Abstract

Single crystal X-ray diffraction data were collected at T = 570, 770, 1070 K on a sample of a P4/n vesuvianite from Val d'Ala (Piedmont, Italy). Structure refinements yielded R values of 6.7, 6.8 and 8.5% for the three temperatures, respectively; the P4/n symmetry was retained throughout. The volume thermal expansion coefficient was 33.7(7) . 10 (super -6) K (super -1) ; different expansion rates were observed along the a and c directions [10.4(3) . 10 (super -6) K (super -1) and 12.9(3) . 10 (super -6) K (super -1) respectively]. The volume expansion is larger than that observed in the structurally similar garnet, due to a higher expansion rate along the c axis. An analysis of the atomic displacement parameters (ADP's) with temperature performed on Ca(2) and Ca(3) atoms did not indicate significant positional disorder. As well no evidence of changes in the cation distribution was found in the Ca(4)a and b and M(2)a and b sites even after prolonged heating at 1070 K.

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