A radiometric dating method based on the decay of 40K to stable 40Ca. This is not a generally useful technique because 40Ca is the most abundant naturally occurring stable isotope of calcium (96.94%). The formation of radiogenic 40Ca atoms in a rock or mineral therefore increases its abundance only slightly. The ratio 40Ca:44Ca (96.94%:2.08%) can be used to determine the amount of radiogenic 40Ca present, although the dominance of naturally occurring 40Ca makes it rather insensitive. Furthermore the determination of the isotopic composition of calcium by mass spectrometry is made difficult by the low efficiency of ionization of calcium atoms in a thermionic source, and by fractionation of isotopes during that process. Because of these disadvantages the 40K:40Ca method of dating is really viable only for minerals that are strongly enriched in potassium and depleted in calcium, such as micas in pegmatite and sylvite in evaporite rocks.