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The blue colouring of beryls from Licungo, Mozambique: an X-ray absorption spectroscopy study at the iron K-edge

Published online by Cambridge University Press:  05 July 2018

M. O. Figueiredo ,
T. Pereira da Silva ,
J. P. Veiga ,
C. Leal Gomes  and
V. De Andrade
M. O. Figueiredo*
Affiliation:
Crystallography and Mineralogy Centre (IICT) and Geological Data Centre (INETI/IGM), Apt. 7586, 2721-866 Alfragide, Portugal CENIMAT, Materials Science Department, FCT, New University of Lisbon, 2829-516 Caparica, Portugal
T. Pereira da Silva
Affiliation:
Crystallography and Mineralogy Centre (IICT) and Geological Data Centre (INETI/IGM), Apt. 7586, 2721-866 Alfragide, Portugal CENIMAT, Materials Science Department, FCT, New University of Lisbon, 2829-516 Caparica, Portugal
J. P. Veiga
Affiliation:
CENIMAT, Materials Science Department, FCT, New University of Lisbon, 2829-516 Caparica, Portugal
C. Leal Gomes
Affiliation:
Research Centre of Geology and Resources Valorization, Minho University, Gualtar Campus, 4710-057 Braga, Portugal
V. De Andrade
Affiliation:
European Synchrotron Radiation Facility (ESRF), B.P. 220, 38043 Grenoble, Cedex, France
*
*E-mail: [email protected]
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Abstract

The colours of natural and synthetic beryl (ideally Be3Al2Si6O18) have been attributed either tothe presence of chromophore ions partially replacing Al and/or Be (Cr3+, Fe2+/3+, Mn2+) or to the occurrence of colour centres related to the presence of multi-atomic groups. However, no full explanation has been proposed for the blue colour in natural beryl gemstones (aquamarine) despite it being well established that bluish-green synthetic aquamarines containing Fe turn blue when heated for 1 h at 400°C.

A X-ray absorption near-edge spectroscopy (XANES) study at the Fe K-edge was performed on blue beryl crystals from Licungo pegmatite (Mozambique) to ascertain the speciation state of this colouring element, reported in blue sapphire (ideally Al2O3) as a mixture of Fe2+ and Fe3+ in octahedral coordination. The general trend of the spectra is the same when synchrotron radiation (SR) impinges along the c axis of the hexagonal crystal, as well as along the a axis with c vertical. Conversely, a shoulder in the absorption edge is noticed when the incident beam is perpendicular to c, with this axis horizontal in the polarization plane of the SR beam, a reverse in the relative intensities of the two post-edge features is simultaneously observed. Pre-edge features accounting for Fe speciation (valence plus coordination) are identical in all three situations. Considering previous approaches which account for polarization, an explanation is suggested for the special effects observed on the Fe K-edge XANES spectra of analysed blue beryl crystals. Ideas on the origin of the blue colour of this mineral are also presented.

Keywords

berylXANESFe K-edgeblue colouring
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 1 , February 2008 , pp. 175 - 178
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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