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

• Main Authors: Sytle M. Antao, Laura A. Cruickshank, Kaveer S. Hazrah
• Format: Article
• Language: English
• Published: MDPI AG 2019-06-01
• Series: Minerals
• Subjects: hausmannite; chemical analysis; crystal structure; structural variations
• Online Access: https://www.mdpi.com/2075-163X/9/6/343

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]_&#931;_1.00Mn³⁺_2.00O₄ compared to Mn²⁺Mn³⁺₂O₄ obtained by refinement. The single-crystal structure refinement in the tetragonal space group <i>I</i>4₁/<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) &#197;, and <i>V</i> = 312.80(7) &#197;³. The Jahn&#8722;Teller elongated Mn³⁺O₆ octahedron of the <i>M</i> site consists of <i>M</i>&#8722;O &#215; 4 = 1.9272(5), <i>M</i>&#8722;O &#215; 2 = 2.2843(7), and an average &lt;<i>M</i>&#8722;O&gt;[6] = 2.0462(2) &#197;, whereas the Mn²⁺O₄ tetrahedron of the <i>T</i> site has <i>T</i>&#8722;O &#215; 4 = 2.0367(8) &#197;. 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_0.99Mg_0.01](Mn_1.52Fe_0.48)O₄. The Rietveld refinement gave <i>R (F</i>²) = 0.0368. The unit-cell parameters are <i>a</i> = <i>b</i> = 5.78144(1), <i>c</i> = 9.38346(3) &#197;, and <i>V</i> = 313.642(1) &#197;³. The octahedron has <i>M</i>&#8722;O &#215; 4 = 1.9364(3), <i>M</i>&#8722;O &#215; 2 = 2.2595(6), and average &lt;<i>M</i>&#8722;O&gt;[6] = 2.0441(2) &#197;, whereas <i>T</i>&#8722;O &#215; 4 = 2.0438(5) &#197;. The refinement gave <i>T</i>(<i>sof</i>) = 0.820(9) Mn²⁺ + 0.180(9) Fe²⁺ and <i>M</i>(<i>sof</i>) = 0.940(5) Mn³⁺ + 0.060(5) Fe³⁺. Samples-a and -b are normal spinels with different amounts of substitutions at the <i>M</i> and <i>T</i> sites. The Jahn&#8722;Teller elongation, &#916;(<i>M</i>&#8722;O), is smaller in sample-b because atom substitutions relieve strain compared to pure Mn₃O₄.