THE RED LUMINESCENCE EMISSION OF FELDSPAR AND ITS WAVELENGTH DEPENDENCE ON K, NA, CA - COMPOSITION

Abstract

Feldspar specimens covering the whole Or-Ab-An ternary have been investigated by cathodoluminescence (CL), photoluminescence (PL), radioluminescence (RL) and radiophosphorescence (RP) spectrometry. A red luminescence emission, which is commonly explained by Fe3+ lattice defects, is a characteristic feature of all the spectra. Different shifts of the peak-wavelength between ~680-750 nm (1.82-1.65 eV) were observed with varying feldspar composition. Despite the dependence of the peak position on the Ca/Na ratio, initially described for CL in the 1970s, there is also a shift induced by changing NaK composition. The observed effects can be explained by known relations that the peak position of the red luminescence emission in feldspars can be affected both by the structural state of the feldspar and the site occupancy of the trivalent iron. In the case of alkali feldspars another factor may influence the peak-shift. The incorporation of the larger potassium ion causes non-linear variations of the cell dimensions and therefore Fe-O bond distance. The behaviour of the red peak-shift dependent on the feldspar composition is not equal for all types of luminescence investigated. This is most likely caused by the different luminescence excitation mechanism.