Quantifying hydrofluoric acid etching of quartz and feldspar coarse grains based on weight loss estimates: implication for ESR and luminescence dating studies

Abstract

A series of hydrofluoric (HF) acid etching experiments were performed on several 50 to 300 μm (nominal sieve opening) quartz and feldspar samples, and average etching thicknesses were subsequently determined from weight loss estimates, following the approach of Bell & Zimmerman (1978). Our results are consistent with previous studies and confirm that both HF experimental conditions (etching time, HF concentration, agitation) and the nature/origin of the minerals have a significant effect on etching rate and magnitude. For the samples considered in this study, the outer 10 μm of quartz (i.e., the usual etching target in most ESR and luminescence dating studies) was removed with 40% HF after 40 min of etching, in fair agreement with previous studies. Similar etching results were achieved in just over 20 min when the quartz samples were constantly agitated during 40% HF treatment. For K-feldspars, 10% HF etching for between 10 and 40 min was required to remove the same thickness, although some variability in etching efficiency was evident between different samples and experimental conditions. For a given experimental setup, the overall variability in etching thickness among quartz samples was found to be of at least 20% after 40– 60 min with 40% HF, and can be as high as ∼35%. A set of 14 K-feldspars from a single site revealed an inherent variability of 14% in etching thickness. As a first order approximation, these data may provide useful insights into the expected variability among samples of different origin and from a given area, respectively. However, we recommend that each laboratory evaluates etching thickness losses for their specific HF experimental setup. Additionally, our results show that the 10 μm etching usually considered in ESR and luminescence dating studies may reduce the alpha contribution of the total dose rate to <5%. Although relatively small, this contribution is nevertheless non-negligible. Based on these results, it may be worth considering an additional alpha dose rate term as part of the ESR and luminescence age calculation when etching depths are < 15 μm.