""The symmetry and crystal chemistry of a (OH,F)-rich spessartine. from Junzico Xijang, Wushan Spessartine Mine,. Yunxiao Co., Fujian Province, P.R. China, with composition. X(Mn2+. 2.87Fe2+. 0.09Ca0.04) Y(Al1.94Fe3+. 0.06) Z[(SiO4)2.52. (OH1.11F0.81)] have been studied by single-crystal X-ray. structure refinement and FTIR spectroscopy. This sample is. tetragonal, space group I41\\\/acd, with a = b = 11.6347(3) Å,. c = 11.6449(3) Å, c\\\/a = 1.006, Rall 3.1% (θmax = 55°). The. two independent tetrahedra differ significantly both in. terms of refined bond-lengths and site-scattering 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 3568 cm-1 and a doublet at 3622-3632. cm-1. Pleochroic curves suggest assignment to a unique OH. dipole, involved in two different hydrogen bonds (O3-. H...O2 and O3-H...O3, the latter showing two different, OH.... H and O-H...F, configurations). The observed space. group and composition imply: (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 XMn3. YAl2. Z[(SiO4)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 spessartine.. Geometric considerations and the very limited increase. 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.""
Boiocchi, M., Bellatreccia, F., DELLA VENTURA, G., Oberti, R. (2012). On the symmetry and atomic ordering in (OH,F)-rich spessartine: towards a new hydrogarnet end-member. ZEITSCHRIFT FUR KRISTALLOGRAPHIE, 227, 385-395 [10.1524/zkri.2012.1487].