gems, Brazil, topaz, imperial topaz, Capão, Minas Gerais, causes of color, spectroscopy, chemistry

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 for 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⁺ and Fe³⁺ is not remarkably influenced by heating the samples up to 600°C.