Original paper

Coexisting 3T and 2M1 polytypes of phengite from Cima Pal (Val Savenca, western Alps): Chemical and polytypic zoning and structural characterisation

Ivaldi, Gabriella; Ferraris, Giovanni; Curetti, Nadia; Compagnoni, Roberto

Abstract

Randomly oriented phengite flakes, up to 15 cm across and 5 cm thick, occur in a metamorphic dyke at Cima Pal (upper Val Savenca, western Alps). The phengite formed in the Sesia zone at quartz-eclogite-facies conditions (T ~ 550°C, P ~ 16-18 kbar) in the presence of hydrous fluid. A greenschist-facies retrogression (~ 4-5 kbar and 500°C) locally altered the eclogite-facies assemblage. The mica flakes show yellowish and greenish zoning. This zoning is superposed by a structural zoning of 3T and 2M1 (more abundant) polytypes. Single-crystal X-ray diffraction data were collected and the structures refined from the following samples. (K0.90Na0.05)(Al1.51Mg0.32Fe0.18Ti0.03)[Si3.40Al0.60O10](OH)2[yellowish 3T; a 5.220(1), c 29.762(9) Å], (K0.95Na0.01)(Al1.40Mg0.34Fe0.27Ti0.03)[Si3.50Al0.50O10](OH)2[greenish 3T; a 5.228(1), c 29.73(1) Å], (K0.98Na0.02)(Al1.55Mg0.24Fe0.21Ti0.02)[Si3.38Al0.62O10](OH)2[yellowish 2M1; a 5.2132(8), b 9.051(2), c 19.937(5) Å, β 95.76(2)°], and (K0.99Na0.02)(Al1.42Mg0.33Fe0.24Ti0.04)[Si3.45Al0.55O10](OH)2[greenish 2M1; a 5.225(1), b 9.057(2), c 19.956(6) Å, β 95.73(2)°]. Tetrahedral cation order is absent in both polytypes, although partial octahedral order is present in the 3T polytype. Colour and limited chemical zoning is attributed to a fluctuation in the fluid-phase composition. The 2M1 polytype with disordered pattern of octahedral cations was probably crystallised from the pre-existing 3T polytype by a solid state reaction. The transformation occurred during the sharp decompression stage leading to the greenschist facies, which triggered nucleation and growth of the 2M1 polytype.

Keywords

phengite-2m1phengite-3tstructure refinementcation orderingcoexistence of polytypes