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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.

Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C.

Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures.

These steps help remove as much atmospheric Next, ​the mineral sample is heated to melting in a vacuum furnace, driving out all the gas.

A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.

With each increment in quality, more subtle sources of error have been found and taken into account.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.Then the gas sample is cleaned of all unwanted gasses such as H A variant of the K-Ar method gives better data by making the overall measurement process simpler.The key is to put the mineral sample in a neutron beam, which converts potassium-39 into argon-39.

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