CATHODOLUMINESCENCE SPECTROSCOPY OF FELDSPARS

Visual observation of feldspars using a cold cathode cathodoluminescence stage reveals that most plagioclase feldspars luminesce in green color, most Potassium feldspars luminesce with a blue color and fenitized feldspars luminesce with a red or near-Infrared (NIR) emission. This technique has been used for petrographic examination of rock samples with much success. Much less is known about the spectral characteristics of the feldspar cathodoluminescence.

The development of compact fiber optic spectrophotometers that are sensitive in the 200-1100nm wavelength range has allowed easy collection of cathodoluminescence spectra from minerals. The broad spectral range available has revealed extensive emissions below and above previously reported spectra in the 400-700nm range. To examine the entire spectral range, a fused quartz window on the cathodoluminescence stage, an UV-IR collimator and an UV-IR fiber cable must be used to couple the spectrophotometer to the cathodoluminescence device. By focusing the collimator on the cold-cathode cathodoluminescence stage sample holder, and area of less than 6mm diameter may be examined. Once properly aligned, the collimator may be used to examine individual mineral grains.

Visually, members of the plagioclase feldspars tend to appear green, members of the Potassium feldspars blue and fenitized feldspars red when exposed to an electron beam. There are considerable spectral variations between the different feldspar compositions; however, certain characteristics are shared by all the different types of feldspars. The visual colors are from a combination of emissions below 550nm (blue), emissions between 500-600nm (green) and emissions above 650nm (red). All the feldspar species can have strong emission in the 700-800nm band (NIR). The degree of fenitization in both plagioclase and Potassium feldspars is evidently related to the intensity of emissions between 650-800nm. Differences in the distribution of emission bands are indicative of compositional differences in plagioclase and the Potassium feldspars.