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One of the isotopes of potassium, 40 K, decays partly by electron capture (a proton becomes a neutron) to an isotope of the gaseous element argon, 40 Ar, the other product being an isotope of calcium, 40 Ca.
In cases where particular minerals are to be dated, these are separated from the other minerals by using heavy liquids (liquids with densities similar to that of the minerals) in which some minerals will float and others sink, or magnetic separation using the different magnetic properties of minerals.
The mineral concentrate may then be dissolved for isotopic or elemental analysis, except for argon isotope analysis, in which case the mineral grains are heated in a vacuum and the composition of the argon gas driven off is measured directly.
Radiometric dating uses the decay of isotopes of elements present in minerals as a measure of the age of the rock: to do this, the rate of decay must be known, the proportion of different isotopes present when the mineral formed has to be assumed, and the proportions of different isotopes present today must be measured.
This dating method is principally used for determining the age of formation of igneous rocks, including volcanic units that occur within sedimentary strata.
This may not always be the case because addition or loss of isotopes can occur during weathering, diagenesis and metamorphism and this will lead to errors in the calculation of the age.
It is therefore important to try to ensure that decay has taken place in a 'closed system', with no loss or addition of isotopes, by using only unweathered and unaltered material in analyses.
Measurement of the concentrations of different isotopes is carried out with a mass spectrometer.
In these instruments a small amount (micrograms) of the sample is heated in a vacuum to ionise the isotopes and these charged particles are then accelerated along a tube in a vacuum by a potential difference.
The decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years.