Alteration of Plagioclase in Granite during Weathering

Secondary minerals of plagioclase in spheroidally weathered granite of the Seranishi-cho, Hiroshima Prefecture, Japan have been examined by the technique of X-ray diffraction, X-ray fluorescence, DTA, TGA, pH measurements and electron microscopic observation. Spheroidally zoning of A, B and C owing to the difference of appearance at outcrop corresponds to the secondary mineral components, that is, allophane or allophane-like substance is found only in the zone C of core-part of granite and the zone B, metahalloysite and halloysite are predominant in the zones A and B that were intensely weathered parts, and kaolinite exists in the zones A and B, but lacks in the zone C. The crystallinity and grain-size of these kaolin minerals increase gradually from near central core-part toward the outer A zone. The probable al-terating sequence of plagioclase in the weathered granite of this district has been roughly assumed as follows:
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