Prograde versus retrograde chlorite-amphibole intergrowths in a calc- silicate rock

Transmission electron microscopy was used to determine structural relationships between chlorite and amphibole and to distinguish prograde from retrograde intergrowths. Prograde chlorite inclusions occur with hk0 zone axes subparallel to the amphibole c axis but have incoherent and fractured boundaries. These relations are consistent with mimetic growth of amphibole in the chlorite foliation but not with topotaxial replacement. Some retrograde chlorite in amphibole occurs with chlorite (001) and amphibole (100) planes parallel and the amphibole c axis parallel to cholorite [100], [1̄10], or [1̄1̄0]. Coherent boundaries occur parallel to chlorite (001) layers, with no apparent misfit strain. Growth ledges of talc-like layers occur along coherent boundaries, showing that chlorite growth can occur by a single-layer mechanism. Boundaries that are not parallel to chlorite layers are strained such that chlorite layers bend to promote coherence. Most retrograde intergrowths are associated with fractures and consist of chlorite with minor amounts of talc and unidentified phyllosilicates. -from Authors