POSITIONAL DISORDER OF OXYGEN IN GARNET - THE CRYSTAL-STRUCTURE REFINEMENT OF SCHORLOMITE

The structure of a crystal of schorlomite garnet with composition {Ca2 .87Mg0.10Na0.04} [Ti1.06Fe0.633+Al0.14Fe0.062+Zr0.04Mg0.04 Mn0.03V0.01 ] (Si2.35Fe0.343+Fe0.312+)O-12, and an a parameter of 12.157(3) Angstr om, has been refined using MoK alpha X-ray-diffraction data to an R(W) of 0.042. The oxygen atom in the structure is modeled by a split isot ropic position, which results from significant occupancy of the tetrah edral site by iron. Lengths of bonds between tetrahedrally coordinated cations and the two oxygen atoms are consistent with silicon-oxygen a nd iron-oxygen distances observed in other garnet structures.