This article is protected by German copyright law. You may copy and distribute this article for your personal use only. Other use is only allowed with written permission by the copyright holder. On the symmetry and atomic ordering in (OH,F)-rich spessartine: towards a new hydrogarnet end-member Massimo Boiocchi I , Fabio Bellatreccia II , Giancarlo Della Ventura II and Roberta Oberti * , III I Centro Grandi Strumenti, Universita ` di Pavia, via Bassi 21, Pavia, Italy II Dipartimento di Scienze Geologiche, Universita ` RomaTre, largo San Leonardo Murialdo 1, Roma, Italy III CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, via Ferrata 1, Pavia, Italy In memoriam Professor Friedrich Liebau Received November 28; 2011 accepted March 14, 2012 Published online: June 18, 2012 Spessartine / (OH,F) in garnets / FTIR / Cation and anion ordering / Single crystal structure analysis / X-ray diffraction Abstract. The symmetry and crystal chemistry of a (OH,F)-rich spessartine from Junzico Xijang, Wushan Spes- sartine Mine, Yunxiao Co., Fujian Province, P.R. China, with composition X (Mn 2þ 2.87 Fe 2þ 0.09 Ca 0.04 ) Y (Al 1.94 Fe 3þ 0.06 ) Z [(SiO 4 ) 2.52 (OH 1.11 F 0.81 )] have been studied by single-crys- tal X-ray structure refinement and FTIR spectroscopy. This sample is tetragonal, space group I4 1 /acd, with a ¼ b ¼ 11.6347(3) A ˚ , c ¼ 11.6449(3) A ˚ , c/a ¼ 1.006, R all 3.1% (q max ¼ 55  ). The two independent tetrahedra differ signifi- cantly both in terms of refined bond-lengths and site-scatter- ing values, the latter allowing to confirm the extent of the (OH,F) substitution and to its preferential ordering at O3, the site occupied by the anions coordinated to Z2. FTIR spectra consist of a prominent peak at 3580 cm 1 and a doublet at 3622–3632 cm 1 . Pleochroic curves suggest as- signment to a unique O––H dipole, involved in two different hydrogen bonds (O3––H  O2 and O3––H  O3, the latter showing two different, O––H  H and O––H  F, config- urations). The observed space group and composition im- ply: (a) that lowering in symmetry is related to the presence of (OH,F) 4 groups; (b) the existence of a new hydrogarnet end-member with ideal composition X Mn 3 Y Al 2 Z [(SiO 4 ) 2 (OH) 4 ] and significant F content, the first related to the pyralspite series; the sample of this work is close to its 50 : 50 solid solution with spessar- tine. Geometric considerations and the very limited in- crease in volume with respect to spessartine suggest that the presence of F is a requisite for the incorporation of significant amount of monovalent anions in pyralspites. 1. Introduction Significant OH substitution is frequent in grossular, andra- dite, uvarovite, i.e, garnets with Ca as X cation and Al, Fe 3þ and Cr 3þ , respectively as Y cations (ugrandites after Winchell, 1933), but occurs rarely and in limited extents in pyrope, almandine, spessartine, i.e, garnets with smal- ler (Mg, Fe, Mn) X cations and Al as Y cation (pyral- spites after Winchell, 1933). During a systematic spec- troscopic (FTIR) study of several silicate garnets, we identified a spessartine sample with particularly intense absorption bands in the OH stretching region. Electron microprobe (EMP) analysis showed the presence of sig- nificant F content, a feature already reported by Smyth et al. (1990) for a spessartine sample with cubic Ia  3d symmetry. In contrast, structure refinement (SRef) showed unambiguously a tetragonal symmetry, I4 1 /acd space group, allowing for the presence of two distinct Z sites, one of which is partly vacant. The same space group had been reported for the rare hydrogarnets henri- termierite, ideally X Ca 3 Y Mn 3þ 2 Z [(SiO 4 ) 2 (OH) 4 ] (Aubry et al., 1969; Armbruster et al., 2001), and holtstamite, ideally X Ca 3 Y (Al > Mn 3þ ) 2 Z [(SiO 4 ) 2 (OH) 4 ] (Ha ˚lenius et al., 2005). In those garnets, the lowering in symmetry has been related to the occurrence of Y Mn 3þ and to the consequent Jahn-Teller distortion of the octahedron. I4 1 / acd symmetry implies the recognition of a new end- member when the monovalent anions (ordered at the O3 site, bonded to the vacant Z2 site with multiplicity 0.5) exceeds 0.50 atoms per formula unit (apfu). The present study is thus the first report of I4 1 /acd symmetry and of SiO 4 vs. (OH) 4 ordering in pyralspites, and implies the existence of a further hydrogarnet end-member with composition X Mn 3 Y Al 2 Z [(SiO 4 ) 2 (OH) 4 ]. This paper is dedicated to the memory of prof. Frie- drich Liebau, in recognition of his outstanding contribu- tions to mineralogy and in particular to classification and nomenclature issues. Z. Kristallogr. 2012, 227, 385–395 / DOI 10.1524/zkri.2012.1487 385 # by Oldenbourg Wissenschaftsverlag, Mu ¨nchen * Correspondence author (e-mail: oberti@crystal.unipv.it) For Supplementary Material see online-version.