The origin of black colouration in onyx agate from Mali
- Autor(en)
- Jens Götze, Lutz Nasdala, Ulf Kempe, Eugen Libowitzky, A Rericha, Torsten W. Vennemann
- Abstrakt
Natural onyx agate from Mali was investigated in an integrated mineralogical and chemical study to revealthe origin of the unusual black colouration. Detailed studies by polarizing microscopy, scanning electronmicroscopy and micro-Raman spectroscopy showed that the colour of the dark bands is related to theincorporation of small particles of carbon (low-crystalline graphite) up to 200 nm in size into thecryptocrystalline silica matrix. The dark bands have carbon contents of 1.88 wt.%. The location of thegraphite particles is closely related to the primary structural banding in the chalcedony.Cathodoluminescence data shows that the banding is interrupted by small fissures containing secondaryhydrothermal quartz. The carbon isotope composition (d13C value of _31.1_0.2%) of the carbonaceousmaterial points to an organic precursor. Both the direct hydrothermal formation of graphite from methaneunder elevated temperature and the graphitization of organic precursors by secondary hydrothermal ormetamorphic overprint are possible explanations for the colour of the dark bands. The graphitization oforganic precursors results in an intense electron spin resonance line at geff = 2.0026.
- Organisation(en)
- Institut für Mineralogie und Kristallographie
- Externe Organisation(en)
- Technische Universität Bergakademie Freiberg, Eberhard Karls Universität Tübingen, Independent researcher
- Journal
- Mineralogical Magazine
- Band
- 76
- Seiten
- 115-127
- Anzahl der Seiten
- 13
- ISSN
- 0026-461X
- DOI
- https://doi.org/10.1180/minmag.2012.076.1.115
- Publikationsdatum
- 2012
- Peer-reviewed
- Ja
- ÖFOS 2012
- 1030 Physik, Astronomie, 105116 Mineralogie
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/95d01986-91d9-406a-a522-68764e852a47