Structural Trends and Solid-Solutions Based on the Crystal Chemistry of Two Hausmannite (Mn3O4) Samples from the Kalahari Manganese Field

The crystal chemistry of two hausmannite samples from the Kalahari manganese field (KMF), South Africa, was studied using electron-probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD) for sample-a, and high-resolution powder X-ray diffraction (HRPXRD) for sample-b, and a synthetic Mn3O4 (97% purity) sample-c as a reference point. Hausmannite samples from the KMF were reported to be either magnetic or non-magnetic with a general formula AB(2)O(4). The EPMA composition for sample-a is [Mn0.882+Mg0.112+Fe0.012+](Sigma 1.00)Mn2.003+O4 compared to Mn2+Mn23+O4 obtained by refinement. The single-crystal structure refinement in the tetragonal space group I4(1)/amd gave R1 = 0.0215 for 669 independently observed reflections. The unit-cell parameters are a = b = 5.7556(6), c = 9.443(1) angstrom, and V = 312.80(7) angstrom(3). The Jahn-Teller elongated Mn3+O6 octahedron of the M site consists of M-O x 4 = 1.9272(5), M-O x 2 = 2.2843(7), and an average [6] = 2.0462(2) angstrom, whereas the Mn2+O4 tetrahedron of the T site has T-O x 4 = 2.0367(8) angstrom. The site occupancy factors (sof) are M(sof) = 1.0 Mn (fixed, thereafter) and T(sof) = 1.0008(2) Mn. The EPMA composition for sample-b is [Mn0.99Mg0.01](Mn1.52Fe0.48)O-4. The Rietveld refinement gave R (F-2) = 0.0368. The unit-cell parameters are a = b = 5.78144(1), c = 9.38346(3) angstrom, and V = 313.642(1) angstrom(3). The octahedron has M-O x 4 = 1.9364(3), M-O x 2 = 2.2595(6), and average [6] = 2.0441(2) angstrom, whereas T-O x 4 = 2.0438(5) angstrom. The refinement gave T(sof) = 0.820(9) Mn2+ + 0.180(9) Fe2+ and M(sof) = 0.940(5) Mn3+ + 0.060(5) Fe3+. Samples-a and -b are normal spinels with different amounts of substitutions at the M and T sites. The Jahn-Teller elongation, Delta(M-O), is smaller in sample-b because atom substitutions relieve strain compared to pure Mn3O4.