Thermal behaviour of davyne-group minerals

Abstract

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, α _a , were 10.2(3)·10^-6, 13.4(7)·10^-6, 15.1(8)·10^-1 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, α _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 α _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 P6₃ and at 943 K in space group P6₃ (R=0.062, with 750 independent reflections) and in space group P6₃/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 P6₃/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.