Chemical composition and vibrational spectra of tungsten-bearing goethite and hematite from Western Rhodopes, Bulgaria

Abstract

The incorporation of tungsten in goethite and hematite as well as the relationships between the two minerals in a supergene environment are considered on the basis of a representative sample from the Grantcharitza tungsten deposit (Western Rhodopes, Bulgaria), studied by scanning electron microscopy, electron probe microanalysis, X-ray powder diffraction and micro-Raman spectroscopy. The sample is a colloform black material consisting predominantly of goethite ( G-I ) with 6.83–10.85 wt.% WO3 and of several products of its alteration, including W-poor goethite ( G-II ) with 1.35–5.72 wt.% WO3, a W-rich ferric iron oxide phase ( Fw ) with 22.19–25.12 wt.% WO3, and hematite ( H-I ) with 1.21–1.72 wt.% WO3. It is shown that G-I is an aggregate of proper goethite and of a W-rich ferric iron oxide phase ( Sw ). The alteration of G-I aggregates includes: (i) selective dissolution of Sw which either re-precipitates, thus forming a separate W-rich phase ( Fw ), or which is almost entirely removed from the aggregate; (ii) replacement of the residual goethite G-II by hematite H-I . An atomic ratio W/Fe of about 0.006 is supposed to correspond to the isomorphic (Fe3+ ⟷ W6+) incorporation of tungsten into goethite and hematite in the sample studied. The Raman spectrum of G-II resembles the already published spectroscopic data on goethite, whereas H-II shows spectral features typical of a highly disordered hematite structure. The presence of W atoms in the structures of hematite H-I and goethite G-II causes the appearance of a low-frequency shoulder of the Raman peak near 400 cm−1. According to the Raman spectroscopy data, the W-rich constituent Sw in G-I aggregates as well as its re-precipitated variety Fw both have a Fe-O network similar to that of goethite. In both materials only a small amount of W atoms occupies Fe-positions, while the major part of tungsten forms W-O self-assembled clusters generating a broad band between 580 and 750 cm−1 in the Raman spectra. As shown by model calculations, tungsten is organised in chain-like clusters of corner-sharing WO6 octahedra with an average W-O-W bond angle of about 150°. In addition, Fw contains isolated WO6 octahedra or corner-sharing-WO6 clusters with an average W-O-W angle of about 180°.