Host: The Mineralogical Society of Japan
Introduction It has been well known, natural garnets exhibit zoning of many elements and especially the distribution of trace heavy elements in its crystal structure attracts interest since such information may include the history of the mineral genesis. SR-XRF imaging was carried out to reveal the distribution of trace heavy elements in garnet from the Sanpou mine, Okayama. From the results, the garnet contained Sn, W and rare earth elements (REE) as trace heavy elements and the zoning of these elements were parallel to garnet crystal shape and their distribution varied from core to rim. The purpose of the present study is what produced such characteristic zoning in garnet structure. So first, we carried out single-crystal X-ray structure analysis to reveal the size of the crystallographic sites of constituent atoms, and then, estimated the sites of trace elements based on their ionic radii. In order to obtain direct information of the site of the trace elements, we have utilized local structure information obtained by XAFS analysis. Result & Discussion Single-crystal X-ray analysis revealed that garnet from the Sanpou mine is grandite garnet (Ca3(Fe0.71Al0.29)2(SiO4)3) and possess a cubic unit cell with space group Ia-3d and lattice constant of a=12.0065(6)Å. Sn K-edge XAFS analysis showed that tetravalent Sn was coordinated by the 6 oxygen and the Sn-O distance was 2.00Å.This results agreed well with the coordination and bond length of the octahedral site determined by the structure analysis, so the substitution of Sn for the octahedral site was apparent.For other elements except for Sn, the crystal-chemical structural environment was examined based on the ionic radii. Element which has ionic radii close to the size of the crystallographic site is able to substitute for the site. Therefore, we compared the ionic radii of the trace heavy elements with the size of each site. First, the sizes of the 3 atomic sites in the garnet were determined using metal-oxygen distance obtained from the structural analysis fixing the ionic radius of oxygen as 1.40Å. The metal-oxygen distance for the dodecahedral, octahedral, and tetrahedral sites were calculated as 1.00Å, 0.60 and 0.25Å, respectively. Then they were compared with the ionic radii after Shannon and Prewitt(1969,1970). As for REE zoning, the zoning position shifted outside with increasing the atomic number. This tendency corresponded to that of the lanthanide contraction, so it seemed that ionic radii played important role in the zoning during the crystal growth.
