TL spectra of mineral mixtures: Discrimination between different components

Abstract

Spectral measurements of the thermoluminescence from a polymineralic sand and its component minerals are reported. The signals from density-separated and chemically separated fractions of the sand are compared in an attempt to select wavelength regions characteristic of the quartz and feldspar mineral components. The data clearly demonstrate that for these materials the uses of conventional broad band optical filters and a blue sensitive photomultiplier tube fail to separate the mineral fractions, despite earlier claims. However, measurements in the near infra-red are characteristic of the feldspar components. Whilst density separations lead to clearly distinguishable spectral features for light, medium and heavy mineral fractions, subsequent HF etching and alpha irradiation emphasized that the TL of the density separated quartz fraction was dominated by the traces of feldspar contamination. Indeed, at the sensitivity level available, all the signal from the quartz fraction arose from traces of feldspar contaminants. The temperatures of the glow peaks are so similar in all mineral fractions that these values are a totally inadequate basis for mineral separation. The X-ray excited luminescence spectra differ from those of thermoluminescence but the blue/green radioluminescence is very similar for all the mineral fractions separated in this study. The similarity of radioluminescence and the glow peak temperatures are attributed to the role of the silicate groups, exciton decay paths and point defects in the silicate networks which are common to all the component minerals of feldspars, quartz, silica, zircon and silicates.