The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried.Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.
The second method is called absolute dating and is done by analysing the amount of radioactive decay in the minerals of the rocks.
Scientists find out the age of a dinosaur fossil by dating not only the rocks in which it lies, but those below and above it.
Then the gas sample is cleaned of all unwanted gasses such as H, nitrogen and so on until all that remains are the inert gasses, argon among them.
Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.
K-Ar analyses cost several hundred dollars per sample and take a week or two.
A variant of the K-Ar method gives better data by making the overall measurement process simpler.
The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.
But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.
The learning curve has been long and is far from over today.
With each increment in quality, more subtle sources of error have been found and taken into account.
Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.