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The origin of black colouration in onyx agate from Mali

Published online by Cambridge University Press:  05 July 2018

J. Götze ,
L. Nasdala ,
U. Kempe ,
E. Libowitzky ,
A. Rericha  and
T. Vennemann
J. Götze*
Affiliation:
TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, D-09596 Freiberg, Germany
L. Nasdala
Affiliation:
Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Wien, Austria
U. Kempe
Affiliation:
TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, D-09596 Freiberg, Germany
E. Libowitzky
Affiliation:
Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Wien, Austria
A. Rericha
Affiliation:
Alemannenstraβe 6, D-14612 Falkensee, Germany
T. Vennemann
Affiliation:
TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, D-09596 Freiberg, Germany
*
*E-mail: goetze@mineral.tu-freiberg.de
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Abstract

Natural onyx agate from Mali was investigated in an integrated mineralogical and chemical study to reveal the origin of the unusual black colouration. Detailed studies by polarizing microscopy, scanning electron microscopy and micro-Raman spectroscopy showed that the colour of the dark bands is related to the incorporation of small particles of carbon (low-crystalline graphite) up to 200 nm in size into the cryptocrystalline silica matrix. The dark bands have carbon contents of 1.88 wt.%. The location of the graphite particles is closely related to the primary structural banding in the chalcedony. Cathodoluminescence data shows that the banding is interrupted by small fissures containing secondary hydrothermal quartz. The carbon isotope composition (δ13C value of –31.1±0.2‰) of the carbonaceous material points to an organic precursor. Both the direct hydrothermal formation of graphite from methane under elevated temperature and the graphitization of organic precursors by secondary hydrothermal or metamorphic overprint are possible explanations for the colour of the dark bands. The graphitization of organic precursors results in an intense electron spin resonance line at geff = 2.0026.

Keywords

agateonyxRaman spectroscopycathodoluminescencecarbon isotopesradiation damage
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 1 , February 2012 , pp. 115 - 127
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2012

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