An improved method for separating quartz from rock using pyrophosphoric acid - 2nd edition

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Ditchburn, R.G.; Durrant, B.; Futter, R.J.; Mawdesley, S.; Zondervan, A. 2019 An improved method for separating quartz from rock using pyrophosphoric acid. 2nd ed. Lower Hutt, N.Z.: GNS Science. GNS Science report 2019/10. 11 p.; doi: 10.21420/1TFF-RQ08


A method that employs hot dehydrated phosphoric acid (pyrophosphoric acid) to isolate quartz from rock for surface exposure dating with 10Be and 26Al has been improved so that up to 250 g of sample can be decomposed reasonably safely. Because pyrophosphoric acid is more selective in dissolving the silicate minerals than the commonly used mixture of dilute hydrofluoric and nitric acids, the yield of quartz is relatively high. However, serious hazards arise from using large volumes of hot concentrated phosphoric acid to decompose rock. When the acid is heated to evaporate water and initiate the reaction, the crushed rock settles to the bottom of the vessel causing the mixture to superheat and "bump" severely. Then, as the reaction progresses, the mixture increases in viscosity and will form a solid gel if allowed to cool. Starting with a greater excess of phosphoric acid to reduce the viscosity is impractical for such large amounts of rock so, to avoid a solid gel at the end of the reaction, the mixture must be diluted with cold water while it is still near 250 °C. This step is particularly dangerous as the water at first instantaneously turns to steam that can eject gel resulting in serious thermal burns even through thick gloves. We have mitigated these hazards by keeping the sample suspended using a mixer throughout the procedure. There is no superheating at the start and water can be added slowly at the end thus quenching the reaction and dispersing the particulate matter. A hazardous reagent, boiling sodium hydroxide solution, has been eliminated. This document, which replaces GNS Science Report 2014/45, covers the safety features of the mixer-heater and outlines the procedure for decomposing rock and recovering pure quartz. (auth)