Pseudomorphic transformation of plagioclases during the weathering of granitic rocks in Galicia (NW Spain)

Abstract

Microscopic study of twelve weathering profiles developed on granitic rocks in Galicia (NW Spain) showed that, in most cases, the transformation of the parent rock into saprolite occurs with pseudomorphic alteration of the plagioclases. It was seen that the transformation and the retention of plagioclases in the alteration profile are directly related to the conservation of the general structure of the rock and this, in its turn, depends on the mineralogical composition and texture. The most unstable constituents, in the Galician environment, are biotites and plagioclases, there being a clear difference in the rate of alteration between the plagioclases and the K-feldspars. This implies that a large part of the fabric of the rock remains unaltered when the plagioclases are completely weathered, so that the pores caused by the weathering of the plagioclases are filled by their own alteration products. With regard to the influence of the texture, it was observed that fine grained rocks are altered more easily than coarse grained ones, especially if they are rich in biotites. In this study we show various examples in which, even with high levels of alteration, the morphological features of the original plagioclase crystals are preserved: twins, original zoning, myrmekitic textures and perthites. The pseudomorphic transformation of plagioclase is independent of the products to which it gives rise, and here we show examples where gibbsite, 1:1 dioctahedral phyllosilicate, or a mixture of both, appear as secondary minerals. The mineralogical nature of a newly formed product depends mainly on the intensity of the leaching to which the profile is subjected, which is influenced by the rainfall in the area where they are found, as well as the size of existing voids in the material.