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

Cubic garnet (space group Ia\overline 3 d) has the general formula X 3 Y 2 Z 3 O 12 , where X , Y and Z are cation sites. In the tetragonal garnet (space group I 4 1 / acd ), the corresponding cation sites are X 1 and X 2, Y , and Z 1 and Z 2. 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 (Mn 2.82 Fe^{2+}_{0.14}Ca 0.04 ) Σ3 Y {Al 1.95 Fe^{3+}_{0.05}} Σ2 Z [(SiO 4 ) 2.61 (O 4 H 4 ) 0.28 (F 4 ) 0.11 ] Σ3 (Sps 94 Alm 5 Grs 1 ) was studied with single-crystal X-ray diffraction and space group I 4 1 / 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 Z 2 site is fully occupied, but the Z 1 site contains vacancies. The occupied Z 1 and Z 2 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 Z 1 site is vacant, a larger [(O 2 H 2 )F 2 ] tetrahedron is formed by two OH and two F anions in the O22 and F11 sites, respectively. This [(O 2 H 2 )F 2 ] tetrahedron is similar to the O 4 H 4 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 Z 2 site that contains the O 4 H 4 tetrahedron. The H atom is bonded to an O3 atom [O3—H3 = 0.73 (2) Å]. Because of O2—Mn 3+ —O2 Jahn–Teller elongation of the Mn 3+ O 6 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).

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