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Siliceous speleothems can be formed in sandstone caves. Recently, opal “biospeleothems” have been found in the World's largest cave in Precambrian sandstones on the Chimantá Tepui in Venezuela. The speleothems, although reminiscent of normal stalactites and stalagmites from limestone caves, are in fact large microbialites. More than a dozen forms were distinguished, but they share a common structure...
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