Abstract: A study of weathered profiles from fresh rock to the surface in granitic rocks and biotite-plagioclase gneiss shows the following: 1. The particle size of the kaolin increases from not subject to estimate in fresh rock to a maximum of approximately 60 µ in the vicinity of the contact between the B horizon and the saprolite, then it decreases to approximately 13 µ in the A horizon. 2. Gibbsite occurs most abundantly when the abrasion pH of rock slurries is greater than 6 or less than 5.5. In outcrop, gibbsite is abundant in the upper soil horizons and in the partially weathered rock but is sparse in the intervening saprolite. 3. The total amount of kaolin increases steadily upward from zero in fresh rock to a maximum in the B horizon then declines sharply in the A horizon.
These data are interpreted to mean that kaolin forms, grows and is stable in saprock, saprolite and B horizon, but it is not stable in the upper B and A horizons or in the very basic environment of incipient weathering.