The thermal behaviour of microsommite (MC), davyne from Vesuvius (DV) and from Zabargad (DZ) was determined by X-ray single crystal data obtained employing a microfurnace connected to a four-circle diffractometer. Upon heating, the a parameter increased linearly, with similar thermal expansion rates for the three samples: the mean linear expansion coefficients, alpha(a), were 10.2(3). 10(-6), 13.4(7). 10(-6), 15.1(8). 10(-6) K-1 for MC, DV and DZ respectively. At about 473 K both MC and DZ showed a discontinuity in the expansion of the c parameter. The mean linear expansion coefficient, alpha(c), changed abruptly from 16(4). 10(-6) K-1 for both minerals below the discontinuity to 2(1). 10(-6) and 3(1). 10(-6) K-1 for MC and DZ, respectively, above the discontinuity. In DV, however, the alpha(c) coefficient was constant between 293 und 827 K and equal to 1(2). 10(-6) K-1. The substructure of MC was refined under room conditions (R=0.055 with 758 independent reflections) in space group P6(3) and at 943 K in space group P6(3) (R=0.062, with 750 independent reflections) and in space group P6(3)/m (R=0.065 with 394 independent reflections). By comparing the structural refinements the discontinuity could be related to the tilting of the tetrahedra connected along c: for T<473 K the temperature increase induced the stretching of such chains through tetrahedral tilting. At 473 K the chains were completely stretched and a purely displacive phase transition occured, with symmetry change from space group P6(3) to P6(3)/m. The further limited increase of the c parameter for T>473 K was due to the small tetrahedral expansion. In DV, where the completely stretched structure was already realized under room conditions, no phase transition occured.
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