Opal-CT
A variety of Opal
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About Opal-CT
Formula:
SiO2 · nH2O
A variety of Opal
Originally considered to be a hydrous form of silica composed of disordered Cristobalite and Tridymite, with water content of about 10 wt% (Segnit & Jones, 1971).
Wilson (2014) and Fröhlich (2020) found no evidence for the presence of cristobalite in some samples of this material, and Fröhlich found the material to be a mixture of non-crystalline (X-ray amorphous) material and poorly crystalline blades of low temperature tridymite. Sedimentary and hydrothermal opal-CT appear slightly different, with sedimentary opal-CT containing monoclinic tridymite and hydrothermal opal-CT containing possibly orthorhombic tridymite (Fröhlich, 2020). The hydrothermal opal-CT is optically fibrous and probably identical to the variety lussatite (Fröhlich, 2020). Curtis et al. (2019) also found some variability in opal-CT, with simple and complex varieties, after studying numerous samples, but they ascribe this difference at least partly to variations in cristobalite content, and also note it can grade into opal-A and opal-C.
Unpublished studies by R. Bottrill confirm that much of this material contains tridymite but no trace of cristobalite, and where it does it may be considered a mixture with opal-C.
Opal-CT can form from very low temperature sea floor environments to hydrothermal deposits to over 100degC, but with time and temperature can convert to chalcedony and more crystalline quartz.
It can show a play of colour, e.g. in Ethiopian opals (Curtis et al., 2019).
It typically consists of packed microscopic (~10-12 micron diameter) spheres (“lepispheres”) made up of tiny blades of monoclinic tridymite and amorphous silica (Frohlich, 2019).
Originally considered to be a hydrous form of silica composed of disordered Cristobalite and Tridymite, with water content of about 10 wt% (Segnit & Jones, 1971).
Wilson (2014) and Fröhlich (2020) found no evidence for the presence of cristobalite in some samples of this material, and Fröhlich found the material to be a mixture of non-crystalline (X-ray amorphous) material and poorly crystalline blades of low temperature tridymite. Sedimentary and hydrothermal opal-CT appear slightly different, with sedimentary opal-CT containing monoclinic tridymite and hydrothermal opal-CT containing possibly orthorhombic tridymite (Fröhlich, 2020). The hydrothermal opal-CT is optically fibrous and probably identical to the variety lussatite (Fröhlich, 2020). Curtis et al. (2019) also found some variability in opal-CT, with simple and complex varieties, after studying numerous samples, but they ascribe this difference at least partly to variations in cristobalite content, and also note it can grade into opal-A and opal-C.
Unpublished studies by R. Bottrill confirm that much of this material contains tridymite but no trace of cristobalite, and where it does it may be considered a mixture with opal-C.
Opal-CT can form from very low temperature sea floor environments to hydrothermal deposits to over 100degC, but with time and temperature can convert to chalcedony and more crystalline quartz.
It can show a play of colour, e.g. in Ethiopian opals (Curtis et al., 2019).
It typically consists of packed microscopic (~10-12 micron diameter) spheres (“lepispheres”) made up of tiny blades of monoclinic tridymite and amorphous silica (Frohlich, 2019).
Unique Identifiers
Mindat ID:
32185 (as Opal-CT)
3004 (as Opal)
3004 (as Opal)
Long-form identifier:
mindat:1:1:32185:7 (as Opal-CT)
mindat:1:1:3004:1 (as Opal)
mindat:1:1:3004:1 (as Opal)
GUID
(UUID V4):
(UUID V4):
db9e1576-012c-46f9-bddd-96bcada88a29 (as Opal-CT)
b3226449-9ed0-47fe-a88a-e53851c6d480 (as Opal)
b3226449-9ed0-47fe-a88a-e53851c6d480 (as Opal)
Pronunciation of Opal-CT
Pronunciation:
| Play | Recorded by | Country |
|---|---|---|
| Jolyon Ralph | United Kingdom |
Chemistry of Opal-CT
Mindat Formula:
SiO2 · nH2O
Elements listed:
Varieties of Opal-CT
Common Associates
Associated Minerals Based on Photo Data:
| 21 photos of Opal-CT associated with Bitumen | |
| 12 photos of Opal-CT associated with Quartz | SiO2 |
| 9 photos of Opal-CT associated with Chalcedony | SiO2 |
| 3 photos of Opal-CT associated with Opal | SiO2 · nH2O |
| 1 photo of Opal-CT associated with Prase Opal | SiO2 · nH2O |
| 1 photo of Opal-CT associated with Cristobalite | SiO2 |
| 1 photo of Opal-CT associated with Edenite | NaCa2Mg5(Si7Al)O22(OH)2 |
| 1 photo of Opal-CT associated with Hematite | Fe2O3 |
| 1 photo of Opal-CT associated with Hornstein | |
| 1 photo of Opal-CT associated with Precious Opal | SiO2 · nH2O |
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.
Opal-CT in petrology
An essential component of rock names highlighted in red, an accessory component in rock names highlighted in green.
Internet Links for Opal-CT
mindat.org URL:
https://www.mindat.org/min-32185.html
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References for Opal-CT
Localities for Opal-CT
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 | |
| Curtis +3 other references |
| Curtis +3 other references |
| Carr et al. (2021) |
Austria | |
| Pristacz et al. (2011) |
| Taucher (2012) |
| Postl et al. (1996) |
| www.markasit070752.ws24.cc (n.d.) |
| Taucher et al. (1998) |
| A.Weiß (1972) |
Brazil | |
| Costa et al. (2016) |
Bulgaria | |
| Zendelska et al. (2018) +1 other reference |
Chile | |
| TESIS PARA OPTAR AL GRADO DE MAGÍSTER ... |
Czech Republic | |
| Kratochvíl |
| Koníčková Š. et al. (2016) |
| Trnka M (2006) |
Ethiopia | |
| Zhao +1 other reference |
Faroe Islands | |
| Caucia et al. (2013) |
| Caucia et al. (2013) |
France | |
| Remi Bornet Collection |
| Remi Bornet Collection |
| Remi Bornet Collection | |
| Aimé Rudel : "Curiosités Géologiques ... |
| Remi Bornet Collection | |
| Remi Bornet Collection | |
| Remi Bornet Collection | |
| Mallard |
| BRUNET (2016) |
| Econ Geol (1989) |
Greece | |
| Voudouris (2005) |
| Helvaci et al. (1993) |
| Godelitsas +1 other reference |
Hungary | |
| Society of Economic Geologists Student Chapter University of Miskolc (2018) |
| Society of Economic Geologists Student Chapter University of Miskolc (2018) |
Iceland | |
| Jones et al. (2007) |
India | |
| Gotze +7 other references |
Indian Ocean | |
| Wang et al. (2017) |
| Wang et al. (2017) | |
Indonesia | |
| Mazot et al. (2007) |
Israel | |
| Sokol et al. (2014, November) |
Italy | |
| Napoleoni et al. (2021) |
| Ghiara M.R. et al. (1999) |
| Rielli et al. (2022) |
Japan | |
| T. Nakanishi et al. (2003) |
Jordan | |
| Pitty et al. (2010) |
| Khoury et al. (1985) |
Madagascar | |
| Popov et al. (2020) |
| Simoni et al. (2010) |
Mexico | |
| Forti (2006) |
| Fritsch et al. (2004) |
| Curtis +3 other references |
| Fritsch et al. (2004) |
Namibia | |
| Saphira Minerals specimens |
| Gerhard Louw photo ID 983450 | |
New Zealand | |
| A. G. Reyes et al. (2004) |
| [var: Lussatite] Sutherland et al. (ed.) |
| Brathwaite et al. (2021) |
Pacific Ocean | |
| Hein et al. (2010) |
Peru | |
| Fritsch et al. (2004) |
| Dill et al. (1997) |
Poland | |
| University of Adelaide |
Russia | |
| Kyle et al. (2007) |
Slovakia | |
| Mesiarkinová et al. (2009) |
| Rusinová P. & Bačík P. (2013) |
Tunisia | |
| S Tunisia +1 other reference | |
Turkey | |
| Helvaci (1998) |
| Temel et al. (2010) | |
| A Summary and Geological Comparison. ... +1 other reference |
| Mutlu et al. (2005) |
UK | |
| Curtis +3 other references |
| Curtis +3 other references |
USA | |
| Eckel et al. (1997) |
| Keller et al. (2006) |
| University of Adelaide - Tate Museum ... |
| Altaner et al. (1990) |
Venezuela | |
| Franco Urbani (2009) |
Les Rois Mine, Mur-sur-Allier, Clermont-Ferrand, Puy-de-Dôme, Auvergne-Rhône-Alpes, France