Chemical composition and vibrational spectral of tungsten-bearing goethite and hematite from western Rhodopes, Bulgaria

The incorporation of W into goethite and hematite in the supergene environment of the Grantcharitza tungsten deposit in the Western Rhodopes is discussed via results from SEM, EPMA, XRD and micro-Raman spectroscopy. The sample is a collofirm black material consisting mainly of goethite with WO (sub 3) 6.83-10.85 wt.% and of several products of its alteration, including a W-poor goethite with WO (sub 3) 1.35-5.72 wt.%, a W-rich ferric iron oxide phase with WO (sub 3) 22.19-25.12 wt.% and hematite with WO (sub 3) 1.21-1.72 wt.%. The presence of W atoms in the structures of hematite and goethite causes the appearance of a low-frequency shoulder in the Raman peak near 400 cm (super -1) . In both minerals, only a small amount of W occupies Fe positions, while the major part of the W forms W-O self-assembled clusters generating a broad band between 580 and 750 cm (super -1) in the Raman spectra. Tungsten is organized in chain-like clusters of corner-sharing WO (sub 6) octahedra with an average W-O-W bond angle of approximately 150 degrees .