Vesuvianite is a well-known rock-forming mineral with complex crystal chemistry and structure. High-symmetry (P4/nnc) vesuvianite occurs in metamorphic calc-silicate rocks mostly in skarn, and formed at high temperatures. Low-symmetry (non-P4/nnc) vesuvianite occurs in rodingites, in veins found in mafic rocks and serpentinites and rarely in metamorphic calc-silicate rocks in which it formed at relatively low temperatures during retrogressive metamorphism. In low-symmetry vesuvianite, two types of possible ordered arrangements (P4/n and P4/nc) exist. There are few research reports of vesuvianite with P4nc symmetry than those of P4/n. An infrequent high-temperature skarn is developed in Kushiro, Tojyo-cho, Hiroshima prefecture, Japan. The formation temperature of the skarn is estimated at about 800°C, although skarnforms generally at about 500°C. Gehlenite and spurrite formed primary at the skarn. Vesuvianite formed by retrograde metamorphism of those minerals. The purpose of this study is to clarify the characteristic of the crystal structure of the vesuvianite from the high-temperature skarn. Special attention is paid on the structural and chemical differences among several structural types of the vesuvianite. 30 small single-crystals were picked up from one hand-specimen that collected in high-temperature skarn in Japan. 20 crystals were ground into spheres, but polygonal shapes of 10 crystals were retained without grinding. These crystals were mounted on fine glass capillaries. The single-crystal X-ray intensity data were collected on a Siements SMART three-circle diffractometer with a CCD detector using graphite-monochromatized MoKα radiation (50KV, 40mA) and were corrected for Lorentz and polarization factors. Subsequent to the X-ray diffraction experiments, chemical analyses were carried out for those crystals. The structural refinements were made using the program SHELX. 30 structure data sets were determined. In the present study, the symmetries of vesuvianites were determined by precise analyses of the systematic absences of reflections. Tojyo vesuvianites are classified into 3 symmetry types. (1): P4/nnc high symmetry (2): Mixtures between P4/n and P4/nc (3): Almost complete P4/nc Moreover (2) and (3) are both divided into two sub-groups by chemical compositions. (2)a: with hydrogarnet-like substitution at tetragonal site (2)b: without hydrogarnet-like substitution (3)a: with boron and F (3)b: without boron Those groups can be briefly classified with cell parameters. Almost complete P4/n structure was not found in this study. It can be concluded that the crystals of those structural types were grown during the several stage of retrograde metamorphism.
