Vibrational spectroscopic study of the arsenate mineral strashimirite Cu8(AsO4)4(OH)4·5H2O—Relationship to other basic copper arsenates
Introduction
The mineral strashimirite Cu8(AsO4)4(OH)4·5H2O is a hydrated hydroxyl divalent copper arsenate [1], [2]. The mineral is of monoclinic symmetry with space group P21/m and may be compared with other hydroxyl copper arsenate minerals, including euchroite Cu2(AsO4)(OH)·3H2O, olivenite Cu2(AsO4)(OH), cornwallite Cu5(AsO4)2(OH)4. Olivenite is monoclinic, space group P21/n and is the most common secondary mineral of the oxidized zone of hydrothermal deposits. Other related copper and arsenate minerals are the minerals cornwallite [Cu5(AsO4)2(OH)4] and clinoclase [Cu3(AsO4)(OH)3]. Each of these minerals occurs in the oxidized zones of copper deposits and olivenite is by far the commonest [3], [4]. Cornwallite and clinoclase are rare secondary minerals that crystallise monoclinic, space group P21/a [3], [5], [6].
The relative stabilities of the basic copper arsenates have been determined using estimated chemical parameters and experimentally determined solubility products [7], [8]. Magalhaes et al. have reported the relative stability of copper arsenate minerals [9]. Normal anhydrous copper(II) arsenate is known as the naturally occurring species lammerite [Cu3(AsO4)2], but it is very rare [10]. The more basic stoichiometries occupy fields at higher pH as expected. Since olivenite is the stable phase under chemical conditions intermediate to those that serve to stabilize cornwallite and clinoclase, paragenetic relationships have been explored [10]. Olivenite is often found with either cornwallite or clinoclase, but not together. The stability of the basic copper arsenate minerals is related to their redox potential and phase fields exist for the related minerals olivenite, cornubite [Cu5(AsO4)2(OH)4], clinoclase and cornwallite. Thus, the structures of these minerals are related and should therefore provide similar spectra, which should differ in terms of the intensity and position of the bands according to the relative mole ratios of Cu/As/OH and the number and site symmetry of the formula units in the unit cell of the minerals.
Whilst the infrared spectra of some minerals have been forthcoming, few comprehensive studies of related minerals such as the basic copper arsenates have been undertaken [11], [12], [13]. The structural investigation of some arsenates and the nature of the hydrogen bond in these structures have been reported [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. It was found that the hydroxyl unit was coordinated directly to the metal ion and formed hydrogen bonds to the arsenate anion [25]. The basic copper arsenates include a significant number of diagenetically related minerals. Raman spectroscopy is important tool for our understanding of diagenetically related minerals such as the basic copper arsenate minerals. The aim of this paper is the study of the Raman spectra of strashimirite on the basis of its expected crystal structure, which is as yet not known. 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.
Section snippets
Minerals
The studied samples of the mineral strashimirite were found at the Zálesí deposit, the Rychlebské hory Mountains, northern Moravia, Czech Republic, and Svätodušná deposit near Lubietová, central Slovakia, Slovak Republic. These strashimirite minerals are labelled sample A and sample B. These samples are deposited in the mineralogical collections of the National Museum Prague. The samples were analysed for phase purity by X-ray powder diffraction and no minor significant impurities were found.
Raman spectroscopy
The Raman spectra of the tetrahedral anions in aqueous systems are well known [30]. The Raman active symmetric stretching ν1 vibration of the arsenate anion is observed at 810 cm−1 and coincides with the position of the triply degenerate Raman and infrared active antisymmetric stretching mode (ν3). The doubly degenerate Raman active symmetric bending mode (ν2) is observed at 342 cm−1 and the triply degenerate Raman and infrared active out-of-plane bending modes (ν4) is observed at 398 cm−1. Of all
Conclusions
Raman spectroscopy has been used to characterise the mineral strashimirite. Two samples from the Czech and Slovak Republics were used. The Raman spectra of the two minerals differed and this difference may probably be connected with anionic isomorphic substitution As ⇔ S for A and As ⇔ P, S for B. Characteristic Raman bands of the (AsO4)3− stretching and bending vibrations were identified and described. Raman bands attributable to the OH stretching vibrations of water and hydroxyl units were
Acknowledgments
The financial and infra-structure support of the Queensland University of Technology, Inorganic Materials Research Program is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. This work was also supported by Ministry of Culture of the Czech Republic (MK00002327201) to Jiří Sejkora.
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