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Three isotopes of potassium exist and tend to occur in a known fixed ratio in mineral‐forming environments. The stable isotopes potassium‐39 and potassium‐41 constitute 93.25 815 and 6.73 025% of all potassium atoms. Radioactive potassium‐40 contributes only 0.0117% of all potassium atoms. The rarity of potassium‐40 means that its initial abundance in minerals or rocks must generally calculated from its known ratio to the other two isotopes that are much easier to measure accurately.

The abundance of argon‐40 in the sample is determined by mass spectrometry. The amount of potassium‐40 remaining in the sample is calculated by subtracting the amount of argon‐40 that has accumulated and the corresponding amount of calcium‐40 that would have formed in the double decay of potassium‐40. On the assumption that no argon‐40 existed in the sample at the time it was formed and that the mineral has behaved as a closed system with no gain or loss of argon‐40, the age of the sample can be calculated. Of course this assumption is not always valid. Two processes that produce excess argon‐40 and therefore anomalously old ages are (1) the incorporation of mantle xenoliths and xenocrysts that contain ungassed argon‐40 and (2) younger lavas that contain argon‐40 bearing gas‐bubble/vesicles. Processes that cause argon‐40 to leak from rocks and minerals, which produces anomalously young ages, include subsequent heating and alteration. For these reasons, care is taken to select samples that are unaltered and unfractured. This often requires extensive sample searching and preparation. Potassium feldspar (sanidine, orthoclase, and microcline) is the most commonly used mineral group, but biotite, muscovite, and illite clays can also be dated.