SEM/TEM-AEM characterization of micro- and nano-scale zonation in phengite from a UHP Dora-Maira marble; petrologic significance of armoured Si-rich domains

Chemically zoned phengite flakes from an impure marble of the ultra-high pressure Brossasco-Isasca unit (Dora-Maira massif, Italian Western Alps) have been investigated in detail, revealing a complex nano-structure within the crystals. Diffraction patterns show that this phengite is a highly ordered 3T polytype. In bright-field TEM images, 60-120 Aa wide areas of mottled contrast cross, at an angle of approximately 40 degrees , the host phengite that represents the matrix. The spatially ordered mottled contrast and AEM analyses are indicative of the presence of high-Si phengite relics (Si 3.61 pfu) within the ordered matrix (Si 3.38 pfu). The higher-Si phengite has c 29.68 Aa, different from that of the lower-Si matrix (c 29.75 Aa). A zoned 3T phengite mantle (3.41-3.31 Si pfu) surrounds the phengite cores showing decreasing values of c from the inner to the outer part (29.78-29.85 Aa). A later 2M (sub 1) polytype with Si 3.22 pfu has been locally detected at the edge of the phengite crystals. Differences in both the octahedral and tetrahedral contents between the two types of phengite nano-domains suggest that the formation of the high-Si phengite is due to an incomplete ionic reaction affecting the prograde phengite at the metamorphic peak. The suggested reaction is phengite(a) + 0.20 Mg (super 2+) + 0.23 Si (super 4+) --> phengite(b) +0.44 Al (super 3+) .