However, if you put it near the core of a nuclear reactor, so that it is bombarded by neutrons, then this will convert it into Ar.
The problem is not universal, as the majority of minerals and rocks dated by K-Ar do not contain the excess argon.
A second problem is that for technical reasons, the measurement of argon and the measurement of potassium have to be made on two different samples, because each measurement requires the destruction of the sample.
If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.
Consequently, the amount of it found in rocks is negligible — unless you subject them to an artificial neutron source.
A crucial point to note is that because K are isotopes of the same element, they have the same chemical properties.