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

摘要

Natural topaz crystals of gem quality and orange-red colour from the Ouro Preto area, Brazil have been analyzed 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 V~(4+). Mn seems to be substituted as Mn~(3+) not detectable by EPR at -band frequencies and Ti as Ti~(4+) which is inactive for both EPR and optical absorption. Cr and Fe have been found in the valence states Cr~(3+) and Fe~(3+), 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 V~(4+), Cr~(3+), Fe~(3+) is not remarkably influenced by heating the samples up to 600 ℃.