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| Titel |
Radio-induced alteration in cordierite - Implications for petrology, gemmology and materials science |
| VerfasserIn |
R. Krickl, L. Nasdala, D. Grambole, R. Kaindl |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021239
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| Zusammenfassung |
| Cordierite is a common metamorphic and magmatic mineral, which is used as petrologic tool
for reconstructing the history of its host rock. Further applications include cordierite
gemstones and the use of synthetic analogs in ceramics. Cordierite is stable over a
wide temperature and pressure range and relatively resistant to chemical alteration;
however, its properties can be significantly changed upon the impact of external
irradiation.
In the course of a comprehensive study, natural radiohaloes in cordierite (a widespread
feature caused by the impact of alpha-particles originating from radioactive inclusions) as
well as artificial analogs produced by implantation of 8.8 MeV He2+ ions were investigated
using modern micro-techniques. Additional irradiation experiments were performed using
O6+ ions, electrons and gamma-rays.
Ion irradiation causes yellow colouration that is strongly pleochroic, and fades at higher
doses. The possibility of radiation-treatment for enhancing the quality of gem-cordierite is
discussed. While samples remain crystalline up to doses of 1016 He2+/cm2, the same
material is fully amorphised when irradiated with the same dose of 30 MeV O6+ ions.
These different observations may help to estimate the performance assessment of
cordierite-ceramics in radiated environments.
A very important result concerning the petrological use of cordierite is the radio-induced
transformation of channel constituents: Inside the irradiated areas the vibrational bands of
CO2 decrease in intensity, whereas two new bands appear at 2135 cm-1 (both IR- and
Raman-active; cf. Nasdala et al., 2006) and 1550 cm-1 (only Raman-activ). They are
assigned to stretching vibrations of carbon monoxide and molecular oxygen, respectively,
thus indicating a radio-chemical transformation 2CO2 → 2CO + O2 in alpha-irradiated
cordierite. This study yields the first spectroscopic evidence for the irradiation-induced
formation of molecular oxygen in cordierite. Polarised vibrational spectra of oriented samples
give evidence that not only CO2 but also CO and O2 are preferentially oriented parallel [100],
most probably being located at the Ch |
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