SPECTROSCOPY OF THE CATION DISTRIBUTION IN THE SCHORLOMITE SPECIES OFGARNET

A homogeneous megacryst of schorlomite was investigated to determine t he valence states of Fe and Ti and the crystallographic sites occupied by these elements. The chemical composition of the specimen was analy zed by electron microprobe, wet-chemical analysis, FTIR, and INAA. The results from X-ray absorption near-edge structure spectroscopy (XANES ) are consistent with exclusively Ti4+ occupying the octahedral site o nly. The tetrahedral site is deficient in Si and the results of low-te mperature Fe-57 Mossbauer spectroscopy indicate that the remainder of the site is occupied by Fe3+ and substantial Fe2+. A spin-allowed inte nsified crystal-held transition of Fe-[4](2+) is present in the near-i nfrared spectrum. The optical absorption spectrum is dominated by an i ntense band centered at 500 nm with a full width of 8000 cm(-1) at hal f maximum peak height; this band is assigned to an Fe2+-Ti4+ intervale nce charge transfer transition between Fe-[4](2+) and Ti-[6]. The cati on site occupancies in this specimen of schorlomite can be expressed b y the following formula: [GRAPHICS] t are evident when their atomic ar rangements are projected down [001]. In that projection, the chains ex ist in (100) planes, with two planes per unit cell. In monazite the pl anes are offset by 2.2 Angstrom along [010], relative to those in xeno time, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional O2' atom to complete the REO(9) polyhedron.