Theoretical analysis indicates that direct alteration of K-feldspar to gibbsite in the zone of weathering probably occurs only under conditions of good soil drainage and only if the pH is above 4.3. The analysis also indicates that gibbsite should not form if the soil water has an initial dissolved silica concentration above 10-4,6 moles/liter. The calculations further show that, in the pH range 2 to 8, K-mica should not form as an initial precipitate in the hydrolysis of K-feldspar but must always be preceded by the precipitation of some kaolinite and/or gibbsite. This result may partly explain the rather infrequent occurrence of K-mica as a direct alteration product of K-feldspar.

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