A re-appraisal of the chemical formula of nealite, Pb4Fe(AsO3)2Cl4 . 2H2O, on the basis of a crystal structure determination

Abstract

A crystal structure determination of the mineral nealite, Pb4Fe(AsO3)2Cl4 . 2H2O, (a = 6.548(4), b = 10.243(8), c = 5.587(3) [A];α = 96.2(1),β = 89.6(1),γ = 97.7(1) [°]; Z = 1; space group\(P\bar 1 - C_i^1\)) was performed using single crystal X-ray data and a direct method strategy; the structural parameters were fitted by least squares techniques. The structure is characterized by isolated FeO2Cl2(H2O)2 octahedra (Fe-O: 2.08 A, 2x, and 2.12 A, 2x, Fe-Cl: 2.60 A, 2x) and AsO3 pyramids (As-O from 1.79 to 1.89 A) which are connected by PbO4Cl4 and PbO5Cl4 polyhedra to a framework (with Pb-O distances from 2.22 to 3.34 A, Pb-Cl distances from 3.10 to 3.54 A). Oscillation photographs, exposed for 20 h, show very faint streaks indicating a doubling of c, probably caused by a partial ordering of a “split” lead atom position. Nealite is the first mineral found to contain a divalent iron atom coordinated to oxygen and chlorine atoms simultaneously.