| Summary: | The crystal structure of two different samples of pyrope garnet, ideally Mg<sub>3</sub>Al<sub>2</sub>Si<sub>3</sub>O<sub>12</sub>, from South Africa was refined using the Rietveld method, space group <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>I</mi><mi>a</mi><mover accent="true"><mn>3</mn><mo>¯</mo></mover><mi>d</mi></mrow></semantics></math></inline-formula>, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Sample 1 from Wesselton Mine is a single cubic phase and is optically isotropic. Electron-probe microanalysis (EPMA) provided an average composition {Mg<sub>2.30</sub>Fe<sup>2+</sup><sub>0.26</sub>Ca<sub>0.42</sub>Mn<sup>2+</sup><sub>0.02</sub>}<sub>∑3</sub>[Al<sub>1.53</sub>Fe<sup>3+</sup><sub>0.06</sub>Cr<sup>3+</sup><sub>0.40</sub>Ti<sup>4+</sup><sub>0.01</sub>Fe<sup>2+</sup><sub>0.01</sub>]<sub>∑2</sub>Si<sub>3</sub>O<sub>12</sub>, which contains a significant amount of Cr cations. The unit-cell parameter (Å) and bond distances (Å) are <i>a</i> = 11.56197(1) Å, average <Mg-O> = 2.2985, Al-O = 1.9101(4), and Si-O = 1.6343(3) Å. Sample 2 from De Beers Diamond Mine has an average composition {Mg<sub>2.33</sub>Fe<sup>2+</sup><sub>0.33</sub>Ca<sub>0.33</sub>Mn<sup>2+</sup><sub>0.01</sub>}<sub>∑3</sub>[Al<sub>1.73</sub>Fe<sup>3+</sup><sub>0.12</sub>Cr<sup>3+</sup><sub>0.06</sub>Ti<sup>4+</sup><sub>0.05</sub>Fe<sup>2+</sup><sub>0.05</sub>]<sub>∑2</sub>Si<sub>3</sub>O<sub>12</sub> and is a fine-scale intergrowth of two cubic phases. The weight percentage, unit-cell parameter (Å), and bond distances (Å) for phase 2a are 62.2(1)%, <i>a</i> = 11.56185(1) Å, average <Mg-O> = 2.3006, Al-O = 1.9080(4), Si-O = 1.6334(4) Å. The corresponding values for phase 2b are 37.8(1)%, <i>a</i> = 11.53896(1) Å, average <Mg-O> = 2.2954, Al-O = 1.9020(6), Si-O = 1.6334(6) Å. The two cubic phases in sample 2 cause the crystal to be optically anisotropic because of strain induce birefringence. The unit-cell parameter and bond distances for sample 1 are similar to those in phase 2a.
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