Weathering of Chlorite and Vermiculite in Ultramafic Rocks of Cabo Ortegal, Northwestern Spain

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 Hercynian-age orogenesis had apparently been partly transformed to high-charge vermiculite during subsequent metamorphism of the rocks. The idealized structural formulae for these minerals are (Al 1.9 Fe 3 + 0.2 Fe 2+ 0.4 Mg 9.2 Cr 0.2 )(Si 5.8 Al 2.2 )O 20 (OH) 16 and X 1.3 (Fe 3+ 0.7 Fe 2+ 0.1 Mg 5.2 Ni 0.1 )(Si 5.8 Al 2.2 )O 20 (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 hydroxy-interlayered 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.