Giant haloes in mica

Abstract

MUCH attention has been given to the origin of ‘giant’ pleo-chroic haloes found in a number of minerals, especially micas^1–6. Normal pleochroic haloes are due to the effects of radiation damage from α-particles emitted from small inclusions at the centre of each halo, and the largest radius of the halo is determined by the range of the α-particle of highest energy, which is that from ²¹²Po in the case of a thorium-rich inclusion These have an initial energy of 8.78 MeV. The giant haloes seem to be of two kinds. In Gentry's sample7 ∼40% have a discrete radius ∼ 54 µm and he reports them to be invariably associated with dense thorium haloes. This group appears properly to be associated with production by the well-known but rare (∼ 1.3×10⁻⁴) pair of long range α-particle groups from 212Po at ∼ 10.5 MeV. The remaining giant haloes have a monotonic distribution in radius extending up to 110 µm; they have been interpreted in terms of diverse processes such as extinct radiations of higher energy, proton knock-ons by α-particles, knock-ons from neutrons, channelling, migration of the parent radioactive material, but we report one natural and simple explanation which seems not to have been considered.