Re-examination of common extraction and purification methods of quartz and feldspar for luminescence dating

Abstract

We tested common mineral extraction and purification procedures used in luminescence dating. Coarse grain quartz and alkali feldspar (KF) were extracted and etched from three samples of different geological setting and mineralogical maturity, using a variety of laboratory procedures. This was followed by particle size distribution measurement, SEM/EDS and XRF analyses, and De determinations employing SAR protocols – all used to characterize the mineral extracts. Etching quartz in 40% HF for 40 min removed all feldspars, with some preferential etching along fissures and grain boundaries. For mineralogically less mature samples, HF reduced average quartz grain size by 30–50 µm, whereas in mature samples extended etching of up to 60 min reduces grain size by ∼10 µm. Ca-fluorides precipitated during HF etching but were fully removed by soaking overnight in 16 % HCl. Extracting quartz by heavy liquids or by magnetic separation (both followed by HF etching) resulted in equally pure quartz, with statistically identical De values. Soaking KF for 40 min in 10 % HF resulted in uneven etching, severe micro-scale erosion along cleavage plains, grain disintegration, substantial precipitation of K-fluorides and the reduction of more than 100 µm in modal grain size, probably due to grain breakage along etched plains. The effects of etching vary greatly between samples from different geological provinces; more mature samples are etched in a more uniform way whereas mineralogically less mature samples are fragmented by etching. The best benefits for KF are obtained by etching in diluted HF for short duration. For quartz, more concentrated HF and longer etching might be considered if grains are to have their entire α-affected zone (20 µm) removed. Since concentrated HF dissolves all feldspar contamination in quartz, there is no need for re-sieving after HF as that would bias the original grain size. Our results provide confidence in standard laboratory quartz sample preparation.