Abstract
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 AB2O4. The EPMA composition for sample-a is [Mn2+0.88Mg2+0.11Fe2+0.01]Σ1.00Mn3+2.00O4 compared to Mn2+Mn3+2O4 obtained by refinement. The single-crystal structure refinement in the tetragonal space group I41/amd gave R1 = 0.0215 for 669 independently observed reflections. The unit-cell parameters are a = b = 5.7556(6), c = 9.443(1) Å, and V = 312.80(7) Å3. The Jahn–Teller elongated Mn3+O6 octahedron of the M site consists of M–O × 4 = 1.9272(5), M–O × 2 = 2.2843(7), and an average <M–O>[6] = 2.0462(2) Å, whereas the Mn2+O4 tetrahedron of the T site has T–O × 4 = 2.0367(8) Å. 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)O4. The Rietveld refinement gave R (F2) = 0.0368. The unit-cell parameters are a = b = 5.78144(1), c = 9.38346(3) Å, and V = 313.642(1) Å3. The octahedron has M–O × 4 = 1.9364(3), M–O × 2 = 2.2595(6), and average <M–O>[6] = 2.0441(2) Å, whereas T–O × 4 = 2.0438(5) Å. 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, Δ(M–O), is smaller in sample-b because atom substitutions relieve strain compared to pure Mn3O4.
Highlights
Numerous studies were carried out on spinels because of their chemical and structural simplicity, their geological importance, and their use as geothermometers, geobarometers, and geospeedometers (e.g., [1,2])
This study examines the crystal chemistry of two different hausmannite samples from the Kalahari manganese field (KMF)
Synchrotron High-Resolution Powder X-Ray Diffraction (HRPXRD). Both samples-b and -c were studied with high-resolution powder X-ray diffraction (HRPXRD) that was performed at beamline 11-BM, Advanced Photon Source (APS), Argonne National Laboratory (ANL)
Summary
Numerous studies were carried out on spinels because of their chemical and structural simplicity, their geological importance, and their use as geothermometers, geobarometers, and geospeedometers (e.g., [1,2]). It seems that substitution of Fe3+ cations at the M site give rise to a magnetic sample. The chemical bonds in spinels influence the cation ordering, the structural distortion, and the magnetic-exchange interactions [23]. In the tetragonal hausmannite structure, the O atoms form a tetragonally distorted cubic close-packed structure where the tetrahedral and octahedral sites are occupied by Mn2+ and Mn3+ cations, respectively [24]. Hausmannite occurs in a few localities such as Ilmanau, Thuringia, Germany and the Kalahari manganese field (KMF), South Africa [14] It is typically ferrimagnetic, but a strong magnetic sample was discovered in the hydrothermally altered KMF [13]. These results are compared with data from the literature (Table 1)
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