In the early 1950s, when the potassium-argon (parent-daughter) method
was being developed by scientists at the University of Chicago, it was
thought that the potash-bearing variety of the mineral feldspar would be
an ideal closed system, because it was usually optically clear and free
of flaws. This widely shared, logical, and perfectly scientific
deduction soon turned out to be quite wrong. The scientific workers
discovered that when feldspar and mica from the same rock (and thus of
the same age) were analyzed side by side, the mica always came out
older! Investigation showed that feldspar “leaked” argon (lost some of
its radiogenic argon) even at room temperature, but the mica retained
all or nearly all of the argon that had been generated in it.