Coexisting 3T and 2M(1) polytypes of phengite from Cima Pal (Val Savenca, western Alps): Chemical and polytypic zoning and structural characterisation

Randomly oriented phengite flakes, up to 15 cm across and 5 em thick, occur in a metamorphic dyke at Cima Pal (upper Val Savenca, western Alps). The p hengite formed in the Sesia zone at quartz-eclogite-facies conditions (T si milar to 550 degreesC, P similar to 16-18 kbar) in the presence of hydrous fluid. A greenschist-facies retrogression (similar to 4-5 kbar and 500 degr eesC locally altered the eclogite-facies assemblage. The mica flakes show y ellowish and greenish zoning. This zoning is superposed by a structural zon ing of 3T and 2M(1) (more abundant) polytypes. Single-crystal X-ray diffrac tion data were collected and the structures refined from the following samp les. (K0.90Na0.05)(Al1.51Mg0.32Fe0.18Ti0.03)(Si3.40Al0.60O10](OH)(2) [yello wish 3T, a 5.220(1), c 29.762(g) Angstrom], (K0.95Na0.01)(Al1.40Mg0.34Fe0.2 7Ti0.03)(Si3.50Al0.50O10](OH)(2) [greenish 3T; a 5.228(1), c 29.73(1) Angst rom], (K0.98Na0.02)(Al1.55Mg0.24Fe0.21Ti0.02)[Si3.38Al0.62O10](OH)(2) [yell owish 2M(1); a 5.2132(8), b 9.051(2), c 19.937(5) Angstrom, beta 95.76(2)de grees], and (K0.99Na0.02)(Al1.42Mg0.33Fe0.24Ti0.04)[Si3.45Al0.55O10](OH)(2) [greenish 2M(1); a 5.225(1), b 9.057(2), c 19.956(6) Angstrom, beta 95.73( 2)degrees]. Tetrahedral cation order is absent in both polytypes, although partial octa hedral order is present in the 3T polytype. Colour and limited chemical zon ing is attributed to a fluctuation in the fluid-phase composition. The 2M(1 ), polytype with disordered pattern of octahedral cations was probably crys tallised from the pre-existing 3 T 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 2M(1) pol ytype.