Extraction and purification of quartz in rock using hot phosphoric acid for in situ cosmogenic exposure dating

Quartz extraction and purification is essential for the successful measurement of 10Be and 26Al by AMS for exposure dating. The common procedure involves repetitive etching by ultrasonic heating and agitation in a dilute aqueous HF solution (2% w/w), which has proven adequate for quartz rich rock (i.e., granite, quartzite). We have developed an alternative method using hot phosphoric acid (250 °C), which preferentially dissolves silicates but not quartz. This method is particularly effective on samples with very low quartz abundance or with cryptocrystalline silica (i.e., greywacke, silcrete, chert) to enhance quartz recovery. We describe the method in detail and present data that compares the efficiency and the benefits of the two methods.