Five spinel single-crystal samples within the ulvöspinel-qandilite series [(Fe2−xMgx)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 Å 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 Å) and only limited variation in T-O (from 2.007 to 2.002 Å). In conjunction with data from the literature, the present study provide a basis for quantitative analyses of the variation in TMg-O bond distance from 1.966 Å for Mg-poor ulvöspinel to 1.990 Å for the qandilite end-member.