Crystal structure of nanlingite the first mineral with a [Fe(AsO3)6] configuration

Abstract

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 4 O 4 cubes, MgF 2 O 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 with 2.21 A. The iron atom (at 3 b , site symmetry 3m) 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 II with a low-spin state. In the configuration of arsenite groups, the distance = 1.73 A of the [Fe(AsO 3 ) 6 ] clusters is shorter than the normal range. The empirical crystal-chemical formula for nanlingite is (Na 0.90 Li 0.10 )∑ 1.00 (Ca 4.41 Li 1.06 Mg 0.17 Na 0.06 Mn 0.05 □ 0.25 )∑ 6.00 (Mg 11.38 Fe 3+ 0.30 Al 0.29 Ti 0.03 )∑ 12.00 (AsO 3 ) 2 [Fe II (AsO 3 ) 6 ](F 13.77 OH 0.16 )∑ 13.93 .