ORDER-DISORDER IN THE OCTAHEDRAL SHEET OF METAMORPHIC 2M1 PHENGITES: A COMBINED EMPA AND AXANES STUDY

Two types of metamorphic phengites are known: one is linked to HP and is 3T, the other is 2M₁: for both the composition is due to rock compositional constraints. This work aims at investigating crystallographic differences between the two phengite types. Six mica samples were studied: 1) one UHP 3T phengite from Dora Maira (P~2.5 Gpa, T~ 750°C); 2) four 2M₁ phengites from orthogneisses from Eastern Alps (P= < 0.6 Gpa, T~ 500-600°C); 3) one 2M₁ ferroan muscovite from a pegmatite in Antarctica (LP, T ~ 500°C). All studied phengites display significant extents of celadonite substitution. EPMA were carried out at Padua using Cameca-Camebax SX50. XAFS spectra were recorded for Fe at SSRL (U.S.A.) by fluorescence detection mode, and for all other atoms at DAPHNE (Frascati, Italy) by transmission mode. We recorded our AXANES spectra on mosaics of flakes carefully separated and oriented flat on a plastic support. All 2M₁ phengites have ^IVAl > 1 apfu; their ^VIatom contents are dominated by Al, with variable Fe (partly Fe³⁺) and Mg. Ti, Mn, and Cr are <0.03 apfu. ^VIOccupancy in them is slighty > 4 apfu, i.e., there is a partial occupancy of the third ^VIsheet site. XAFS spectra show complex patterns, which reflect an ordered cation distribution in the ^VIsheet (and, for Al, in the ^IVsheet as well). By contrast, the 3T phengite have ^IVAl just < 1 apfu), and Al and Mg are dominant ^VIconstituents, being Ti, Cr and Fe (Fe²⁺) present only at the trace level. XAFS spectra show one broad peak only, indicating Fe²⁺ in a fully disordered distribution. This is also pointed out by the absence of other XANES features at high-energy i.e., before the onset of EXAFS oscillations. In conclusion: i) 3T phengites from Dora Maira massif, related to UHP, displays complete octahedral disorder; ii) 2M₁ phengites unrelated to HP suggest Al and Mg/Fe order over the M1 and (M2,M3) sites in a regular way, as does the 2M₁ muscovite from Antarctica pegmatite.