The formation of opal in marine reptile bones and wood

This item is only available electronically.The age of precious opal and the mechanisms that result in formation as opposed to the ubiquitous common opal are poorly understood. Until now, there has been no research on the replacement of biominerals in vertebrate bones by opal. As the microtexture, mineralogy and chemistry of bones are well-known, they provide a unique opportunity to study the mechanism of precious opal deposition. In this article chemical and textural features of Andamooka opalised plesiosaur bones were compared with those in non-opalised ichthyosaur bones from Moon Plain and a recent dolphin bone. Opalised wood samples from Nevada and White Cliffs were also studied to compare with bone opalisation and different depositional environments (sedimentary vs volcanogenic). The cellular form of a continuous irregular framework of silica was retained in the wood samples. The mineralogy of the wood samples reflects their depositional environment, where opal-CT and opal-C is dominant in volcanic deposits (Nevada) and opal-A in sedimentary deposits (White Cliffs). Comparison of the Nevada wood to Post-Archean average shale (PAAS) shows that it is rich in most trace elements with the exception of Y and U. The high amount of trace elements is a reflection of its volcanic origin. In contrast, the opalised wood from White Cliffs is depleted in most trace elements with the exception of Co. Cracks were observed in both the opalised wood and bone samples which allowed the void space required to form precious opal. The opalised wood from White Cliffs and the opalised plesiosaur bones from Andamooka are chemically very similar and reflect similar compositions for the opalising fluids. The Haversian system was preserved in the non-opalised ichthyosaur bone but not in the opalised bones. The ichthyosaur bone is comprised mostly of carbonate-hydroxylapatite but in the opalised bones the major mineral is quartz. Modern dolphin bone consists of bioapatite with water and organic material: its trace element composition is broadly similar to the ichthyosaur bone from Moon Plain but is richer in Sr, Zn and Co. When normalized to PAAS, the ichthyosaur bone is depleted in all trace elements with the exception of Sr, which is likely a product of the carbonate-rich mineralogy. Like the ichthyosaur bones, the opalised bones are also depleted in trace elements, with the exception of Co and Zn. There is no evidence of remnant bioapatite in the opalised bone, a finding consistent with the chemical analyses that show only trace amounts of Ca and no P. The level of microstructural preservation in the opalised bone suggests that opalisation is not a closely coupled dissolution-reprecipitation reaction and that there was a fluid filled space between the reaction fronts which allowed the opal silica spheres to form and settle within a comparatively small space (100 µm). An alternative interpretation is that the fibrous quartz filled the osteon canals before opalisation and that the bioapatite was then dissolved away leaving a hollow cast that filled slowly with opal.Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 200

The formation of precious opal: Clues from the opalization of bone

Copyright © 2008 Mineralogical Association of CanadaThe 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 SiO₂ (88.59 to 92.69 wt%), with minor amounts of Al₂O₃ (2.02 to 4.41 wt%) and H₂O (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 μm), 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.Benjamath Pewkliang, Allan Pring and Joël Brugge