Any alteration or fracturing means that the potassium or the argon or both have been disturbed.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.
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.
One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits.
Clay minerals are less than 2 micrometres thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice.
After the recrystallization of magma, more Ca being the most abundant isotope.