Specimens Orthoclase is a polymorph of other minerals that share the same chemistry, but have different crystal structures. If positive identification between these minerals can not be made by field methods, then the specimen may simply be referred to as a potassium feldspar or K-spar .
Plagioclase feldspars lack potassium, are light colored and are usually striated.
The other k-spar minerals are sanidine , microcline and anorthoclase . Orthoclase is the more common of the k-spars. The differences between these minerals are minor in hand samples. Orthoclase does not show the lamellar twinning that is common in microcline and is occassionally present as striations on cleavage surfaces. Sanidine and anorthoclase usually have a flattened crystal habit.