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 Rm, with a = 10.2114(10), c = 25.689(3) Å, V = 2319.2(6) Å 3, to R1 = 0.021, wR2 = 0.049. The chemical formula of nanlingite is Na(Ca, Li, Na)6(Mg, Fe)12(AsO3)2[Fe(AsO3)6] F14 with Z = 3 according to the crystal structure refinement.

The complex framework is formed by edge- and corner-sharing of distorted NaF8 and CaF4O4 cubes, MgF2O4 octahedra, and AsO3 trigonal pyramids. The lithium atom occupies (22 %) the split site at 0.58(2) Å from the main Ca site and exhibits a five-fold coordination with <Li–(O,F) > 2.21 Å. The iron atom (at 3b, site symmetry 3̅m) is surrounded by six As atoms of AsO3 groups at a distance of 2.40 Å with the lone-pair electrons pointing towards the central atom. This [Fe(AsO3)6] cluster is a novel configuration in the crystal chemistry of minerals. According to Mössbauer spectra and electron energy-loss spectroscopy data the oxidation state of Fe in nanlingite is mainly FeII with a low-spin state. In the configuration of arsenite groups, the distance <As(2)–O>= 1.73 Å of the [Fe(AsO3)6] clusters is shorter than the normal range. The empirical crystal-chemical formula for nanlingite is (Na0.90Li0.10)∑1.00(Ca4.41 Li1.06 Mg0.17 Na0.06 Mn0.050.25)∑6.00(Mg11.38 Fe3+0.30Al0.29Ti0.03)∑12.00 (AsO3)2 [Fe II (AsO3)6](F13.77OH0.16)∑13.93.

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