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 1003 cm-1 is assigned to the (SO4)2- ν1 symmetric stretching mode. Shoulders to this band are found at 997 and 1010 cm-1. The low intensity Raman bands at 1128, 1120 and even 1132 cm-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 624 cm-1 are assigned to the ν4 SO42- bending modes. The ν2 (SO 4)2- bending modes are observed at 460 and 494 cm -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. © 2014 Elsevier B.V. All rights reserved.