Twinning and exsolution in an antiperthite

An antiperthite feldspar (composition of the main part An27.2 Ab69.2Or3.6) has been studied by x-rays and transmission electron microscopy. Complex twinning and exsolution on very fine scale are described for the first time for this compositional range. Evidence is given for a distinct intermediate region between the plagioclase and the potash feldspar.