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

You do not currently have access to this article.