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Clay minerals are less than 2 μm thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice.
In 2013, the K–Ar method was used by the Mars Curiosity rover to date a rock on the Martian surface, the first time a rock has been dated from its mineral ingredients while situated on another planet.
It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar).
To obtain the content ratio of isotopes in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is volatilized in vacuum.
The potassium is quantified by flame photometry or atomic absorption spectroscopy.
Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites.
In these materials, the decay product is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes).