A natural Fe- and Ti-rich phlogopite-1M of volcanic origin with an average composition (K0.86Na0.08Ba0.04) (Mg1.97Al0.17Mn0.01Fe 0.292+ Fe 0.373+ Ti0.19)(Si2.72Al1.28)O10.66(OH1.22F0.12) 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 [αa = 1.43(5) · 10−5 K−1, αb = 1.47(4) · 10−5 K−1, αc = 1.99(19) · 10−5 K−1, αV = 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.

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