Natasha Morrison

B.Sc. (Honours) Thesis

An Investigation into UV Fluorescence in Feldspar Group Minerals

(PDF - 2.5 Mb)

Feldspar group minerals are among the most abundant mineral groups in the Earth’s lithosphere and have been documented, studied and analyzed for a wide range of petrologic purposes. The methods used to examine feldspars include cathodoluminescence, petrographic examinations including Michel-Levee compositions, scanning electron microscope and electron microprobe analysis (SEM and EMP), ion microprobe and laser ablation ICP-MS analysis, studies of isotope ratios and X-Ray Diffraction (XRD). As a result the physical properties and paragenesis of feldspars are well understood, however UV fluorescence is one property which has long been described but has not been fully quantified. In this study, the fluorescence of a selected suite of feldspar group minerals was examined to investigate the link between fluorescence and crystal chemistry through determination of major and trace elements. The methods used included petrographic characterization, UV fluorescing imaging and image analysis, electron microprobe analysis, and crystal-structure modeling. Samples from Canada and the United States were used in this study focusing on compositions of the following alkali feldspars: microcline (including amazonite) and orthoclase (including adularia); and plagioclase feldspars (albite, oligoclase, labradorite, bytownite and anorthite). Features such as intercrystalline impurities were also examined. Fluorescence is generally thought to be controlled by activator elements, typically metal cations (Fe2+, Fe3+, Ti and Mn) or REEs. Electrons within these elements become excited by photons at UV wavelengths which cause electron excitation, i.e. a jump to a higher orbital. The de-excitation of this electron then causes a loss in energy that is released as light, at a different wavelength than that the photon which was originally absorbed. In the case of UV fluorescence these emitted photons are visible to the human eye. Integrating results from all of the methods show that different compositions of feldspar fluoresce with similar colors but at strongly variable intensities. Microprobe data, combined with x-ray mapping and UV imaging, showed that there was a significantly higher intensity of fluorescence in K-feldspars, while plagioclase group feldspars have a much lower intensities. Orthoclase and microcline fluoresced very strongly, albite less so, and anorthite showed the lowest fluorescence intensity. However there was a negative correlation between trace element-activator contents and fluorescence intensity. Taking crystal structure into consideration, the unit cell parameters increase in K-feldspar. This along with the monoclinic structure may allow greater photon channeling and thus increases the intensity of fluorescence. Future work employing more sensitive trace-element analysis and UV spectroscopy and XRD are recommended to fully understand the link between fluorescence and crystal chemistry.

Keywords: Feldspar, Fluorescence, Geochemistry, K-feldspar, Plagioclase, Ultra Violet light
Pages: 71
Supervisor: Grant Wach