CoesiteSiO2Mon. 2/m : B2/b
4.DA.40StishoviteSiO2Tet. 4/mmm (4/m 2/m 2/m) : P42/mnm
4.DA.45KeatiteSiO2
4.DA.50SeifertiteSiO2Orth. mmm (2/m 2/m 2/m) : Pbcn

Related Minerals - Hey's Chemical Index of Minerals GroupingHide

7.8.1QuartzSiO2Trig. 3 2 : P31 2 1
7.8.2CoesiteSiO2Mon. 2/m : B2/b
7.8.3TridymiteSiO2Tric. 1
7.8.4StishoviteSiO2Tet. 4/mmm (4/m 2/m 2/m) : P42/mnm
7.8.5CristobaliteSiO2Tet. 4 2 2 : P41 21 2
7.8.6LechatelieriteSiO2
7.8.7Silhydrite3SiO2 · H2OOrth.
7.8.9MogániteSiO2Mon.

Fluorescence of OpalHide

Yellow to green (uranyl)

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.
Industrial Uses:
Gemstones

Opal in petrologyHide

References for OpalHide

Reference List:
Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
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.
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., Yang, B.X. (1994) X-ray absorption spectroscopy of silicon dioxide (SiO2) polymorphs: the structural characterization of opal. American Mineralogist: 79: 622-632.
Weise, C., publisher (1996) Opal - extraLapis Nr.10. Christian Weise Verlag, München.
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. http://mgu.bg/sessions/08/1/kostovri3.pdf
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, F., Ghisoli, C., Marinoni, L., Bordoni, V. (2013) Opal, a beautiful gem between myth and reality. Neues Jahrbuch für Mineralogie - Abhandlungen: 190: 1-9.
Eckert, J., Gourdon, O., Jacob, D.E., Meral, C., Monteiro, P.J.M., Vogel, S.C., Wirth, R., Wenk, H.-R. (2015) Ordering of water in opals with different microstructures. European Journal of Mineralogy: 27: 203-213.

Internet Links for OpalHide

Localities for OpalHide

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.

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