Phlogopite of volcanic origin: an in-situ low and high temperature neutron powder diffraction study combined with infrared spectroscopy

A natural Fe- and Ti-rich phlogopite-1M of volcanic origin with an average composition (K0.86Na0.08Ba0.04) (Mg 1.97Al0.17 Mn0.01Fe2+ 0.29Fe3+0.37Ti0.19)(Si 2.72Al1.28)O10.66 (OH1.22F 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 [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 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.