Crystal structures of near-end-member phlogopite at high temperatures and heat-treated Fe-rich phlogopite: The influence of the O,OH,F site

The crystal structure of end-member phlogopite-1M from White Well, Australi a, was determined by refinement using single-crystal X-ray data to 600 degr ees C (R = 0.069, wR = 0.096 at 600 degrees C). Cell parameters were refine d at 20, 150, and thereafter at 50 degrees C intervals to 600 degrees C. Th e rate of expansion of the metric unit-cell dimensions is linear. The expan sivity of the c dimension is 1.81 x 10(-5) degrees C-1, and 1.40 and 1.34 x 10(-5) degrees C-1 for the a and b dimensions, respectively (compared to f luorophlogopite in a previous study: 24 degrees to 600 degrees C, alpha(a) = 0.86, alpha(b) = 0.75, and alpha(c) = 1.81 x 10(-5) degrees C-1). The OH- rich octahedra (M1, M2) expand without changing shape significantly at elev ated temperatures. in contrast to F-rich octahedra in fluorophlogopite, whi ch become elongate approximately along the c axis. The interlayer site beco mes elongate in phlogopite with increasing temperature relative to fluoroph logopite. although the site shows general flattening with increasing temper ature. We also refined the room-temperature structure of a sample of Fe-ric h phlogopite-1M sample From SilverCrater, near Bancroft, Ontario, and the s ame phlogopite after heat treatment at 904 degrees C for 24 hours (untreate d: R = 0.039, wR = 0.043; heat-treated:R = 0.039, wR = 0.047). In contrast to earlier studies, there was no change in Fe sits occupancy from the octah edra to the tetrahedra. The octahedra (M1, M2) and the intralayer site ate flattened in both the heated and unheated samples, but the heated sample sh ows significant flattening in all these sites overthe unheated sample.