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.
\[\mathrm{(K_{0.90}Na_{0.05})\ (Al_{1.51}Mg_{0.32}Fe_{0.18}Ti_{0.03})\ [Si_{3.40}Al_{0.60}O_{10}](OH)_{2}\ [yellowish\ 3\mathit{T};\ \mathit{a}\ 5.220(1),\ \mathit{c}\ 29.762(9)\ {\AA}]}\]
\[\mathrm{(K_{0.95}Na_{0.01})\ (Al_{1.40}Mg_{0.34}Fe_{0.27}Ti_{0.03})\ [Si_{3.50}Al_{0.50}O_{10}](OH)_{2}\ [greenish\ 3\mathit{T};\ \mathit{a}\ 5.228(1),\ \mathit{c}\ 29.73(1)\ {\AA}]}\]
\[\mathrm{(K_{0.98}Na_{0.02})\ (Al_{1.55}Mg_{0.24}Fe_{0.21}Ti_{0.02})\ [Si_{3.38}Al_{0.62}O_{10}](OH)_{2}\ [yellowish\ 2\mathit{M}_{1};\ \mathit{a}\ 5.2132(8),\ \mathit{b}\ 9.051(2),\ \mathit{c}\ 19.937(5)\ {\AA},\ {\beta}\ 95.76(2){^\circ}]}\]
, and
\[\mathrm{(K_{0.99}Na_{0.02})\ (Al_{1.42}Mg_{0.33}Fe_{0.24}Ti_{0.04})\ [Si_{3.45}Al_{0.55}O_{10}](OH)_{2}\ [greenish\ 2\mathit{M}_{1};\ \mathit{a}\ 5.225(1),\ \mathit{b}\ 9.057(2),\ \mathit{c}\ 19.956(6)\ {\AA},\ {\beta}\ 95.73(2){^\circ}]}\]

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.

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