PAUL D STEWART / SCIENCE PHOTO LIBRARY PAUL D STEWART / SCIENCE PHOTO LIBRARY
Cyrenopsis australiensis bivalve from the Coober Pedy Opal Field (Lower Cretaceous) of South Australia. Polished to show the precious opal 'fire' of irridescence. Coober Pedy is world-famous for its precious opal (hydrated amorphous form of silica - SiO2Â·nH2O). Most of the opalized fossils here are marine bivalves, but opalised belemnites, ichthyosaurs, and plesiosaurs also occur. Opals can form at low temperature when the cavity is replaced by a jelly of hydrated silica. Over time the silica spheres grow, settling to the bottom of the gel and solidifying. At microscopic scales, precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. It was shown by J. V. Sanders that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.
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