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A Study of a Deep Weathering Profile on Granite in Peninsular Malaysia: III. Alteration of Feldspars¹

ABSTRACT

Feldspar grains or their pseudomorphs from the different weathering zones are studied with the SEM. Weathering commences with dissolution to form voids in the grains. Close to the rock, the first product is allophanic material present as globules adhering to the voids in the grains. In the zone the product is halloysite with some amorphous silica spherules. The surface of the pseudomorph has a hummocky appearance suggesting an amorphous intermediary phase prior to halloysite formation. Kaolinite formation commences in the _p zone and gibbsite in the _m zone. Admixtures of kaolinite and halloysite may be found on the same pseudomorph, but admixture of gibbsite with another secondary mineral is not encountered.

Biotite follows a similar trend of alteration. It is concluded that irrespective of the primary weatherable mineral, the type of secondary mineral found is a function of the microenvironment.

¹ Contribution from the Geological Institute, Krijgslaan, 271, Gent, Belgium. Work supported in part by a grant from the National Science Foundation, Belgium.

² Assistant and Graduate Student, respectively.

Received for publication July 1, 1977. Accepted for publication October 12, 1977.