Xonotlite in rodingite assemblages from the Ronda peridotites, Betic Cordilleras, southern Spain

Xonotlite, ideally Ca6Si6O17(OH)(2), occurs as a major phase in the leucocratic dyke complex that intruded the Ronda peridotites, near the village of Carratraca, Malaga, Spain. Xonotlite occurs as acicular grains in veins parallel to the dykes, as tabular grains along the reaction zone between the serpentinite host-rock and the leucocratic dykes, and as acicular grains replacing hydrogrossular in the matrix of the dykes. It is associated with albite, pectolite and hydrogrossular. Electron-microprobe analyses give a formula (Ca-6.05)(Si5.96Al0.01)O-17(OH)(2) for the acicular and tabular xonotlite, and (Ca-6.11)(Si5.92Al0.03)O-12(OH)(2) for xonotlite replacing hydrogrossular. X-ray powder-diffraction patterns of xonotlite show the presence of two polytypes: M2a2b2c and Ma2b2c. This unusual association (xonotlite, hydrogrossular, pectolite) was produced by the rodingitization of the leucocratic dykes during the serpentinization of the ultramafic massif at low-temperature conditions, 300degrees to 350degreesC; it was associated with the production of lizardite in the surrounding serpentinites.