UNIQUE CATHODOLUMINESCENCE (CL) FABRIC OF BLUE QUARTZ IN A LLANITE DIKE, LLANO COUNTY, TEXAS

Blue quartz phenocrysts in llanite, a Precambrian hypabyssal rhyolite dike, Llano County, Texas, display unique SEM-CL fabrics (Seyedolali et al., 1997). The phenocrysts have a distinctive blue luminescence color, and they exhibit CL fabrics unlike those of any other quartz examined to date.

Llanite samples display quartz phenocrysts with dark CL-bands, 15 to 229 µm wide, which are characteristic of deformed quartz. The CL fabric resembles a brick-like structure, the walls of which are nonluminescent and appear to be healed fractures or crystal defects. Alternatively, the bands could be the result of alpha quartz growth on earlier beta forms. Relict zoning is commonly overprinted by CL deformation bands, suggesting late deformation.

SEM-CL revealed red, irregular, low intensity 5 to 57 µm CL halos around phenocrysts, resembling overgrowths on quartz. In addition, nonluminescent quartz and K-feldspar fill embayments in some phenocrysts. Furthermore, fine quartz (19 to 97 µm) with red and low intensity CL is present in the rock groundmass. These characteristics may have been produced by metasomatism.

The centers of larger phenocrysts are blue, but become cloudy-white and chatoyant during rotation in reflected light. Margins are commonly clear or brown and do not change color with rotation. Smaller phenocrysts are entirely brown or colorless. The chatoyant “flash” is due to reflection from 1-3 sets of microscopic linear elements that become visible (in reflected light) at certain angles and that tend to coincide with blue color. Blue, chatoyant centers of the larger phenocrysts are relatively soluble in strong HF acid; they become porous and chalky white, whereas brownish margins remain unaltered. The reflective linear elements, possibly mineral inclusions or structural defects, may enhance HF solubility but their relation to blue color is uncertain.

Work is in progress to clarify the relationship between the linear elements and blue color and to characterize the mode of occurrence of the deformation bands. SEM-CL is a powerful technique for studying the origin of quartz and evolution of magmatic dikes.