Stress-induced alteration of sudoite: structural and chemical modifications

The purpose of this study was to investigate the structural and chemical modifications of phyllosilicates that occur under natural conditions, using the progressive deformation of chlorite (sudoite) present in quartz-rich veins from the Internal Zone of the Rif range (Morocco) as the model system. Signs of chlorite deformation include kinks, chevron-like folds, and fractures. The samples also contain later, undeformed grains, which sealed the fractures or grew with (001) perpendicular to the compressive stress. Deformation-induced structural changes consist mainly of basal cleavages associated with ordered replacement of brucite sheets by hydrated layers, thus leading to irregular microdomains of mixed-layer chlorite-vermiculite and sudoite. Such structural modifications represent a mechanism for accommodating the compressive stress. Structural changes were accompanied by minor chemical ones, which lead from di,tri-chlorite (sudoite) to phases with a more trioctahedral character (mixed-layer chlorite-vermiculite). The hydration reaction occurred throughout a topotactic replacement of the pre-existing sudoite grains. Later, undeformed grains consist of mixed-layer chlorite-vermiculite intergrown with retrogressive kaolinite and minor Fe oxide, and are interpreted as having formed through a dissolution-precipitation process, during deformation. Retrogression of sudoite probably occurred during the latest stage of exhumation, in low-temperature conditions.