Plagioclase Feldspar

Description: A group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium is called plagioclase feldspar. Members of the plagioclase group are the most common rock-forming minerals. They are important to dominant minerals in most igneous rocks and important constituents of many metamorphic rocks, such as gneiss.

Chemical Formula: Ranges from NaAlSi3O8 (Albite) to CaAl2Si2O8 (Anorthite)

Crystal Form: Rare, varies from tabular and parallel in Albite composition to short and prismatic in Anorthite.

Crystal System: Triclinic

Color: Typically white to gray, may also range from colorless, through shades of off-white, to yellowish, pink, reddish brown or nearly black.

Streak: Colorless (harder than the streak plate)

Luster: Crystals are vitreous (glass-like) to porcelaneous (porcelain-like)

Fracture: Conchoidal

Cleavage: One perfect cleavage and one good cleavage that meet at nearly 90° (right angles)

Hardness: 6 to 6.5 (harder than glass)

Density/Specific Gravity: Ranges from 2.6 - 2.8 g/cm3, depending on the amount of calcium.

Magnetism: N/A

Taste: N/A

Hydrochloric acid: N/A

Radioactivity: N/A

Fluorescence: N/A

Distinguishing Physical Properties:

  • Cleavage: Perfect cleavage, with cleavage faces intersecting at right angles

  • Appearance: Striations often present on cleavage faces

  • Luster: Commonly milky, pearly, translucent luster

  • Crystal Form: Again, well-defined crystals are extremely rare, usually plagioclase is distributed unobviously within rocks.

Photograph Attribution: Plagioclase Feldspar specimen photograph by Sean C. Murphy, 2020.