Smectite formation produced by weathering in a coarse granite saprolite in Galicia (NW Spain)

Weathering of a calcalkaline granite was studied in the south of Galicia (NW Spain) where the average annual precipitation is 1400 mm and the average annual temperature is 12°C. The original rock contains perthitic K-feldspars, plagioclases with inclusions of muscovite and opaque minerals, quartz and chloritized biotite, with apatite, zircon, sphene and opaques as accessories. In the saprolite the structure of the rock is preserved, the plagioclases show up to grade 4 weathering and the biotites, between 2 and 3. Weathering in quartz and potassium feldspar crystals is manifested only by fracturing. Biotite changes following the parallel linear model and its weathering products are interstratified biotite–vermiculite and iron oxyhydroxides. The plagioclases change to a microgranular material by pseudomorphic transformation. This material, which substitutes the plagioclase, includes small clearly delimited units which retain zones with the optic characteristics of muscovite. X-ray diffraction analyses of microsamples show that they are formed by 1:1 diocthaedral phyllosilicate, smectite and a small quantity of mica. From these facts we concluded that smectite is formed inside the plagioclase crystals, and probably originates from the inclusions of muscovite contained in these crystals, as it is suggested by the microscopic study which shows the increase in volume which occurs when the crystals of muscovite are transformed.