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: (K1.96Na0.13Ca0.01)(Mg3.15Fe2.592+Ti0.17Mn0.09)(Si5.98Al1.92Ti0.10)O-20[(OH)(1.47)F-1.98], was determined by Rietveld refinement using high-resolution neutron powder diffraction at in situ temperatures ranging from 20 to 900degreesC. The room-temperature structure of the samples heated to between 400 and 900degreesC using an electric furnace in air was also refined. The crystal structures were refined to an R-P of 2.98-5.06% and R-wp of 3.84-6.77%. For the in situ beating experiments in a vacuum, the unit-cell dimensions increased linearly to 600degreesC. The linear expansion coefficient for the c axis was 1.65 x 10(-5),C-1, while those for the a and b dimensions were 4.44 x 10(-6)degreesC(-1) and 5.21 x 10(-6)degreesC(-1), respectively. Accordingly, the increase in the unit-cell volume up to 600degreesC 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 (3.76degrees at 20degreesC to 1.95degrees at 600degreesC) 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 400degreesC. The octahedral sheet thickness and mean <M-O> distance decreased linearly due to oxidation of octahedral Fe. However, the interlayer separation and mean <K-O> distance decreased at temperatures over 400degreesC. At 400degreesC, 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 (3.57degrees at 400degreesC to 6.15degrees at 900degreesC).