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)O20[(OH)1.47F1.98], 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°C. 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 <M–O> distance decreased linearly due to oxidation of octahedral Fe. However, the interlayer separation and mean <K–O> 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).