Structural investigation of low-symmetry vesuvianite collected from Tojyo, Hiroshima, Japan: Implications for hydrogarnet-like substitution

摘要

The crystal structure of a unique type of vesuvianite with the space group PAIn [a = 15.576(2), c = 11.835(2) A] formed by the retrograde alteration of gehlenite in high-temperature skarn at Kushiro (Tojyo, Shobara Hiroshima) southwestern Japan, is determined by single-crystal X-ray diffraction (R1 = 3.63%). Significant vacancies (29%) occur only on one of the two pseudo-equivalent Z1 sites (Z1b). In addition, the Si-0 distance at the Z1b site remarkably increases up to 1.707(3) A, indicating the existence of a hydrogarnet-like substitution, while Z1a, the other pseudo-equivalent Si site, has only 5% vacancies associated with an Si-O distance of 1.636(3) A. The total refined Si site occupancy of 17.7 atoms per formula unit is in excellent agreement with the results of electron-microprobe analyses.