The difference between potassium feldspar and plagioclase

Date: Feb. 22, 2022

The plagioclase has better semihedral or euhedral crystals than the potassium feldspar, so the plagioclase is mostly columnar or long plate, while the potassium feldspar is mostly other-shaped columnar. Especially in the same rock, this law is more clear at a glance.

Basic plagioclase is generally narrow plate; medium acid plagioclase is generally wide plate; potassium feldspar is granular or rectangular.

The most reliable identification marks for potassium feldspar

Step-like obvious cleavage;

It has Kasbah double crystals, which is shown as a common single rectangular particle showing different light and dark halves under reflected light, and there seems to be a straight line in the middle;

It has a striped structure, and these stripes are arranged in a slightly oriented crystal and are also distributed in an irregular water system. The color of the stripes is often lighter than that of the main feldspar.

Identification features of plagioclase

It has a straight and smooth cleavage surface, and the step shape is not obvious;

Polylamellar twinning is sometimes seen, that is, narrow fissures parallel to the long axis are observed on the smooth cleavage plane of the plagioclase.