| Title | The octahedral sheet of metamorphic 2M1-phengites; a combined EMPA and AXANES study |
| Author | Cibin, G.; Cinque, G.; Marcelli, A.; Mottana, A.; Sassi, R. |
| Author Affil | Cibin, G., Universita di Roma Tre, Dipartimento di Scienze Geologiche, Rome, Italy. Other: University of Maine; Louisiana State University; University of Padova, Italy; Istituto Nazionale di Fisica Nucleare, Italy; Rutherford Appleton Laboratory, United Kingdom; Universita di Padova, Italy |
| Source | Petrologic mineralogy, the study of minerals in context; a memorial in honor of Charles V. Guidotti, prefaced by M.D. Dyar, E.S. Grew, D.J. Henry and F.P. Sassi. American Mineralogist, 93(2-3), p.414-425, . Publisher: Mineralogical Society of America, Washington, DC, United States. ISSN: 0003- 004X |
| Publication Date | Feb. 2008 |
| Notes | In English. 103 refs. GeoRef Acc. No: 287517 |
| Index Terms | Antarctica; Italy--Dora Maira Massif; Alps--Eastern Alps; Alps; celadonite; chemical composition; Cottian Alps; crystal chemistry; Dora Maira Massif; Eastern Alps; electron probe data; Europe; geologic barometry; geologic thermometry; gneisses; granites; igneous rocks; Italy; metamorphic rocks; mica group; muscovite; orthogneiss; pegmatite; phengite; Piedmont Alps; Piemonte Italy; plutonic rocks; sheet silicates; silicates; Southern Europe; spectra; Western Alps; Western Europe; X-ray spectra; XAFS spectra; XANES spectra |
| Abstract | Two types of metamorphic phengites are known: one is linked to high pressure and is 3T; the other is 2M1, 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 approximately 4.3 GPa, T approximately 730°C); (2) five 2M1 phengites from medium-P orthogneisses in the Eastern Alps metamorphic basement, Italy (P<=0.7 GPa, T approximately 500-600°C); and (3) one 2M1 ferroan muscovite from pegmatite in Antarctica (P<=0.2 GPa, T approximately 500°C). All micas display significant extents of celadonite substitution. In particular, the 2M1-phengite formulae (calculated on the basis of 11 O) have 0.68‹IVAl‹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 carefully 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 Fe3+ and little angle-dependent intensity variations. The 2M1-ferroan muscovite from Antarctica displays the same AXANES behavior as 2M1-phengites. By contrast, the ultrahigh-pressure 3T-phengite from Dora Maira (having IVAl = 0.42 apfu, and Al and Mg as the dominant octahedral constituents) has XAFS spectra that differ significantly. Not only is the IVAl feature strongly reduced, in agreement with the increased Si content, but also Fe XAFS spectra show one broad feature only, indicating that all Fe is Fe2+ in a fully disordered distribution with no angle- dependent variations. We conclude that this 3T-phengite is actually contaminated by exsolved Fe-bearing pyrope platelets, which cannot be resolved under SEM examination; by contrast, the 2M1-phengites, unrelated to high-pressure, suggest Al/Fe3+ order over the M1 and (M2, M3) sites, as also does the 2M1 pegmatitic muscovite. |
| URL | http://hdl.handle.net/10.2138/am.2008.2548 |
| Publication Type | journal article |
| Record ID | 84723 |