Crystal structure refinements of tetragonal (OH,F)-rich spessartine and henritermierite garnets

Abstract

Cubic garnet (space group Ia\overline 3 d) has the general formula X 3 Y 2 Z 3O12, where X, Y and Z are cation sites. In the tetragonal garnet (space group I41/acd), the corresponding cation sites are X1 and X2, Y, and Z1 and Z2. In both space groups only the Y site is the same. The crystal chemistry of a tetragonal (OH,F)-rich spessartine sample from Tongbei, near Yunxiao, Fujian Province, China, with composition X (Mn2.82Fe^{2+}_{0.14}Ca0.04)Σ3 Y {Al1.95Fe^{3+}_{0.05}}Σ2 Z [(SiO4)2.61(O4H4)0.28(F4)0.11]Σ3 (Sps94Alm5Grs1) was studied with single-crystal X-ray diffraction and space group I41/acd. The deviation of the unit-cell parameters from cubic symmetry is small [a = 11.64463 (1), c = 11.65481 (2) Å, c/a = 1.0009]. Point analyses and back-scattered electron images, obtained by electron-probe microanalysis, indicate a homogeneous composition. The Z2 site is fully occupied, but the Z1 site contains vacancies. The occupied Z1 and Z2 sites with Si atoms are surrounded by four O atoms, as in anhydrous cubic garnets. Pairs of split sites are O1 with F11 and O2 with O22. When the Z1 site is vacant, a larger [(O2H2)F2] tetrahedron is formed by two OH and two F anions in the O22 and F11 sites, respectively. This [(O2H2)F2] tetrahedron is similar to the O4H4 tetrahedron in hydrogarnets. These results indicate ^{X}{{\rm Mn}^ {2+}_{3}}\,^{Y}{\rm Al}_{2}^{Z}[({\rm SiO}_{4})_{2}({\rm O}_{2}{\rm H}_{2})_{0.5}({\rm F}_{2})_{0.5}]_{\Sigma3} as a possible end member, which is yet unknown. The H atom that is bonded to the O22 site is not located because of the small number of OH groups. In contrast, tetragonal henritermierite, ideally ^{X}{\rm Ca}_{3}\,^{Y}{\rm Mn}^{3+}_{2}\,^{Z}[({\rm SiO}_{4})_{2}({\rm O}_{4}{\rm H}_{4})_1]_{\Sigma3}, has a vacant Z2 site that contains the O4H4 tetrahedron. The H atom is bonded to an O3 atom [O3—H3 = 0.73 (2) Å]. Because of O2—Mn3+—O2 Jahn–Teller elongation of the Mn3+O6 octahedron, a weak hydrogen bond is formed to the under-bonded O2 atom. This causes a large deviation from cubic symmetry (c/a = 0.9534).

Crystal Chemistry of Birefringent Uvarovite Solid Solutions

“…Each sample contains multiple cubic garnet phases, but one crystal fragment is an isotropic cubic phase. This work is part of a large study on garnet-group minerals [35][36][37][38][39]. Briefly, this study indicates that intergrowth of multiple cubic phases in a crystal of uvarovite causes strain-induced optical anisotropy.…”

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

“…Backscattered electron (BSE) images of sample-a revealed the presence of hausmannite within henritermierite garnet (see Figure 8 in [12] (Table 5) Table 7). The difference in Jahn-Teller elongation of 0.034(1) Å between samples-a and -b is one of the most significant structural difference between these two samples.…”

“…The hausmannite sample-a used in this study is a black crystal that occurs with henritermierite garnet from Wessels X Mine, Kalahari manganese field (KMF), Northern Cape Province, South Africa. The hausmannite crystals are intergrown with tetragonal garnet, henritermierite, ideally [12]. Sample-a was obtained from the Royal Ontario Museum (ROM #M54234).…”

“…The oxygen atoms are indicated by red spheres. The hausmannite structure is similar to the henritermierite garnet structure that also consists of Jahn-Teller elongated Mn 3+ O 6 octahedra and SiO 4 , instead of Mn 2+ O 4 , tetrahedra (seeFigure 6in[12]).…”

WinGrt, a Windows program for garnet supergroup minerals

“…3) with representative garnet supergroup mineral analyses selected from literature (e.g. Munno et al 1980;Galuskina et al 2010;Nagashima and Armbruster 2012;Galuskina et al 2013;Grew et al 2013;Antao 2014;Ma et al 2014;Antao and Cruickshank 2018). WinGrt calculates cations (apfu) of garnet supergroup mineral analyses (see rows 38-68 in Fig.…”