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

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_0.17 Mn_0.01Fe²⁺ _0.29Fe³⁺_0.37Ti_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 [α_a = 1.43(5)·10 ^-5 K^-1, α_b = 1.47(4)·10 ^-5 K^-1, α_c = 1.99(19)·10 ^-5 K^-1, α_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 Ml sites, respectively. As a consequence, Fe²⁺ and Fe³⁺ are preferentially ordered at the Ml 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.