The Only Sure Way to Identify a Plagioclase Feldspar from a Potassium

Multiple Choice

The only sure way to identify a plagioclase feldspar from a potassium feldspar on large enough specimen to see with the naked eye is to____________ .

A) find wormy exsolution lamellae going obliquely across the cleavages
B) find multiple parallel twinning striations on a cleavage face
C) compare their colours, plagioclase is always darker
D) measure their exact cleavage angles, plagioclases have 120°