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: Ca 1.00 (Mg 1.24 Ca 0.69 0.06 Mn 0.01) Σ2.00 Ca 2.00 [(AsO 3 OH) 2.13 (AsO 4) 1.84 (PO 4) 0.03 ] Σ4.00 •4H 2 O. 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) Å 3. 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 (AsO 3 OH) 2units. Raman bands at 1656, 1578 cm-1 and infrared bands at 1652, 1601 cm-1 are assigned to the ν 2 (δ) H 2 O 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 ν 1 (AsO 4) 3symmetric stretching, ν 3 (AsO 4) 3antisymmetric stretching, ν 1 (AsO 3 OH) 2symmetric stretching, and ν 3 (AsO 3 OH) 2antisymmetric 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 ν 4 (δ) (AsO 4) 3and (HOAsO 3) 2bending vibrations. Raman bands at 389, 360, 346 and 302 cm-1 are related to the ν 2 (δ) (AsO 4) 3and (HOAsO 3) 2bending 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.

Krupičkaite, Cu6[AsO3(OH)]6·8H2O, a new copper arsenate mineral from Jáchymov (Czech Republic)

“…Raman bands at 372 and 330 cm -1 are related to the ν 2 (δ) (HOAsO 3 ) 2bending vibrations. Raman band at 209 cm -1 is assigned to the ν (OH⋅⋅⋅O) stretching vibrations and those at 144 and 96 cm -1 to lattice modes (Vansant et al 1973;Čejka et al 2011;Sejkora et al 2010bSejkora et al , 2020.…”

Dobšináite, Ca2Ca(AsO4)2·2H2O, a new member of the roselite group from Dobšiná (Slovak Republic)

“…Raman bands 474, 447 and 416 cm -1 are attributed with the ν 4 (AsO 4 ) 3bending vibrations. Raman bands at 366, 333 and 295 cm -1 are related to the ν 2 (AsO 4 ) 3bending vibrations and those below 200 cm -1 to the lattice modes (Vansant et al 1973;Yang et al 2011;Sejkora et al 2020). In the area of ν 1 and ν 3 (AsO 4 ) 3vibrations (1000-700 cm -1 ), the Raman spectrum of dobšináite is close to other As-dominant members of the roselite group and distinctly differs from those of As-dominant members of the fairfieldite group (Fig.…”