The crystal structure of chalcophanite, ZnMn3O7.3H2O

Abstract

Chalcophanite, ZnM_n30~.3I-I~O, crystallizes in the triclinie system with a = 7.54, b = 7"54, c-8"22 A, ~-90 °, fl-~ 117"2 °, ~ ~-120 °, space group P1. The structure determination, based upon Fourier projections, was aided by first considering that of a prominent sub-cell with mono-clinic symmetry, space group G2/m, with am = 4.95, bm= 2"85, Cm-7"17 A, tim = 103 ° 30', related to the real cell by the index transformation monoclinic-> triclinie = @, ~, 0/~, @, 0/~, ~, 1. The structure consists of single sheets of water molecules grouped in open double hexagon rings alternating with double oxygen layers in which six of every seven octahedral sites are occupied by manganese. Zinc atoms linked to the triads of oxygens bounding the vacancies in the manganese layer complete their octahedral coordination with triads of water molecules. A scheme of additional bonding between sheets, based upon hydrogen bridges between water and oxygen, is described. The electron-density projections afford direct evidence of fractional amounts of interstitial zinc ions, leading to better agreement between the X-ray evidence and the chemical and density data.