Three optical absorption bands, A1, A2, and A3, are associated with trapped hole centers that develop when quartz containing Al3+ in a substitutional Si4+ site is subjected to ionizing radiation. Studies of the directional anisotropy of the A2 and A3 optical bands in the quartz basal plane show that they may interchange orientations from crystal to crystal in major rhombohedral growth; this contradicts an earlier theory that the anisotropy results from site selectivity of Al3+ occurring only in minor rhombohedral growth. Four crystallographic directions have been found for the maximum intensity of A2 and/or A3: , , , and . The removal of basal-plane anisotropy at ∼500°C reported by others was confirmed and is attributed to the homogenization of interstitial atoms providing charge compensation for substitutional Al3+.
Plots of Nf for A3 vs. A1, A3 vs. A2, and A2 vs. A1 with bleaching temperature all form straight lines, suggesting a close relationship among the centers related to these optical bands. Since the center related to A3 is known to involve a single trapped hole, it is concluded that all of these centers involve only one trapped hole.