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Potassium 40 dating range
The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.
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 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.
The atoms in some chemical elements have different forms, called isotopes.
Potassium-40 on the other hand has a half like of 1.25 billion years and is common in rocks and minerals.
With each increment in quality, more subtle sources of error have been found and taken into account.
So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.
These effects must be corrected, and the process is intricate enough to require computers.
Ar-Ar analyses cost around 00 per sample and take several weeks.