Molecular structure of the arsenate mineral chongite from Jáchymov – a vibrational spectroscopy study

Abstract

We have undertaken a study of the arsenate mineral chongite from the second world occurrence, which is Jáchymov (Czech Republic). Chongite occurs as colourless to white crystalline spherical and hemispherical aggregates up to 0.3 mm across composed of rich crusts on strongly weathered fragments of rocks and gangue. The chemical composition of chongite agrees well with the general stoichiometry of the hureaulite group of minerals and corresponds to the following empirical formula: Ca1.00(Mg1.24Ca0.69 0.06Mn0.01)Σ2.00Ca2.00[(AsO3OH)2.13(AsO4)1.84(PO4)0.03]Σ4.00·4H2O. Chongite is monoclinic, space group C2/c, with the unit-cell parameters refined from X-ray powder diffraction data: a 18.618(5), b 9.421(2), c 9.988(2) Å, β 96.86(2)o and V 1739.4(7) Å3. Raman bands at 3456, 3400, 3194 cm–1 and infrared bands at 3450, 3348, 3201 and 3071 cm–1 are assigned to the ν OH stretching structurally distinct differently hydrogen bonded water molecules. Raman bands at 2887, 2416 cm–1 and infrared bands at 2904, 2783 cm–1 are connected to ν OH stretching in hydrogen bonded (AsO3OH) units. Raman bands at 1656, 1578 cm–1 and infrared bands at 1652, 1601 cm–1 are assigned to the ν2 (δ) H2O bending vibrations of structurally distinct hydrogen bonded water molecules bonded in the structure by H-bonds of various strength. A Raman band at 1284 cm–1 and infrared bands at 1091 and 1061 cm–1 may be connected to the δ As–OH bending vibrations. The most prominent Raman bands at 902, 861, 828, 807, 758 cm–1 and infrared bands at 932, 899, 863, 815, 746 cm–1 are attributed to overlapping ν1 (AsO4) symmetric stretching, ν3 (AsO4) antisymmetric stretching, ν1 (AsO3OH) symmetric stretching, and ν3 (AsO3OH) antisymmetric stretching vibrations. Raman band at 693 cm–1 and infrared bands at 721, 634 cm–1 are assigned to δ AsOH bend and molecular water libration modes. Raman bands 506, 469, 451, 436 cm–1 and infrared bands at 503, 466 and 417 cm–1 are connected with the ν4 (δ) (AsO4) and (HOAsO3) bending vibrations. Raman bands at 389, 360, 346 and 302 cm–1 are related to the ν2 (δ) (AsO4) and (HOAsO3) bending vibrations. Raman bands at 275 and 238 cm–1 are assigned to the ν (OH⋅⋅⋅O) stretching vibrations and those at 190, 162, 110, 100 and 75 cm–1 to lattice modes.