Granitic or syenite rocks tend to have more sodium-rich plagioclase. Plagioclase does not survive extensive weathering, generally breaking down to clay minerals. Some plagioclase, however, may occur in greywackes or arkoses formed by rapid erosion and burial of material. Metamorphic rocks may be rich in plagioclase, particular gneisses and granulite and amphibolites. Nearly pure calcium-rich plagioclase may occur in some marbles.
Space permits only a few of the many occurrence of plagioclase in Wisconsin to be reported below.
Plagioclase is abundant in the Mineral Lake Intrusion of the Mellen complex, associated with olivine and pyroxene. Local coarse pegmatitic layers exist, such as in a quarry in the NW NW sec. 4 T.44N. R.3W, in SE sec. 11 T.44N. R.3W., west of Mellen. Good exposures also occur in roadcuts along a gravel road in SE NE sec.