| Summary: | 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 Mn<sub>3</sub>O<sub>4</sub> (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<sub>2</sub>O<sub>4</sub>. The EPMA composition for sample-a is [Mn<sup>2+</sup><sub>0.88</sub>Mg<sup>2+</sup><sub>0.11</sub>Fe<sup>2+</sup><sub>0.01</sub>]<sub>Σ</sub><sub>1.00</sub>Mn<sup>3+</sup><sub>2.00</sub>O<sub>4</sub> compared to Mn<sup>2+</sup>Mn<sup>3+</sup><sub>2</sub>O<sub>4</sub> obtained by refinement. The single-crystal structure refinement in the tetragonal space group <i>I</i>4<sub>1</sub>/<i>amd</i> gave R1 = 0.0215 for 669 independently observed reflections. The unit-cell parameters are <i>a</i> = <i>b</i> = 5.7556(6), <i>c</i> = 9.443(1) Å, and <i>V</i> = 312.80(7) Å<sup>3</sup>. The Jahn−Teller elongated Mn<sup>3+</sup>O<sub>6</sub> octahedron of the <i>M</i> site consists of <i>M</i>−O × 4 = 1.9272(5), <i>M</i>−O × 2 = 2.2843(7), and an average <<i>M</i>−O>[6] = 2.0462(2) Å, whereas the Mn<sup>2+</sup>O<sub>4</sub> tetrahedron of the <i>T</i> site has <i>T</i>−O × 4 = 2.0367(8) Å. The site occupancy factors (<i>sof</i>) are <i>M</i>(<i>sof</i>) = 1.0 Mn (fixed, thereafter) and <i>T</i>(<i>sof</i>) = 1.0008(2) Mn. The EPMA composition for sample-b is [Mn<sub>0.99</sub>Mg<sub>0.01</sub>](Mn<sub>1.52</sub>Fe<sub>0.48</sub>)O<sub>4</sub>. The Rietveld refinement gave <i>R (F</i><sup>2</sup>) = 0.0368. The unit-cell parameters are <i>a</i> = <i>b</i> = 5.78144(1), <i>c</i> = 9.38346(3) Å, and <i>V</i> = 313.642(1) Å<sup>3</sup>. The octahedron has <i>M</i>−O × 4 = 1.9364(3), <i>M</i>−O × 2 = 2.2595(6), and average <<i>M</i>−O>[6] = 2.0441(2) Å, whereas <i>T</i>−O × 4 = 2.0438(5) Å. The refinement gave <i>T</i>(<i>sof</i>) = 0.820(9) Mn<sup>2+</sup> + 0.180(9) Fe<sup>2+</sup> and <i>M</i>(<i>sof</i>) = 0.940(5) Mn<sup>3+</sup> + 0.060(5) Fe<sup>3+</sup>. Samples-a and -b are normal spinels with different amounts of substitutions at the <i>M</i> and <i>T</i> sites. The Jahn−Teller elongation, Δ(<i>M</i>−O), is smaller in sample-b because atom substitutions relieve strain compared to pure Mn<sub>3</sub>O<sub>4</sub>.
|
|---|
