Weathering of Chlorite and Vermiculite in Ultramafic Rocks of Cabo Ortegal, Northwestern Spain1

P. Buurman, E. L. Meijer and J. H. van Wijck2
Department of Soil Science and Geology, Agricultural University P.O. Box 37, 6700 AA Wageningen, The Netherlands
1 Publication No. 926 of the Department of Soil Science and Geology.
2 Present address: Technical Centre for the Ceramic Industry, P.O. Box 40, 6994 ZG De Steeg, The Netherlands.

Abstract: 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 (Al1.9Fe3+0.2Fe2+0.4Mg9.2Cr0.2)(Si5.8Al2.2)O20(OH)16 and X1.3(Fe3+0.7Fe2+0.1Mg5.2Ni0.1)(Si5.8Al2.2)O20(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 Fe3+-rich smectite, which probably formed by precipitation from solution.

Key Words: Chlorite • Hydroxy-Al • Serpentinite • Smectite • Vermiculite • Weathering • X-ray powder diffraction

Clays and Clay Minerals; June 1988 v. 36; no. 3; p. 263-269; DOI: 10.1346/CCMN.1988.0360308
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)