title: Vibrational spectroscopic characterization of the sulphate-halide mineral sulphohalite - implications for evaporites.
abstract: The mineral sulphohalite - Na6(SO4)2FCl is a rare sodium halogen sulphate and occurs associated with evaporitic deposits. Sulphohalite formation is important in saline evaporites and in pipe scales. Sulphohalite is an anhydrous sulphate-halide with an apparent variable anion ratio of formula Na6(SO4)2FCl. Such a formula with oxyanions lends itself to vibrational spectroscopy. The Raman band at 1003cm(-1) is assigned to the (SO4)(2-) ν1 symmetric stretching mode. Shoulders to this band are found at 997 and 1010cm(-1). The low intensity Raman bands at 1128, 1120 and even 1132cm(-1) are attributed to the (SO4)(2-) ν3 antisymmetric stretching vibrations. Two symmetric sulphate stretching modes are observed indicating at least at the molecular level the non-equivalence of the sulphate ions in the sulphohalite structure. The Raman bands at 635 and 624cm(-1) are assigned to the ν4 SO4(2-) bending modes. The ν2 (SO4)(2-) bending modes are observed at 460 and 494cm(-1). The observation of multiple bands supports the concept of a reduction in symmetry of the sulphate anion from Td to C3v or even C2v. No evidence of bands attributable to the halide ions was found.
authors.affiliation: School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia. Electronic address: r.frost@qut.edu.au.
authors.forename: Ray L
authors.initials: RL
authors.lastname: Frost
authors.affiliation: Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35,400-000, Brazil.
authors.forename: Ricardo
authors.initials: R
authors.lastname: Scholz
authors.affiliation: School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia.
authors.forename: Andrés
authors.initials: A
authors.lastname: López
authors.affiliation: School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia.
authors.forename: Frederick L
authors.initials: FL
authors.lastname: Theiss
mesh_terms: D006219:Halogens / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013059:Spectrum Analysis, Raman; D013431:Sulfates / Q000737:chemistry*
chemical_list: D006219:Halogens; D008903:Minerals; D013431:Sulfates; C012036:sodium sulfate
keywords: Chloride; Evaporite; Fluoride; Raman spectroscopy; Sulphate; Sulphohalite