APPLICATIONS OF 405NM LASER EMISSION SPECTROSCOPY FOR MINERALOGY AND PETROGRAPHY

The study of emission optical spectra from minerals has become more accessible to researchers with the development of compact fiber optic spectrophotometers. This type of unit, properly configured, can be used to investigate Cathodoluminescence emission spectra in a very wide variety of minerals in the wavelength range of 200-1100nm. The spectrophotometers can also be used with high intensity 405nm laser diodes, originally developed for Blu-Ray optical recording devices, to examine fluorescence emission spectra that are activated by the high intensity of the laser beam.

One application for the lasers is the examination of syenites for sodalite content. Sodalite is generally difficult to observe in a hand specimen because it is usually blends in with the feldspars. In a thin section, isotopic grains of sodalite are easily overlooked. Illumination of a specimen or thin section with a 405nm laser demonstrates that sodalites will typically luminesce bright orange. Many members of the sodalite group that are non-responsive to normal ultraviolet light will respond to the laser. Use of a yellow filter to reduce the scattered laser light enhances this effect and allows easy detection and quantification of any sodalite present in the syenite.

Combined with a spectrophotometer, the lasers can detect the presence of Mn, REE and Cr in a wide variety of minerals. Notable examples are found in members of the apatite group (Mn and REE) and kyanite (Cr). Differences in spectra appear to be indicative of mineral provenance and may eventually lead to methods for nondestructive geochemical analysis of grains in rocks and sediments.