Raman spectroscopic study of the hydroxy‐arsenate‐sulfate mineral chalcophyllite Cu₁₈Al₂(AsO₄)₄(SO₄)₃(OH)₂₄·36H₂O

Abstract

The mixed anion mineral chalcophyllite Cu 18 Al 2 (AsO 4 ) 4 (SO 4 ) 3 (OH) 24 ·36H 2 O has been studied by using Raman and infrared spectroscopies. Characteristic bands associated with arsenate, sulfate and hydroxyl units are identified. Broad bands in the OH stretching region are observed and are resolved into component bands. Estimates of hydrogen bond distances were made using a Libowitzky function. Both short and long hydrogen bonds were identified. Two intense bands at 841 and ∼814 cm −1 are assigned to the ν 1 (AsO 4 ) 3− symmetric stretching and ν 3 (AsO 4 ) 3− antisymmetric stretching modes. The comparatively sharp band at 980 cm −1 is assigned to the ν 1 (SO 4 ) 2− symmetric stretching mode, and a broad spectral profile centred upon 1100 cm −1 is attributed to the ν 3 (SO 4 ) 2− antisymmetric stretching mode. A comparison of the Raman spectra is made with other arsenate-bearing minerals such as carminite, clinotyrolite, kankite, tilasite and pharmacosiderite.

Raman spectroscopy of green minerals and reaction products with an application in Cultural Heritage research

“…Chalchophyllite (Cu 18 Al 2 (AsO 4 ) 4 (SO 4 ) 3 (OH) 24 · 36H 2 O) is another emerald green copper mineral, containing sulphate, arsenate and hydroxyl anions. The stretching bands related to these species are found at 979, 843 (broad and asymmetric) and 3400–3600 cm −1 respectively . The bending vibrations and lattice modes are in good agreement with published data; however, symmetry lowering produces multiple features in the region 620–110 cm −1 (Figure S1, spectrum c; Table ).…”

Raman and infrared spectroscopic characterization of the arsenate‐bearing mineral tangdanite– and in comparison with the discredited mineral clinotyrolite

“…One possible assignment is that these bands are due to the stretching vibrations of the OH units. Intense infrared bands for clinotyrolite [4] observed at 3083, 3280, 3403 and 3501 cm À1 were assigned to water-stretching vibrations. It should be noted that the Raman spectrum of tangdanite could not be obtained.…”

“…The sample fluoresced very badly, and it simply was not possible to obtain the spectral data in the OH stretching region in the Raman spectrum. The Raman spectra in the OH stretching region of clinotyrolite [4] showed bands at 3369 and 3476 cm À1 , with lower-intensity bands at 3043 and 3132 cm À1 . The authors postulated that all bands in the Raman spectra of clinotyrolite are associated with waterstretching vibrations.…”

“…Although the mineral clinotyrolite is now discredited, a vibrational spectroscopic study of this mineral species has been undertaken. [4] The authors have undertaken studies of mixed anion minerals containing arsenate and sulphate. [4][5][6] The aim of this paper is to present the Raman and infrared spectra of tangdanite and to relate the spectra to the structure of the mineral, the molecular structure.…”

“…[4] The authors have undertaken studies of mixed anion minerals containing arsenate and sulphate. [4][5][6] The aim of this paper is to present the Raman and infrared spectra of tangdanite and to relate the spectra to the structure of the mineral, the molecular structure. This research reported here is part of systematic studies on the vibrational spectra of minerals of secondary origin in the oxide supergene zone and their synthetic analogs.…”