A REAPPRAISAL OF THE CHEMICAL FORMULA OF NEALITE, PB4FE(ASO3)2CL4.2H2O, ON THE BASIS OF A CRYSTAL-STRUCTURE DETERMINATION

A crystal structure determination of the mineral nealite, Pb4Fe(AsO3)2 Cl4.2H2O, (a = 6.548(4), b = 10.243(8), c = 5.587(3) [angstrom]; alpha = 96.2(1), beta = 89.6(1), gamma = 97.7(1) [degrees]; Z = 1; space gr oup P1BAR-C(i)1) was performed using single crystal X-ray data and a d irect 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 angstrom, 2x, and 2.12 angstrom, 2x, Fe-C l: 2.60 angstrom, 2x) and AsO3 pyramids (As-O from 1.79 to 1.89 angstr om) which are connected by PbO4Cl4 and PbO5Cl4 polyhedra to a framewor k (with Pb-O distances from 2.22 to 3.34 angstrom, Pb-Cl distances fro m 3.10 to 3.54 angstrom). Oscillation photographs, exposed for 20 h, s how very faint streaks indicating a doubling of c, probably caused by a partial ordering of a ''split'' lead atom position. Nealite is the f irst mineral found to contain a divalent iron atom coordinated to oxyg en and chlorine atoms simultaneously.