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Opal

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Queensland, Australia
© Antonio Borrelli

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Formula:
SiO
 
2
·nH
 
2
O
Colour:Colorless, white, ...Hardness:5½ - 6½
Name:Fron the Sanskrit upala, 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 composed of Cristobalite and/or Tridymite and amorphous silica (closely packed spheres form a diffraction grating to create Precious Opal).

Opal is broken down into four types:

Opal-CT Cristobalite-Tridymite
Opal-C Cristobalite
Opal-AG Amorphous - (Gem)
Opal-AN

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:75.2.1.1

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

Physical Properties of Opal

Lustre:Vitreous, Waxy, Greasy, Dull
Diaphaneity (Transparency):Transparent, Opaque
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 (measured):1.9 - 2.3 g/cm3

Chemical Properties of Opal

Formula:
SiO
 
2
·nH
 
2
O
Essential elements:H, O, Si
All elements listed in formula:H, O, Si

Relationship of Opal to other Species

Related Minerals - Nickel-Strunz Grouping):
4.DA.05Quartz
SiO
 
2
4.DA.05Seifertite
SiO
 
2
4.DA.10Tridymite
SiO
 
2
4.DA.15Cristobalite
SiO
 
2
4.DA.20Mogánite
SiO
 
2
4.DA.25Melanophlogite
46SiO
 
2
·6(N
 
2
,CO
 
2
)·2(CH
 
4
,N
 
2
)
4.DA.30Lechatelierite
SiO
 
2
4.DA.35Coesite
SiO
 
2
4.DA.40Stishovite
SiO
 
2
4.DA.45Keatite
Related Minerals - Hey's Index Grouping:
7.8.1Quartz
SiO
 
2
7.8.2Coesite
SiO
 
2
7.8.3Tridymite
SiO
 
2
7.8.4Stishovite
SiO
 
2
7.8.5Cristobalite
SiO
 
2
7.8.6Lechatelierite
SiO
 
2
7.8.7Silhydrite
3SiO
 
2
·H
 
2
O
7.8.9Mogánite
SiO
 
2

Other Names for Opal

Synonyms:
Common OpalGel-CristobaliteGel-pristobaliteHalf-OpalHemiopal
Indivisible QuartzIridotLechosopalLechososNeslite
Nevada OpalOpaleÓpaloOpalusPaederos
PotchRumaniteSemiopalVidriteVirgin Valley Opal
White Cliffs Opal
German names:
LivitLjarditNeslitRumanitVidrit
Hebrew names:
אופל
Latin names:
Achates unguium colore oculus mundi
Spanish names:
NeslitaRumanitaVidrita
Traditional Chinese names:
白石
Varieties:
AlumocalciteAmatiteAmber OpalBandfire OpalBlack Opal
Bone OpalBoulder OpalCachalong OpalCherry OpalClaro Opal
Contra Luz OpalCrystal OpalDiatomiteFioriteFire Opal
ForcheriteGeyseriteGold OpalHarlequin OpalHoney Opal
Hungarian OpalHydrophaneIsopyreJasper OpalJelly Opal
Lechosos OpalLemon OpalLevin OpalLluvisnando opalMascareignite
MeniliteMexican Fire OpalMilk OpalMoss OpalMother of Opal
Mother of Pearl OpalMountain OpalOnyx OpalOpal MatrixOpal-AG
Opal-ANOpal-AgateOpal-COpal-CTOpal-Jasper
Painter BoulderPineapple OpalPinfire OpalPipe OpalPitch Opal
Prase OpalPrecious Fire OpalPrecious OpalPyrophaneQuinzite Opal
Radiolite OpalRed Flash OpalResin OpalShell OpalSiliceous sinter
Slocum StoneVianditeWash OpalWax OpalWhite Opal
Wood OpalYowah Nut

Other Information

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.
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.

Anal. Chem. Acta (1994): 286: 107.

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).

Internet Links for Opal

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    Localities for Opal

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