Opal-AG
This page is currently not sponsored. Click here to sponsor this page.
About Opal-AG
Formula:
SiO2 · nH2O
Lustre:
Vitreous, Sub-Vitreous, Greasy
Member of:
Name:
A stands for amorphous and G for gel-like.
The origin of the word "opal" 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 in the modern sense.
The origin of the word "opal" 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 in the modern sense.
A 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.
"Precious Opal" shows a play of colours ("opalescence") in white light that is due to diffraction from the regular packing of silica spheres of roughly equal size (Jones et al., 1964; Sanders, 1964). Although the packing of the spheres may be regular, there is neither short-range nor long-range order in this material, and the X-ray powder diffraction pattern is characterized by a distinct broad hump and a possible weak second hump indicative of material that is "X-ray amorphous" (Jones and Segnit, 1971). Since the packing of the spheres is similar to the structure of a gel, the subscript G ("gel-like") has been added (Flörke et al., 1991; Graetsch, 1994). Not all precious opal is opal-AG: The diffracting structure and the resulting opalescence may be preserved to some degree when opal-AG transforms to opal-CT by crystallization (Sanders, 1975).
Opal that shows no play of colours is also composed of the same spherical clusters, but they are non-uniform in size and do not pack in an orderly manner, thus cancel out any possible diffraction of the light (Jones et al., 1964; Sanders, 1964). This type of opal is termed collectively "potch opal" and includes the massive varieties. Porous varieties of inorganic origin such as geyserite and materials composed of tests of microorganisms such as diatomite and radiolarite are also recognized. The X-ray powder diffraction pattern is essentially identical for of all these types of opal, regardless of their origin.
Precious opal
Opal that shows no play of colours is also composed of the same spherical clusters, but they are non-uniform in size and do not pack in an orderly manner, thus cancel out any possible diffraction of the light (Jones et al., 1964; Sanders, 1964). This type of opal is termed collectively "potch opal" and includes the massive varieties. Porous varieties of inorganic origin such as geyserite and materials composed of tests of microorganisms such as diatomite and radiolarite are also recognized. The X-ray powder diffraction pattern is essentially identical for of all these types of opal, regardless of their origin.
Unique Identifiers
Mindat ID:
32187 (as Opal-AG)
3004 (as Opal)
3004 (as Opal)
Long-form identifier:
mindat:1:1:32187:5 (as Opal-AG)
mindat:1:1:3004:1 (as Opal)
mindat:1:1:3004:1 (as Opal)
Similar Names
Pronunciation of Opal-AG
Pronunciation:
| Play | Recorded by | Country |
|---|---|---|
| Jolyon Ralph | United Kingdom |
Physical Properties of Opal-AG
Vitreous, Sub-Vitreous, Greasy
Transparency:
Transparent, Translucent
Comment:
greasy to vitreous on freshly broken surfaces
Chemistry of Opal-AG
Mindat Formula:
SiO2 · nH2O
Elements listed:
Synonyms of Opal-AG
Relationship of Opal-AG to other Species
Member of:
Other Members of Silica:
| Chibaite | SiO2 · n(CH4, C2H6, C3H8, i-C4H10) (n = 3/17 (max)) | Iso. m3(2/m3) : Fd3 |
| Coesite | SiO2 | Mon. 2/m : B2/b |
| Cristobalite | SiO2 | Tet. 422 : P41212 |
| 'Keatite' | SiO2 | Tet. 422 : P43212 |
| Lechatelierite | SiO2 | Amor. |
| Melanophlogite | 46SiO2 · 6(N2,CO2) · 2(CH4,N2) | Tet. 4/mmm(4/m2/m2/m) |
| Mogánite | SiO2 | Mon. 2/m |
| 'Opal-AN' | SiO2 · nH2O | |
| 'Opal-C' | SiO2 · nH2O | |
| 'Opal-CT' | SiO2 · nH2O | |
| Quartz | SiO2 | Trig. 32 : P3121 |
| 'Quartz-beta' | SiO2 | Hex. 622 : P6422 |
| Stishovite | SiO2 | Tet. 4/mmm(4/m2/m2/m) : P42/mnm |
| Tridymite | SiO2 | Tric. 1 |
| 'β-Cristobalite' | SiO2 | Iso. m3m(4/m32/m) : Fd3m |
| 'β-Tridymite' | SiO2 |
Common Associates
Other Information
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Internet Links for Opal-AG
mindat.org URL:
https://www.mindat.org/min-32187.html
Please feel free to link to this page.
Please feel free to link to this page.
Search Engines:
Mineral Dealers:
References for Opal-AG
Reference List:
JONES, J. B., SANDERS, J. V., SEGNIT, E. R. (1964) Structure of Opal. Nature, 204 (4962). 990-991 doi:10.1038/204990a0
Jones, J. B., Segnit, E. R. (1971) The nature of opal I. nomenclature and constituent phases. Journal of the Geological Society of Australia, 18 (1) 57-68 doi:10.1080/00167617108728743
Localities for Opal-AG
Showing 6 localities.
symbol to view information about a locality.
The Locality List
- This locality has map coordinates listed.
- This locality has estimated coordinates.
ⓘ - Click for references and further information on this occurrence.
? - Indicates mineral may be doubtful at this locality.
- Good crystals or important locality for species.
- World class for species or very significant.
(TL) - Type Locality for a valid mineral species.
(FRL) - First Recorded Locality for everything else (eg varieties).
All localities listed without proper references should be considered as questionable.
Australia | |
| www.mindat.org (n.d.) |
| Identified by mine guide |
Belgium | |
| Mélon et al. (1976) |
France | |
| |
Japan | |
| PXRD by Alfredo Petrov |
USA | |
| Gissler et al. (2024) |
Saddle Mountain, Maricopa County, Arizona, USA