Silicates: Plagioclase Feldspars

Composition: NaAlSi₃O₈-CaAl₂Si₂O₈

Crystal system: Triclinic; crystals tabular, also as irregular grains or cleavable masses

5.21.1
cleavable plagioclase
NC USA

Properties:

• Vitreous to silky lustre
• White to grey color; also blue to green
• White streak
• Knife does not scratch (Mohs Hardness 6)
• Average density
• Two perfect cleavages at close to 90^o
• Cleaves rather than fractures
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Feldspars are one of the most common minerals in the earth's crust . Plagioclase occurs mainly in mafic and intermediate igneous rocks such as basalt and andesite.

5.21.2
cleavable plagioclase
NC USA

Minor colorless or white plagioclase intergrown with pink microcline common in felsic igneous rocks is termed perthite. The plagioclase feldspars are distinguished from the alkali feldspars by the common presence of perthite structure in the latter and the ubiquitous presence of lamellar twinning in the former. Specialised characterisation is often required to distinguish the plagioclase feldspar varieties.

Feldspars are important industrial minerals used mainly in the manufacture of glass and ceramics.