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

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 (F_w) 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 (S_w). The alteration of G-I aggregates includes: (i) selective dissolution of S_w which either re-precipitates, thus forming a separate W-rich phase (F_w), 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 S_w in G-I aggregates as well as its re-precipitated variety F_w 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, F_w contains isolated WO₆ octahedra or corner-sharing-WO₆ clusters with an average W-O-W angle of about 180°.