Minerals Topics

 

Major Mineral Groups (Part 5)

Silicates (continued: the feldspars)

Plagioclase Feldspar Pronounce:

• Plagioclase feldspars occur in several varieties, including albite (NaAlSi3O2) and anorthite (CaAl2Si2O2).

• Plagioclase feldspar is identified by its non-metallic, vitreous to pearly luster; white to dark gray color; and two good cleavages at nearly 90o.  Plagioclase feldspar has striations on some cleavage surfaces.

• Its hardness is 6.

• Its specific gravity is 2.6 to 2.8.

• Plagioclase feldspar is used in ceramics, glass, enamel, false teeth, and scouring powders.

Plagioclase

Plagioclase feldspars are made up of sodium, calcium, aluminum, silicon, and oxygen.  Albite is a white plagioclase feldspar.  Anorthite is a dark gray plagioclase feldspar.  These occur in igneous and metamorphic rocks. (Photograph by Parvinder Sethi)

Orthoclase Feldspar Pronounce:

• Orthoclase feldspar has a composition of KAlSi3O8.

• Orthoclase feldspar is identified by its non-metallic, vitreous luster; white, pink, or blue-green color; and two good cleavages at nearly 90o. 

• Its hardness is 6.

• Its specific gravity is 2.6.

• Orthoclase feldspar is used in ceramics, glass, and enamel.

Orthoclase

Orthoclase feldspars are made up of potassium, aluminum, silicon, and oxygen.  They occur as white to salmon pink crystals in igneous and metamorphic rocks. (Photograph by Parvinder Sethi)



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