Crystal structure of nanlingite-the first mineral with a [Fe(AsO_3)_6] configuration

【摘要】：正Nanlingite was first described as a new mineral from a dolomitic limestone along a contact between greisenized granite and dolomitic limestone in the Nanling area,Hunan Province,China.The structure was solved and refined on single-crystal X-ray data in space group R 3 m,with a = 10.2114(10),c = 25.689(3) A,V = 2319.2(6) A~3,to R1 = 0.021,wR2 = 0.049.The chemical formula of nanlingite is Na(Ca,Li,Na)_6(Mg,Fe)_(12)(AsO_3)_2[Fe(AsO_3)_6]F_(14) with Z=3 according to the crystal structure refinement. The complex framework is formed by edge- and corner-sharing of distorted NaF_8 and CaF_4O_4 cubes,MgF_2O_4 octahedra,and AsO_3 trigonal pyramids.The lithium atom occupies(22%) the split site at 0.58(2) A from the main Ca site and exhibits a five-fold coordination withLi-(O,F)2.21 A.The iron atom(at 3b,site symmetry 3 m) is surrounded by six As atoms of AsO_3 groups at a distance of 2.40 A with the lone-pair electrons pointing towards the central atom.This[Fe(AsO_3)_6]cluster is a novel configuration in the crystal chemistry of minerals.According to Mossbauer spectra and electron energy-loss spectroscopy data the oxidation state of Fe in nanlingite is mainly Fe with a low-spin state.In the configuration of arsenite groups,the distance

【作者单位】：Institut fr Mineralogie und Kristallographie Universitt Wien,Althanstr.14,1090 Wien,Austria
【基金】：supported by the National Natural Science Foundation of China(No.40872044)
【分类号】：P573