The octahedral sheet of metamorphic 2M1-phengites: A combined EMPA and AXANES study

• Published in: American Mineralogist
• Main Authors: Augusto Marcelli, Gianfelice Cinque, Annibale Mottana, Giannantonio Cibin, Raffaele Sassi
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2008
• Subjects: Geochemistry and Petrology; Geophysics; Antarc*; Antarctica
• Online Access: https://www.openaccessrepository.it/record/141975; https://doi.org/10.2138/am.2008.2548

Two types of metamorphic phengites are known: one is linked to high pressure and is 3T; the other is 2M{sub 1}, and its composition is linked to rock-compositional constraints. This work investigates the octahedral sheet crystal-chemical differences between the two phengite types. Seven dioctahedral micas were studied: (1) one 3T phengite from an ultrahigh-pressure metagranitoid in the Dora Maira massif, Italy (P {approx} 4.3 GPa, T {approx} 730 C); (2) five 2M{sub 1} phengites from medium-P orthogneisses in the Eastern Alps metamorphic basement, Italy (P {le} 0.7 GPa, T {approx} 500-600 C); and (3) one 2M{sub 1} ferroan muscovite from pegmatite in Antarctica (P {le} 0.2 GPa, T {approx}500 C). All micas display significant extents of celadonite substitution. In particular, the 2M{sub 1}-phengite formulae (calculated on the basis of 11 O) have 0.68 < {sup IV}Al < 0.82 atoms per formula unit (apfu); octahedral atoms are dominated by Al (1.6-1.8 apfu), with minor and variable Fe (0.20-0.35 apfu) and Mg (0.05-0.17 apfu), and very minor Ti, Mn, and Cr. Total octahedral occupancies are slightly above 2.00 apfu, i.e., there seems to be partial occupancy of the third M site. For all micas, we recorded XAFS spectra on mosaics of carefullymore » separated flakes oriented flat on a plastic support that could be rotated so as to account for the polarization of the synchrotron radiation beam, and we processed them on the basis of the AXANES theory. Spectra show angle-dependent absorption variations for Al and Fe, which can be deconvoluted and fitted by dichroic effects. Pre-edges consistently show most Fe to be Fe{sup 3+} and little angle-dependent intensity variations. The 2M{sub 1}-ferroan muscovite from Antarctica displays the same AXANES behavior as 2M{sub 1}-phengites. By contrast, the ultrahigh-pressure 3T-phengite from Dora Maira (having {sup IV}Al = 0.42 apfu, and Al and Mg as the dominant octahedral constituents) has XAFS spectra that differ significantly. Not only is the {sup IV}Al feature strongly reduced, ...