Crystal chemistry of the ulvospinel-qandilite series

Abstract

Five spinel single-crystal samples within the ulvospinel-qandilite series [(Fe 2− x Mg x )TiO 4 , 0.15 x 2+ and Fe 2+ , while the octahedrally coordinated site (M) is populated by Ti 4+ , Mg 2+ , Fe 2+ , and minor amounts of Fe 3+ . The inverse cation distribution is characterized by parallel substitution of Mg 2+ for Fe 2+ 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 Mg 2+ at the M site rather than the T site. In fact, the substitution of Mg 2+ for Fe 2+ 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 T Mg-O bond distance from 1.966 A for Mg-poor ulvospinel to 1.990 A for the qandilite end-member.