Radio-induced alteration in cordierite – Implications for petrology, gemmology and materials science

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 most 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; cf. Nasdala et al., 2006) and 1550 cm^-1 (Raman only). 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 Ch0 position in the large channel cavities. Carbon monoxide and O₂ molecules seem to coexist in artificially irradiated cordierite without any sign of back reaction. Investigations on natural CO₂ -bearing cordierite from different localities show that the 2CO₂ → 2CO + O₂ transformation is ubiquitous in radiohaloes.
Up to date, the detection of CO in cordierite has solely been ascribed to a reducing environment during crystal formation (Khomeko & Langer, 2005). In contrast, this present study shows that CO can also be produced by the action of radioactivity. This, as well as the production of radiogenic O₂ , has to be taken into account in reconstructing palaeofluid compositions from molecules trapped in the structural channels of cordierite.

Khomenko VM, Langer K (2005): Carbon oxides in cordierite channels: Determination of CO₂ isotopic species and CO by single crystal IR spectroscopy. Am Mineral 90: 1913-1917
Nasdala L, Wildner M, Wirth R, Groschopf N, Pal DC, Möller A (2006): Alpha particle haloes in chlorite and cordierite. Mineral Petrol 86: 1-27