Crystal structures of biotite at high temperatures and of heat-treated biotite using neutron powder diffraction

The crystal structure of biotite-1M from Bancroft, Ontario, with the formula: (K1.96 Na0.13 Ca0.01)(Mg3.15 Fe2+2.59 Ti0.17 Mn0.09)(Si5.98 Al1.92 Ti0.10)O20[(OH)1.47 F1.98], was determined by Rietveld refinement using high-resolution neutron powder diffraction at in situ temperatures ranging from 20 to 900°C. The room-temperature structure of the samples heated to between 400 and 900°C using an electric furnace in air was also refined. The crystal structures were refined to an RP of 2.98-5.06% and Rwp of 3.84-6.77%. For the in situ heating experiments in a vacuum, the unit-cell dimensions increased linearly to 600#DDGC. The linear expansion coefficient for the c axis was 1.65×10-5°C-1, while those for the a and b dimensions were 4.44×10-6°C-1 and 5.21×10-6°C-1, respectively. Accordingly, the increase in the unit-cell volume up to 600°C occurred mainly along the c axis, resulting from the expansion in the K coordination sphere along that direction. Results for all K-O bonds were analyzed in terms of the lattice component and an inner component of the structural strain. The ditrigonal distortion decreased (3.76° at 20°C to 1.95° at 600°C) with temperature, because the shorter bonds expanded and the longer bonds contracted. The increase in the interlayer separation and the decrease in the interlayer octahedral flattening angle confirmed that the c-dominated expansion occurred in the interlayer region. In the case of the ex situ-heated samples, the cell dimensions decreased sharply at temperatures over 400°C. The octahedral sheet thickness and mean distance decreased linearly due to oxidation of octahedral Fe. However, the interlayer separation and mean distance decreased at temperatures over 400°C. At 400°C, dehydroxylation began to increase and interlayer regions became more constricted. The overall cell parameters decreased rapidly with increasing temperatures due to dehydroxylation. The large inner strain components in the K-O bonds also resulted in an increase in the considerable ditrigonal distortion (3.57° at 400°C to 6.15° at 900°C).