The Structure of the Colour Centres in Smoky Quartz

A model is proposed to account for the properties of the colour centres produced by irradiation by X-rays in quartz. The hyperfine structure of the paramagnetic resonance spectrum shows that each centre is associated with an aluminium atom, and the model puts the aluminium atom at a silicon site in the lattice with the unpaired electron in a localized orbital on one of the nearest oxygen atoms. The axes of the ‘g’ tensor and the variation of g with direction are predicted unambiguously, as is the axis of the quadrupole interaction. The magnitude and axes of the magnetic hyperfine structure are not predicted, but they are explained in terms of the model. The energy levels deduced theoretically using the paramagnetic resonance data are used to give a tentative interpretation of the optical absorption spectrum. Finally an account is given of the effects to be expected if there is an interaction between the two different colour centres that can be associated with a particular aluminium atom.