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High-temperature treatment, hydrogen behaviour and cation partitioning of a Fe–Ti bearing volcanic phlogopite by in situ neutron powder diffraction and FTIR spectroscopy

¹ Dipartimento Geomineralogico, Università’ di Bari, via E. Orabona 4, 70125 Bari, Italy
² Dipartimento di Scienze Mineralogiche e Petrologiche, Università degli Studi di Torino, via Valperga Caluso 37, 10125, Torino, Italy
³ Dipartimento Scienze della Terra, Università degli Studi di Milano, via Botticelli 23, 20133 Milano, Italy
⁴ National Research Council, IDPA, Section of Milan, via Botticelli 23, 20133 Milano, Italy
⁵ Institute Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex, France

^* Corresponding author, e-mail: f.scordari{at}geomin.uniba.it

A natural Fe- and Ti-rich phlogopite-1M of volcanic origin with an average composition (K_0.86Na_0.08Ba_0.04) (Mg_1.97Al_0.17Mn_0.01Fe _0.29²⁺ Fe _0.37³⁺ Ti_0.19)(Si_2.72Al_1.28)O_10.66(OH_1.22F_0.12) was studied using in situ low/high temperature neutron powder diffraction and the full-profile Rietveld refinement technique. Data sets were collected at 10, 298, 673 and 873 K on the D2B high resolution diffractometer at the ILL (Institute Laue-Langevin) Facility.

The cell parameters expand linearly from room temperature up to 873 K [_a = 1.43(5) · 10^–5 K^–1, _b = 1.47(4) · 10^–5 K^–1, _c = 1.99(19) · 10^–5 K^–1, _V = 4.90(12) · 10^–5 K^–1]. Site-occupancy refinements show that in the octahedral sheet Ti and Al tend to order into the M2 and M1 sites, respectively. As a consequence, Fe²⁺ and Fe³⁺ are preferentially ordered at the M1 and M2 sites, respectively. The H position was accurately refined at 10 K. The large components of the atomic displacement tensor of H suggest that hydrogen disorders over two symmetry mirror-related sites, around the average position (x,1/2,z). Hydrogen can also act as an "atomic probe" of the Al—Si distribution over the tetrahedral sites. A comparison with the high-temperature results previously obtained using single-crystal X-ray diffraction on the same sample shows that, in the present study, dehydrogenation/oxidation processes involving hydroxyl group coordinated to Fe²⁺ have occurred at a very modest rate, as confirmed by FTIR spectroscopy measurements.

Key-words: phlogopite, neutron powder diffraction, FTIR, cation partitioning, thermal expansion, hydrogen behaviour.