NATURE AND CHARACTERISTICS OF 1/1 PHYLLOSILICATES FROM WEATHERED GRANITE, CENTRAL SPAIN

The nature and characteristics of 1:1 phyllosilicates (crystallinity, order and proportion of kaolinite, disordered kaolinite and 7 angstrom-halloysite) in deep palaeoeweathering profiles developed on granitic rocks in Central Spain have been studied. The profile displays a lower part of non-silicified saprolite and an upper part of silicified saprolite, with or without a sedimentary silicified cover. The <2 mum fraction was studied using XRD, TG-DTA and IR techniques. Towards the top of the profile there is a progressive decrease in both 1:1 phyllosilicates and kaolinite contents and an increase in opal. In the upper part (silicified saprolite) two weathering facies can be distinguished: the 'white facies', in which the crystallinity of kaolinite is greater than in the 'red facies. The kaolinite in the upper parts of the profile is probably a remnant of that existing in the non-silicified saprolite, whereas disordered kaolinite and 7 angstrom-halloysite are, at least in part, a product of transformation and/or neoformation under more surface-like conditions.