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But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.
Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.That is, a fresh mineral grain has its K-Ar "clock" set at zero.
Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.