Structural aspects of oxidation-dehydrogenation in staurolite

The oxidation-dehydrogcnation mechanism innatural staurolitc has been investigated bysingle-crystal structure refinement of crystals heated in air al aseries oftemperatures up to 950°C. Crystal structures were refined in the space group CUm to R indices of 1-2% using graphite-monochromatcd MoKa X-radialion. With increasing temperature ofannealing, the site popula tions changed dramatically. The degree ofAl-Dorder over the MQA) and MOB) sites decreases with increasing temperature; this is accompanied by areduction in the pangle, the latter being ameasure ofthe spontaneous strain. Thus with increasing temperature, the degree of order inthe structure decreases, and the structure approaches orthorhombic symmetry. However, a new cation site, TO), becomes imponant inthe heated structure, appearing with very low occupancy inthe structure annealed at 500°C: with increasing temperature of annealing, the occupancy of the site increases dramatically (to 1.10 apfu) as the occupancies of the other sites decrease, particularly MQA), MOB) and T(2). The 7T3) site isoccupied byAland Fc3\ and is tetrahcdrally coordinated by the O(M), 0(1 fl) and 2 x 0(3) oxygen atoms. The oxidation-dehydrogenation proceeds via oxidation ofFc** at 7(2), with the loss ofan associated hydrogen atom at one ofthe coordinating 0(1/1) or O(lfl) hydroxyl anions: this oxidation is accompanied by major rearrangement of cations that involves all cation sites except 7*(l). The presence ofcations at the 7T3) site in natural staurolitc ishence an indicator of postcrystallization oxidation-dehydrogcnation.