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

Abstract

A natural Fe- and Ti-rich phlogopite-1 M of volcanic origin with an average composition (K 0.86 Na 0.08 Ba 0.04 ) (Mg 1.97 Al 0.17 Mn 0.01 Fe 0.29 2+ Fe 0.37 3+ Ti 0.19 )(Si 2.72 Al 1.28 )O 10.66 (OH 1.22 F 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, Fe 2+ and Fe 3+ 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 Fe 2+ have occurred at a very modest rate, as confirmed by FTIR spectroscopy measurements.