Navegando por Autor "Eeckhout, Sigrid G."
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Item Effect of heat treatment on tourmaline from Brazil.(2006) Castañeda, Cristiane; Eeckhout, Sigrid G.; Costa, Geraldo Magela da; Botelho, Nilson Francisquini; Grave, Eddy DeThe crystal-chemical behaviour of tourmaline from Arac¸uaı´, Minas Gerais state, Brazil, when subjected to heating in air atmosphere has been studied by several techniques, including EMPA, UV–Vis, TGA, and Mo¨ ssbauer spectroscopy. The tourmaline samples are typically intermediate members of the elbaite-schorl series. The origin of colour and of its change after treatment has been discussed in terms of local disorder, presence of metal transition elements, oxidation of ferrous iron at the octahedral site, and simultaneous trap of the excess electron. These findings may be used to enhance the colour in tourmaline crystals or generate wanted colour changes.Item Spectroscopic studies of spessartine from Brazilian pegmatites.(2002) Eeckhout, Sigrid G.; Castañeda, Cristiane; Ferreira, Ana Claudia Mousinho; Sabioni, Antônio Claret Soares; Grave, Eddy De; Vasconcelos, Daniela Cordeiro LeiteMössbauer spectra (MS) at room temperature (RT) and Fourier-transform infrared (FTIR) spectra in the OH stretching region were acquired for natural spessartine-almandine garnet samples from different Brazilian pegmatites, including the complex, zoned Alto Mirador pegmatite, the simple, zoned Escondido pegmatite, and the simple Poaiá pegmatite. From MS, it is obvious that in the samples from the Alto Mirador pegmatite, ferrous iron is present at the dodecahedral site and ferric iron at the octahedral site of the garnet structure. The Fe2+/Fe3+ ratio is comparable for all samples from this pegmatite, i.e., ~10%, implying the same geological history, namely similar oxygen fugacities at the moment of garnet formation. In the simple pegmatites, however, almost no ferric iron was detected. On the basis of the multi-band FTIR spectra the nature of the point defects involved in the process of hydrogen uptake in spessartine-almandine garnet has been elucidated, if not completely clarified. The behavior of the absorption bands provides no evidence for multiple incorporation mechanisms, hence the hydrogarnet substitution is proposed to be the only mechanism for the incorporation of hydrogen in the samples studied. As geological setting is difficult to separate from chemical composition, FTIR spectra have the potential to provide useful information on some aspects of the geological history of the samples.