Opal and chalcedony speleothems on quartz sandstones in the Sydney region, southeastern Australia

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Abstract
Speleothems of silica are far rarer than those of calcite but occur in a range of types including stalactites, stalagmites and flowstones. This study has found a wider range and far greater number of silica speleothems on the quartz sandstones of the Sydney region than the small number of previous accounts had suggested. Speleothems on the Sydney region sandstones are composed of multiple layers of amorphous opal‐A and cryptocrystalline chalcedony. Silica slowly dissolved from detrital and diagenetic quartz and kaolinite clays of the host arenites is redeposited as opal‐A at the sandstone surface when groundwater evaporates. This amorphous silica converts over time by Ostwald‐type paragenesis to the cryptocrystalline form, but the expected intermediate opal‐CT phase has not been detected. The crystallisation of chalcedony at earth‐surface temperatures is generally believed to take an extremely long time and its presence makes these speleothems very significant, especially as it is reported in only a small number of silica speleothems elsewhere. Furthermore, a similar paragenetic silica‐‘ripening’ mechanism may also be involved in the low‐temperature earth‐surface formation of other crystalline silica deposits such as silcrete duricrusts and pedogenic quartz. Additional closely coupled laboratory and field investigations into the processes that control silica paragenesis under earth‐surface conditions are sorely needed.