First, the sample is sieved to obtain the target grain size (coarse grains: 90 - 200 µm; fine grains: 4 – 11 µm). Subsequently, it is treated with HCl and H2O2 to destroy carbonates and oxidise organic matter.
a) Coarse grains:
Heavy liquids are used to separate quartz from feldspar and heavy mineral grains, which are characterised by different mineral densities. Obtained quartz grains are then treated with HF to remove the grains’ outer layer of ca. 20 µm, which allows neglecting the alpha-irradiation in the dose rate calculations. Subsequently, etching with HCl is necessary to eliminate fluorides that may have precipitated.
b) Fine grains:
Stoke’s Law is applied to obtain grains in the target range 4-11 µm. This law describes the varying settling rates in fluids for grains of different diameter. The result is a polymineral mixture. To obtain pure quartz, this mixture is treated with H2SiF6 which dissolves everything else than quartz. This last step can go along with an 80% loss of sample material.