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.
Thermal behavior of davyne-group minerals
BONACCORSI, ELENA;MERLINO, STEFANO
1995-01-01
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, 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.