The cause of colour of the blue alexandrites from Malacacheta, Minas Gerais, Brazil

Pinheiro, M.V.B.; Basilio, M.S.; Krambrock, K.; Dantas, M.S.S.; Paniago, R.; Assuncao, A.L.; Pedrosa, S.A.C.

Journal of Gemmology 27(3): 161-170

2000


ISSN/ISBN: 1355-4565
Accession: 020300124

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Abstract
A rare and very attractive variety of chrysoberyl, a blue alexandrite from Malacacheta, Minas Gerais (Brazil), commercially called "peacock-blue" alexandrite, was investigated by electron paramagnetic resonance (EPR), optical absorption, photoluminescence and X-ray photoelectron spectroscopy (XPS). The peacock-blue alexandrite is known for its strong pleochroism ranging from blue to green or greenish-yellow, as well as its strong colour-changing effect showing blue or greenish-blue in daylight and reddish-purple in incandescent light. The results show that the rare bluish colours of the peacock-blue alexandrite are related to a critical balance between Fe (super 3+) and Cr (super 3+) substituting for Al (super 3+) in the sites with mirror symmetry, while the concentration of Cr (super 3+) in the sites with inversion symmetry is negligible.