Micromorphology of a weathered granite near the Ría de Arosa (NW Spain)

A detailed study has been made of several weathering profiles on the late-Hercynian Caldas de Reyes granite, NW-Spain. The field examination has been complemented by laboratory studies of large thin sections of hardened weathered material in conjunction with X-ray diffraction analyses. Three groups of features have been investigated by these methods. (a) General structure of regoliths. — They consist of a saprolite, mostly covered by colluvium. The latter can be distinguished from the saprolite by field methods, heavy mineral content and fabric analyses. The saprolites generally display spheroidal weathering. Microfabric analyses showed that spheroidal weathering is conditioned by micro-crack systems. Weathering starts in the joints; it was found that water transport occurred along joint planes, even those of sizes which could not be detected with the naked eye. Reducing conditions could be deduced from the colour and mineral content along these fine channelways, but in later stages oxidizing conditions prevail. Oscillating groundwater has affected the formation of the secondary minerals along the joints, but not in the surrounding saprolite. The soils formed on the regoliths are entic- and orthic haplumbrepts. The umbric epipedon may reach a thickness of 1 40 metres above an altitude of 250 metres. (b) Mineral transformations. — Kaolinite and metahalloysitc are the most common secondary minerals in weathered granites, metamorphic rocks and in an estuarine terrace. In one locality large red pleochroic secondary muscovite has been observed to form. Gibbsite may also form in a highly alkaline environment and where water movement is very restricted in micro-cracks of weathered feldspars. Secondary minerals (gibbsite and microcrystalline material) may form from plagioclase upon weathering, whereas the weathering products of microcline contain no secondary minerals. Interlamellar crystallized kaolinite or metahalloysite between exfoliated biotite-vermiculite lamellae can be observed during the weathering of biotite. The interlamellar crystallization of kaolinite or metahalloysite is not apparent between exfoliated muscovite lamellae. Minute droplets containing titanium, derived from the weathering of biotite crystals and their sagenite inclusions, are commonly found along the original cleavages of exfoliated biotite. Sometimes anatase has been observed to form out of these droplets. (c) Fabric analyses. — Fabric analyses have been performed to the regoliths according to Brewer's (1964) method, but because it was applied to deeper saprolites his terminology had to be supplemented with various new terms. Certain fabrics (skelsepic plasmic fabrics) are common in colluvium but not in saprolites and neither in soils.