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Spectroscopic study of natural gem quality "Imperial"-Topazes from Ouro Preto, Brazil

¹ Institut für Mineralogie der Philipps-Universität, Hans-Meerwein-Straße, D-35032 Marburg, FRG
² Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Sciences of Ukraine, Paladin Ave. 34, 03191 Kyiv, Ukraine

^* Corresponding author, e-mail: rager{at}mailer.uni-marburg.de

Natural topaz crystals of gem quality and orange-red colour from the Ouro Preto area, Brazil have been analysed chemically and investigated by optical absorption and electron paramagnetic resonance measurements to elucidate the reason of the special orange-red colour. In all topazes the transition elements V, Mn, Ti, Cr and Fe were found. V occurs as V⁴⁺. Mn seems to be substituted as Mn³⁺ not detectable by EPR at X-band frequencies and Ti as Ti⁴⁺ which is inactive for both EPR and optical absorption. Cr and Fe have been found in the valence states Cr³⁺ and Fe³⁺, respectively. The orange-red colour is related to two electron centres with g_eff-values of 1.96 and 1.91, respectively, because both the electron centres and the absorption bands responsible for the orange-red colour can be bleached by thermal treatment whereas the EPR of V⁴⁺, Cr³⁺, Fe³⁺ is not remarkably influenced by heating the samples up to 600 °C.

Key-words: topaz, EPR, optical absorption, colour..