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Coexisting 3T and 2M₁ polytypes of phengite from Cima Pal (Val Savenca, western Alps)

Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, I-10125 Torino, Italy

^* E-mail: ferraris{at}dsmp.unito.it

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.

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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.

Key-words: phengite-2M₁, phengite-3T, structure refinement, cation ordering, coexistence of polytypes..

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