Weathering of Chlorite a N D Vermiculite in Ultramafic Rocks of Cabo Ortegal, Northwestern Spain 1

-Chloritic veins in serpentinite and their weathering products were analyzed by X-ray powder diffraction (XRD) and X-ray fluorescence spectrometry (XRF). Chlorite formed during the Hercynianage orogenesis had apparently been partly transformed to high-charge vermiculite during subsequent metamorphism of the rocks. The idealized structural formulae for these minerals are (mlL9Fea+0.zFe2+o.4Mg9.2Cr0 .z ) (S is . sA12.2)O20(OH)l 6 and Xl.3(Fe3+o.7Fe2+o.~Mg5.zNio. l ) (Sis . sA12.E)OEo(OH)4, respectively. This transformation appears to have taken place by the removal of the hydroxy-interlayer from the chlorite without major effect on the rest of the structure. It is not clear whether other hydroxyinterlayered vermiculites containing less tetrahedral aluminum were intermediate weathering products or inherited minerals. The ultimate weathering product of chlorite and vermiculites was a Fe 3+-rich smectite, which probably formed by precipitation from solution. Key Words--Chlorite, Hydroxy-A1, Serpentinite, Smectite, Vermiculite, Weathering, X-ray powder diffraction.