Potassium is a common element found in many materials, such as feldspars, micas, clay minerals, tephra, and evaporites.
The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors.
The long half-life of 40K allows the method to be used to calculate the absolute age of samples older than a few thousand years.
[3] Argon, being a noble gas, is a minor component of most rock samples of geochronological interest: It does not bind with other atoms in a crystal lattice.
Despite 40Ca being the favored daughter nuclide, it is rarely useful in dating because calcium is so common in the crust, with 40Ca being the most abundant isotope.
Thus, the amount of calcium originally present is not known and can vary enough to confound measurements of the small increases produced by radioactive decay.
In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required.
Ar–Ar dating is a similar technique that compares isotopic ratios from the same portion of the sample to avoid this problem.
[10] Clay minerals are less than 2 μm thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice.