Radiohaloes in feldspar group minerals

External irradiation of minerals with alpha-particles emitted from radionuclide bearing inclusions may lead to significant alteration of the physical and chemical properties of the host material. These altered regions are commonly referred to as radiohaloes. In the vast majority of reported cases, haloes can be detected using an optical microscope, because they show significantly changed optical absorption and birefringence behaviour in comparison to the unaltered host (e.g. Paul Ramdohr (1960): Geol. Rundschau, 49, 253-263). However, there are minerals which commonly show no change in optical absorption upon alpha-irradiation. One well known example is quartz, where radiohaloes can only be revealed by cathodoluminescence (CL) imaging, as they show an intensified, orange emission (e.g. Jens Götze, Michael Plötze, Dirk Habermann (2001): Mineral Petrol, 71, 225-250). In the course of CL-studies on radiohaloes in quartz (Robert Krickl, Lutz Nasdala, Jens Götze, Dieter Grambole, Richard Wirth (2008): Eur J Mineral, 20, 517- 522), hitherto unreported alpha-induced alteration effects in alkali-feldspars and plagioclases were discovered and characterised.
In contrast to quartz, the newly discovered “negative CL-haloes” in feldspars exhibit a lower emission intensity as compared to the unaltered host. Cathodoluminescence spectra show, that this decrease is mainly caused by a strong intensity loss of bands attributed to the O^– / 2 ²⁷Al centre. In some cases a new, probably radiogenic band at ~570–600 nm can be observed.
Radiohaloes in feldspars seem to be quite common and were detected in a number of rocks from different localities. Haloes are not only found around radioactive inclusions but also along cracks, indicating the former circulation of radionuclide bearing solutions. The radioactive origin of the reported features is confirmed by several observations: Haloes are only found at the contact zones to radionuclide bearing phases like for example monazite. Measured outer radii of haloes (and sometimes rings within haloes) are in very good agreement with penetration depths of natural alpha-particles calculated by Monte-Carlo-simulations. Finally, micro-Raman spectroscopic investigations indicate the presence of structural damage which increases with decreasing distance from the radionuclide bearing inclusion, thus correlating with the presumed point defect density distribution in radiohaloes. Amorphised regions within haloes are rare, though they exist in some cases. Glassy feldspar seems to be unstable against secondary alteration and is most often found to be recrystallised and chemically altered. Evidence on different resistance against secondary alteration within different feldspar minerals will be discussed.
The results on natural radiohaloes are confirmed by artificial irradiation experiments: Implantation of a sanidine single crystal with 8.8 MeV He²⁺ ions (corresponding to alpha-particles produced in the decay of ²¹²Po) results in analogous decrease in CL-intensity, showing systematic dependence on irradiation dose. Raman spectroscopic investigations show significant broadening of vibrational bands, indicating significant disturbance of the short range order in the crystal structure. However, crystalline long range order seems to be preserved up to doses of 10¹⁶ He²⁺/cm². The measured extent of broadening correlates very well with calculated point defect distribution curves resulting from Monte-Carlo-simulations. In the case of natural and artificial alpha-irradiation no change in optical absorption could be detected. However, irradiation with electrons and gamma-rays resulted in a markedly yellow-brown colouration of the same sample material that was subjected to He-implantation. For these treatments obviously induce different effects, they are not adequate methods to simulate alpha-particle haloes in these minerals.