K—Ar dating was instrumental in the development of the geomagnetic polarity time scale. 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. When 40 K decays to 40 Ar argon , the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal.
Potassium naturally occurs in 3 isotopes: Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar accumulated to the amount of 40 K remaining. In these materials, the decay product 40 Ar is able to escape the liquid molten rock, but starts to accumulate when the rock solidifies recrystallizes. 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. Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample.