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

Author： Ventruti Gennaro Levy Davide Pavese Alessandro Scordari Fernando Suard Emmanuelle

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0935-1221

Source： European Journal of Mineralogy, Vol.21, Iss.2, 2009-03, pp. : 385-396

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Abstract

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_0.01Fe²⁺_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 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.