Abstract

Using theoretical calculations, Garrels and Mackenzie proposed that evaporation of spring waters from Californian granites would cause the following minerals to precipitate (in this order): calcite, sepiolite, opal. Speleothems from granite caves in California confirm that calcite and then poorly crystallized sepiolite are the first minerals to precipitate and show clearly that sepiolite can form from low-magnesium solutions after relatively little evaporative concentration. Halloysite is found associated with the speleothems, verifying Garrels and Mackenzie's identification of kaolinite-group clays as the most important weathering products of the granite. Birnessite is present in stream deposits in one cave, probably being derived from manganese mineralization upstream of the cave.

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