TEM evidence for high-temperature (300 degrees C) smectite in multistage clay-mineral pseudomorphs in pelitic rocks (Rif, Morocco)

Optically homogeneous pseudomorphs occur in high-pressure metapelites from the Alpine Rif chain, Morocco, along with talc and kyanite. Further charact erization using optical microscopy, electron microprobe, single-crystal X-r ay diffraction, and HRTEM reveals a complex metamorphic replacement sequenc e of a previous high-pressure metamorphic mineral, possibly magnesiochlorit oid. Within the pseudomorphs, three zones are recognized based on the occurrence of distinct layer silicates, which crystallized under different P-T condit ions along the retrograde metamorphic P-T path. These layer silicates are, from core to rim: (I) a relic mixture of margarite and chlorite, (2) orient ed chlorite-smectite intergrowths +/- pyrophyllite, and (3) chlorite and ab undant smectite (at the periphery of the pseudomorph). These pseudomorphs represent an uncommon record of the retrograde metamorph ic P-T path because the earliest smectite formed while the rocks still expe rienced P-T conditions in the stability field of pyrophyllite. This observa tion suggests that clay minerals may crystallize as regional metamorphic pr oducts, and that their stability field might not be restricted to late shal low-depth alteration.