The present rate of delivery of dissolved silica to the ocean would produce a much greater volume of sediments than is observed in the geologic column if all this silica were precipitated as free SiO 2 . If the silica and bicarbonate in the oceans react to form CO 2 and reactive silica, further reactions can take place that form aluminum silicates of the general clay mineral type and the CO 2 is cycled back to the atmosphere. This may be termed 'reverse weathering' and implies that the bulk of allogenic clay minerals entering sea water should not be much altered in composition.

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