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The formation of precious opal; clues from the opalization of bone

Benjamath Pewkliang, Allan Pring and Joel Brugger
The formation of precious opal; clues from the opalization of bone
The Canadian Mineralogist (February 2008) 46 (1): 139-149


The composition and microstructure of opalized saurian bones (Plesiosaur) from Andamooka, South Australia, have been analyzed and compared to saurian bones that have been partially replaced by magnesian calcite from the same geological formation, north of Coober Pedy, South Australia. Powder X-ray-diffraction analyses show that the opalized bones are composed of opal-AG and quartz. Major- and minor-element XRF analyses show that they are essentially pure SiO (sub 2) (88.59 to 92.69 wt%), with minor amounts of Al (sub 2) O (sub 3) (2.02 to 4.41 wt%) and H (sub 2) O (3.36 to 4.23 wt%). No traces of biogenic apatite remain after opalization. The opal is depleted in all trace elements relative to PAAS. During the formation of the opal, the coarser details of the bone microstructure have been preserved down to the level of the individual osteons (scale of around 100 mu m), but the central canals and the boundary area have been enlarged and filled with chalcedony, which postdates opal formation. These chemical and microstructural features are consistent with the opalization process being a secondary replacement after partial replacement of the bone by magnesian calcite. They are also consistent with the opal forming first as a gel in the small cavities left by the osteons, and the individual opal spheres growing as they settle within the gel. Changes in the viscosity of the gel provide a ready explanation for the occurrence of color and potch banding in opals. The indication that opalization is a secondary process after calcification on the Australian opal fields is consistent with a Tertiary age for formation.

ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 46
Serial Issue: 1
Title: The formation of precious opal; clues from the opalization of bone
Affiliation: South Australian Museum, Department of Mineralogy, Adelaide, South Aust., Australia
Pages: 139-149
Published: 200802
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 38
Accession Number: 2008-080706
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
S40°00'00" - S16°00'00", E130°00'00" - E152°00'00"
Secondary Affiliation: University of Adelaide, AUS, Australia
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200823
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