San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide
Orthoclase feldspar.  Collection of the San Diego Natural History Museum.  Photo credit: Tim Murray.

Feldspars, Potash

Feldspar is from the obsolete German word feldspath, feld meaning "field" and spath meaning "spar."

Description and Occurrence

Feldspar is a large group of aluminum silicate minerals that can be divided into two subgroups: orthoclase, which are the potash (potassium aluminium silicates and includes orthoclase, microline and adularia, and plagioclase, which are the sodium and calcium aluminum silicates and includes oliogoclase, albite, and labradorite. Both types usually forms as prismatic or tabular crystals, often twinned. Other crystal habits include massive, lamellar, and granular. Feldspars may be white, coloress, pale yellow, reddish, pink, blue, or gray.

Feldspars are the most abundant of all minerals and make up nearly half of the Earth's crust. They are an important material of many igneous and metamorphic rocks. Orthoclase commonly occurs in granite, and large crystals are common in pegmatites.

Orthoclase is used widely in porcelain and glass manufacturing.

Field Notes: Feldspar cleaves in two directions at 90°. Twinned crystals are common.


Physical Properties

Color Streak Transparency Luster Hardness Cleavage Fracture Specific gravity Crystal form
white, colorless,pale yellow, reddish, pink, blue, or gray white transparent to translucent vitreous 6-6.5 good, in two directions at 90° uneven, conchoidal 2.5-2.6 monoclinic

Photo: Feldspar from Oceanview Mine, Pala, California.
Courtesy of Jeff Swanger, Gem and Mineral Exploration Company
Photo credit: Walter Mroch, Gem and Mineral Exploration Company

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