Question

Difference between perthite and antiperthite

Answer 1

Perthite is K-feldspar with small discrete albitic plagioclase that has become ex-solved from the K-feldspar. Antiperthite is Albitic plagioclase with small discrete K-feldspar that has become ex-solved from the albite. 

To distinguish the difference between perthite & antiperthite is to determine which is the predominant feldspar. If K-feldspar predominates, it is a perthite. If Albite predominates, it is an antiperthite. It then becomes a matter of distinguishing K-feldspar from albite i.e. narrowness of twinning, shapes of crystal laths etc.