Raman spectroscopy of hydrogen‐arsenate group (AsO₃OH) in solid‐state compounds: copper mineral phase geminite Cu(AsO₃OH)·H₂O from different geological environments

Abstract

The participation of hydrogen-arsenate group (AsO 3 OH) 2− in solid-state compounds may serve as a model example for explaining and clarifying the behaviour of As and other elements during weathering processes in natural environment. The mineral geminite, a hydrated hydrogen-arsenate mineral of ideal formula Cu(AsO 3 OH)·H 2 O, has been studied by Raman and infrared spectroscopies. Two samples of geminite of different origin were investigated and the spectra proved quite similar. In the Raman spectra of geminite, six bands are observed at 741, 812, 836, 851, 859 and 885 cm −1 (Salsigne, France), and 743, 813, 843, 853, 871 and 885 cm −1 (Jáchymov, Czech Republic). The band at 851/853 cm −1 is assigned to the ν 1 (AsO 3 OH) 2− symmetric stretching mode; the other bands are assigned to the ν 3 (AsO 3 OH) 2− split triply degenerate antisymmetric stretching mode. Raman bands at 309, 333, 345 and 364/310, 333 and 345 cm −1 are attributed to the ν 2 (AsO 3 OH) 2− bending mode, and a set of higher wavenumber bands (in the range 400-500 cm −1 ) is assigned to the ν 4 (AsO 3 OH) 2− split triply degenerate bending mode. A very complex set of overlapping bands is observed in both the Raman and infrared spectra. Raman bands are observed at 2289, 2433, 2737, 2855, 3235, 3377, 3449 and 3521/2288, 2438, 2814, 3152, 3314, 3448 and 3521 cm −1 . Two Raman bands at 2289 and 2433/2288 and 2438 cm −1 are ascribed to the strong hydrogen bonded water molecules. The Raman bands at 3235, 3305 and 3377/3152 and 3314 cm −1 may be assigned to the ν OH stretching vibrations of water molecules. Two bands at 3449 and 3521/3448 and 3521 cm −1 are assigned to the OH stretching vibrations of the (AsO 3 OH) 2− units. The lengths of the O-H· · ·O hydrogen bonds vary in the range 2.60-2.94 Å (Raman) and 2.61-3.07 Å (infrared). Two Raman and infrared bands in the region of the bending vibrations of the water molecules prove that structurally non-equivalent water molecules are present in the crystal structure of geminite.

Vibrational spectra of the rare‐occurring complex hydrogen arsenate minerals pharmacolite, picropharmacolite, and vladimirite: Dominance of Raman over IR spectroscopy to discriminate arsenate and hydrogen arsenate units

“…Here, we found that the Raman spectra of the studied minerals below 1000 cm −1 perfectly illustrate the number of the present crystallographically different AsO 4 and AsO 3 OH units in their structures and it might serve as an indicator to deduce the number of these units for each mineral (Figure ). In accordance with the tentative assignments found in great number of published articles for Raman spectral behavior of hydrogen arsenate minerals, the highest wavenumber bands are attributed to antisymmetric ν 3 (AsO 3 OH) modes followed by one or two stronger bands from symmetric ν 1 (AsO 3 OH) modes at somewhat lower wavenumbers. These bands in the pharmacolite mineral, which consist of structurally equivalent (AsO 3 OH) 2− units, were registered at 891, 862, and 841 cm −1 (Figure a) bearing close resemblance with their position in literature data (Table ) .…”

“…The IR spectrum of vladimirite has not been previously presented nor interpreted. However, the band assignment can be inferred from the pharmacolite discussion and following the previously published literature results from structurally similar hydrogen arsenate minerals . The strongest maximum at 3542 cm −1 originates from OH stretchings of the non‐hydrogen‐bonded (or only weakly hydrogen bonded) OH oscillators of the water molecule.…”

“…The structure consists of layers joined by hydrogen bonds formed by both water molecules and OH groups within the layers. The reported vibrational spectra of pharmacolite and some arsenate minerals and compounds containing the AsO 3 OH groups will facilitate the interpretation of the spectra of the studied picropharmacolite and vladimirite. However, neither infrared (IR) liquid nitrogen temperature (LNT) spectra of the minerals have been reported (providing important information for hydrogen bonding) nor theoretically simulated spectra have been derived.…”

A Raman spectroscopic study of the ‘cave’ mineral ardealite Ca2(HPO4)(SO4)·4H2O

“…Raman spectroscopy has proved very useful for the study of minerals 12–25. Indeed Raman spectroscopy has proved most useful for the study of diagentically related minerals as often occurs with minerals containing sulfate and phosphate groups.…”

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

“…Raman spectra of some hydrogen-arsenate ions-containing minerals were studied, e.g. burgessite (Čejka et al 2011), pharmacolite (Frost et al 2010), geminite (Sejkora et al 2010) and giftgrubeite (Meisser et al 2019). Raman and infrared spectra of štěpite have been interpreted in detail by Plášil et al (2013).…”