Effect of impurities on cathodoluminescence of tridymite and cristobalite

Cristobalite and tridymite in andesite and obsidian exhibit bright blue emissions in color cathodoluminescence (CL) images. CL spectra of cristobalite and tridymite show emission bands at 400 and 430 nm, respectively. Their CL intensity decreases with an increase in the electron irradiation time, similar to the short-lived blue luminescence reported in hydrothermal quartz. The reduction rate of the CL intensity of hydrothermal quartz is small as compared to cristobalite and tridymite. The initial CL intensity of both minerals immediately after the electron irradiation is positively correlated with the Al2O3 content. This suggests that the blue emissions of cristobalite and tridymite can be attributed to the [AlO4/M] defect center (M: H, Li, Na, and K). The electron irradiation of cristobalite and tridymite diffuses the monovalent cations of H, Li, and Na and destroys their crystal structure, resulting in short-lived blue luminescence. The CL intensity after 600 s of electron irradiation depends on the K2O impurity content. In contrast to other monovalent cations, the K cation is not diffused because of its large ionic radius. The CL emissions of cristobalite and tridymite after 600 s of electron irradiation can be attributed to the [AlO4/M] defect (M: K). CL images of cristobalite in obsidian from Utah, USA, show a heterogeneous distribution of CL intensity, with oscillatory CL zoning on the rim and radial lath-shaped textures with bright CL superimposed on weak luminescence background. This implies that lath-shaped textures with bright emissions might have crystallized from rhyolitic magma at temperatures higher than the recrystallization temperature minerals in the other areas.