TEM evidence for high-temperature (300°C) smectite in multistage clay-mineral pseudomorphs in pelitic rocks (Rif, Morocco)
Agard, Philippe; Jullien, Michel; Goffé, Bruno; Baronnet, Alain; Bouybaouène, Mohammed
European Journal of Mineralogy Volume 11 Number 4 (1999), p. 655 - 668
published: Jul 16, 1999
manuscript accepted: Mar 4, 1999
manuscript received: Sep 29, 1998
ArtNo. ESP147051104007, Price: 29.00 €
Abstract Optically homogeneous pseudomorphs occur in high-pressure metapelites from the Alpine Rif chain, Morocco, along with talc and kyanite. Further characterization using optical microscopy, electron microprobe, single- crystal X-ray diffraction, and HRTEM reveals a complex metamorphic replacement sequence of a previous highpressure metamorphic mineral, possibly magnesiochloritoid. Within the pseudomorphs, three zones are recognized based on the occurrence of distinct layer silicates, which crystallized under different P-T conditions along the retrograde metamorphic P-T path. These layer silicates are, from core to rim: (1) a relic mixture of margarite and chlorite, (2) oriented chlorite-smectite intergrowths ± pyrophyllite, and (3) chlorite and abundant smectite (at the periphery of the pseudomorph). These pseudomorphs represent an uncommon record of the retrograde metamorphic P-T path because the earliest smectite formed while the rocks still experienced P-T conditions in the stability field of pyrophyllite. This observation suggests that clay minerals may crystallize as regional metamorphic products, and that their stability field might not be restricted to late shallow-depth alteration.