On the symmetry and atomic ordering in (OH,F)-rich spessartine: towards a new hydrogarnet end-member

Abstract

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 (Mn 2+ 2.87 Fe 2+ 0.09 Ca 0.04 ) Y (Al 1.94 Fe 3+ 0.06 ) Z [(SiO 4 ) 2.52 (OH 1.11 F 0.81 )] have been studied by single-crystal X-ray structure refinement and FTIR spectroscopy. This sample is tetragonal, space group I4 1 /acd, with a = b = 11.6347(3) A, c = 11.6449(3) A, c/a = 1.006, R all 3.1% (θ max = 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 03, 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 assignment to a unique O―H dipole, involved in two different hydrogen bonds (O3―H···O2 and O3―H···03, the latter showing two different, O―H···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 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 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.