SPINELS RENAISSANCE: THE PAST, PRESENT, AND FUTURE OF THOSE UBIQUITOUS MINERALS AND MATERIALS Crystal chemistry of the ulvospinel-qandilite series

Five spinel single-crystal samples within the ulvospinel-qandilite series [(Fe2-xMg2)TiO4 0.15 < x < 0.94] were synthesized and structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. Site populations, derived from structural and chemical analysis, show that the tetrahedrally coordinated site (T) is exclusively populated by Mg2+ and Fe2+, while the octahedrally coordinated site (M) is populated by Ti4+, Mg2+, Fe2+, and minor amounts of Fe3+. The inverse cation distribution is characterized by parallel substitution of Mg2+ for Fe2+ at both the T and M sites along the series. The variation in the unit-cell parameter from 8.527 to 8.495 A is mainly related to the occurrence of Mg2+ at the M site rather than the T site. In fact, the substitution of Mg2+ for Fe2+ yields significant variations in M-O (from 2.045 to 2.034 A) and only limited variation in T-O (from 2.007 to 2.002 A). In conjunction with data from the literature, the present study provide a basis for quantitative analyses of the variation in Mg-T-O bond distance from 1.966 angstrom for Mg-poor tilvospinel to 1.990 A for the qandilite end-member.