Vibrational spectra of synthetic minerals of the alunite and crandallite type

Raman Spectroscopy Coupled with Reflectance Spectroscopy as a Tool for the Characterization of Key Hydrothermal Alteration Minerals in Epithermal Au-Ag Systems: Utility and Implications for Mineral Exploration

Raman spectroscopy of fine-grained hydrothermal alteration minerals, and phyllosilicates in particular, presents certain challenges. However, given the increasingly widespread recognition of field portable visible-near infrared-shortwave infrared (Vis-NIR-SWIR) spectroscopy as a valuable tool in the mineral exploration industry, Raman microspectroscopy has promise as an approach for developing detailed complementary information on hydrothermal alteration phases in ore-forming systems. Here we present exemplar high-quality Raman and Vis-NIR-SWIR spectra of four key hydrothermal alteration minerals (pyrophyllite, white mica, chlorite, and alunite) that are common in precious metal epithermal systems, from deposits on the island of Newfoundland, Canada. The results reported here demonstrate that Raman microspectroscopy can accurately characterize pyrophyllite, white mica, chlorite, and alunite and provide details on their compositional variation at the microscale. In particular, spectral differences in the 1000-1150 cm-1 white mica Raman band allows the distinction between low-Tschermak phases (muscovite, paragonite) and phases with higher degrees of Tschermak substitution (phengitic white mica composition). The peak position of the main chlorite Raman band shifts between 683 cm-1 for Mg-rich chlorite and 665 cm-1 for Fe-rich chlorite and can be therefore used for semiquantitative estimation of the Fe2+ content in chlorite. Furthermore, while Vis-NIR-SWIR macrospectroscopy allows the rapid identification of the overall composition of the most abundant hydrothermal alteration mineral in a given sample, Raman microspectroscopy provides an in-depth spectral and chemical characterization of individual mineral grains, preserving the spatial and paragenetic context of each mineral and allowing for the distinction of chemical variation between (and within) different mineral grains. This is particularly useful in the case of alunite, white mica, and chlorite, minerals with extensive solid solution, where microscale characterization can provide information on the alteration zonation useful for mineral exploration and provide insight into mineral deposit genesis.

Arsenic immobilization from aqueous solution by the precipitation of the pseudo-octahedral arsenate-substituted natroalunite solid solutions

A series of the arsenate-substituted natroalunite solid solutions were synthesized by hydrothermal precipitation at 200 °C and pH of 4 and characterized to investigate the AsO₄ substitution for SO₄ in natroalunite as the base of a possible immobilization method for arsenic. The AsO₄ substitution in natroalunite increased when the [AsO₄/(AsO₄ + SO₄)]_aq molar ratios of the initial aqueous solutions increased and the maximum substitution reached ~67% molar for the [AsO₄/(AsO₄ + SO₄)]_aq = 0.26. The XRD analysis confirmed that all hydrothermal synthetical solids for the [AsO₄/(AsO₄ + SO₄)]_aq ≤ 0.26 were characteristic of natroalunite-type phases. The AsO₄ substitution in the natroalunites increased the c lattice parameters, owing to the difference between the SO distance and the AsO distance in the crystal structures. For the [AsO₄/(AsO₄ + SO₄)]_aq = 0.28 at 200 °C and pH of 4, a mixture of natroalunite, amorphous arsenate phase and Na₂SO₄ was formed. The crystals of the arsenate-substituted natroalunites were regular ditrigonal scalenohedron (pseudo-octahedron). The Raman spectra were characterized by two bands centered upon 899-917 cm^-1 and 981-997 cm^-1, which represented the symmetric stretching vibration v₁(AsO₄) and the antisymmetric stretching vibration v₃(AsO₄), respectively. The infrared bands around 868-897 cm^-1 were assigned to the symmetric stretching vibration v₁(AsO₄). The thermal decomposition of the arsenate-substituted natroalunites showed three separated endothermic steps, namely the loss of H₃O⁺, the loss of OH^- and the loss of SO₃ + (As₂O₃ + O₂). The solubility products [K_sp] and the Gibbs free energies of formation [ΔG_f^o] for the arsenate-substituted natroalunites decreased from 10^-81.21 to 10^-109.16 and from -4714.49 kJ/mol to -5352.95 kJ/mol with the increase of the AsO₄ substitution from 0% to 67%, respectively.

Characterization of alunite supergroup minerals by Raman spectroscopy

Raman spectroscopy has been used to study the molecular structure of different natural minerals of the alunite supergroup (AB(3)(XO(4))(2)(OH)(6)), with A=K(+), Na(+), Ca(2+), Sr(2+), Ba(2+), B=Al(3+), Fe(3+) and X=S(6+), P(5+). The influence of the ions, in A-, B- and X-sites, is highlighted in the Raman spectra by variations in the position of certain vibrations and is discussed in association with published crystallographic data in order to describe the observed differences. It was found that A-site substitutions are characterized by wavenumber shifts of the vibrations involving hydroxyl groups. The positions of these vibrational bands vary linearly with the ionic radius of the ions in this site. B-site substitutions induce shifts of all bands due to structural modifications that lead to differences in the chemical environment around the hydroxyl and XO(4) groups and changes in B-O bond lengths. A correlation showed that these shifts correlate well with the ionic radii of the B-ions. The spectra of compounds containing both sulfate and phosphate groups are described by numerous vibration bands caused by a complex elemental composition and a symmetry change of the XO(4) groups. This study has also made it possible to generalize substitution effects on the wavenumbers of several vibrations and show that Raman spectroscopy could be a powerful tool for identifying and distinguishing minerals of the alunite supergroup.

Vibrational spectroscopic analysis of the mineral crandallite CaAl3(PO4)2(OH)5·(H2O) from the Jenolan Caves, Australia

The mineral crandallite CaAl(3)(PO(4))(2)(OH)(5)·(H(2)O) has been identified in deposits found in the Jenolan Caves, New South Wales, Australia by using a combination of X-ray diffraction and Raman spectroscopic techniques. A comparison is made between the vibrational spectra of crandallite found in the Jenolan Caves and a standard crandallite. Raman and infrared bands are assigned to PO(4)(3-) and HPO(4)(2-) stretching and bending modes. The predominant features are the internal vibrations of the PO(4)(3-) and HPO(4)(2-) groups. A mechanism for the formation of crandallite is presented and the conditions for the formation are elucidated.

Raman spectroscopic investigation on Jarosite-Yavapaiite stability

The stability of synthetic Jarosite (KFe(3)(SO(4))(2)(OH)(6)) at low temperature and reduced atmospheric pressure has been studied by Raman spectroscopy. Jarosite remains stable between 8 and 295 K, provided that the sample is not exposed to reduced atmospheric pressure. When exposed to reduced atmospheric pressure (2.0x10(-2) Torr), however, the conversion of Jarosite into a different mineral is readily detected at room temperature by the appearance of a new Raman peak. The Raman shift of this peak (1032 cm(-1)) matches with that of Yavapaiite (KFe(SO(4))(2)), which can be obtained by thermal decomposition of Jarosite above 473 K. These studies provide a better understanding of the stability of Jarosite subjected to conditions similar to that on the surface of Mars.