Diagnostic Features
Hardness: 6
Streak: White to none
Specific Gravity: 2.7
Luster: Nonmetallic
Cleavage: Present in 2 nearly orthogonal directions
Fracture: Conchoidal (rounded)
Other: Plagioclase crystals are generally white, with a penetrating fabric parallel to the cleavage planes. It is harder than glass.
Identification Tips
1. Plagioclase resembles orthoclase feldspar in nearly every respect. Plagioclase generally tends to vary from white to gray whereas orthoclase is generally tan to salmon.
2. Plagioclase is a principal constituent of mafic igneous rocks such as gabbro whereas orthoclase is a principal constituent of felsic igneous rocks such as granite.
