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 (sub 0.86) Na (sub 0.08) Ba (sub 0.04) ) (Mg (sub 1.97) Al (sub 0.17) Mn (sub 0.01) Fe (sub 0.29) (super 2+) Fe (sub 0.37) (super 3+) Ti (sub 0.19) ) (Si (sub 2.72) Al (sub 1.28) )O (sub 10.66) (OH (sub 1.22) F (sub 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[alpha (sub a) =1.43(5).10 (super -5) K (super -1) , alpha (sub b) =1.47(4).10 (super -5) K (super -1) , alpha (sub c) =1.99(19).10 (super -5) K (super -1) , alpha (sub V) =4.90(12).10 (super -5) K (super -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 (super 2+) and Fe (super 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 (super 2+) have occurred at a very modest rate, as confirmed by FTIR spectroscopy measurements.