Opal

Opal

Queensland, Australia

Opal, etc.

Fornaccia, Fornovolasco, Vergemoli, Apuan Alps, Lucca Province, Tuscany, Italy

Opal-AN

Mitchell Co., North Carolina, USA

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

IMA status:

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

Strunz 8th ed.:

4/D.01-80

Nickel-Strunz 10th (pending) ed.:

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 ed.:

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

mindat.org URL:

http://www.mindat.org/min-3004.html
Please feel free to link to this page.

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

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:

n_α = 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

All elements listed in formula:

H, O, Si - search for minerals with similar chemistry

Analytical Data:

Contains 2 - 10% water.