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 site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.The rock sample to be dated must be chosen very carefully.Any alteration or fracturing means that the potassium or the argon or both have been disturbed.These effects must be corrected, and the process is intricate enough to require computers.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.
With each increment in quality, more subtle sources of error have been found and taken into account.
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These steps help remove as much atmospheric Ar from the sample as possible before making the measurement.
Next, the mineral sample is heated to melting in a vacuum furnace, driving out all the gas.
Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.