Feldspars

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Mineral class: Tectosilicate
Composition: Feldspars are aluminum silicates with varying proportions of potassium (K), sodium (Na) or calcium (Ca). There are two broad categories of feldspars:
Potassium feldspars, KAlSi₃O₈. Potassium feldspars are polymorphic, displaying different crystal forms despite having the same chemical composition. They are also called potash or alkali feldspars.
Orthoclase. Usually colorless or white. Monoclinic crystals. When mixed with quartz and kaolin fuses at a relatively low temperature to form porcelain.
Microcline. Triclinic crystals. White or pink colors are common. A blue-green variety is called Amazonite, found in Brazil, India, and at Pike's Peak, Colorado.

Plagioclase feldspars are sodium/potassium aluminum silicates, with potassium and sodium ions instead of calcium. They form a solution series from Albite (all sodium) to Anorthite (all calcium), where intermediate members of the series each have increasing concentrations of calcium and decreasing amounts of sodium:
NaAlSi₃O₈ = Sodium feldspar is Albite.
Oligoclase, (Na, Ca)AlSi₃O₈
Andesine, (Na, Ca)AlSi₃O₈
Labradorite, (Ca, Na)AlSi₃O₈
Bytownite, (Ca, Na)AlSi₃O₈
CaAl₂Si₂O₈ = Calcium feldspar is Anorthite.

Hardness: 6
Color: White, pink, grey, green
Streak: White
Cleavage: 2 directions at 90 degree angles. Orthoclase cleavage surfaces are striated; plagioclase surfaces are not striated.
Environment: Feldspars account for 60 percent of the continental crust, but are virtually absent from ocean beds. Common in stream gravel.
Comments: Scratched by quartz but not by knife.
At the surface of the Earth, particularly in warm, humid environments, feldspars weather (by hydrolysis) to form Kaolin, a fine-grained, white clay which is the main ingredient of Kaopectate. The large positive ions of the feldspars (K⁺, Na⁺, Ca2⁺) are replaced by hydrogen ions (H⁺) from water, yielding Al₂Si₂O₅(OH)₄. Kaolin is found as bedded masses in soils and sedimentary rocks. It is soft (Mohs 2.5 or less) and has an earthy odor, particularly if moistened.

Mineral class:	Tectosilicate
Composition:	Feldspars are aluminum silicates with varying proportions of potassium (K), sodium (Na) or calcium (Ca). There are two broad categories of feldspars: Potassium feldspars, KAlSi₃O₈. Potassium feldspars are polymorphic, displaying different crystal forms despite having the same chemical composition. They are also called potash or alkali feldspars. Orthoclase. Usually colorless or white. Monoclinic crystals. When mixed with quartz and kaolin fuses at a relatively low temperature to form porcelain. Microcline. Triclinic crystals. White or pink colors are common. A blue-green variety is called Amazonite, found in Brazil, India, and at Pike's Peak, Colorado. Plagioclase feldspars are sodium/potassium aluminum silicates, with potassium and sodium ions instead of calcium. They form a solution series from Albite (all sodium) to Anorthite (all calcium), where intermediate members of the series each have increasing concentrations of calcium and decreasing amounts of sodium: NaAlSi₃O₈ = Sodium feldspar is Albite. Oligoclase, (Na, Ca)AlSi₃O₈ Andesine, (Na, Ca)AlSi₃O₈ Labradorite, (Ca, Na)AlSi₃O₈ Bytownite, (Ca, Na)AlSi₃O₈ CaAl₂Si₂O₈ = Calcium feldspar is Anorthite.
Hardness:	6
Color:	White, pink, grey, green
Streak:	White
Cleavage:	2 directions at 90 degree angles. Orthoclase cleavage surfaces are striated; plagioclase surfaces are not striated.
Environment:	Feldspars account for 60 percent of the continental crust, but are virtually absent from ocean beds. Common in stream gravel.
Comments:	Scratched by quartz but not by knife. At the surface of the Earth, particularly in warm, humid environments, feldspars weather (by hydrolysis) to form Kaolin, a fine-grained, white clay which is the main ingredient of Kaopectate. The large positive ions of the feldspars (K⁺, Na⁺, Ca2⁺) are replaced by hydrogen ions (H⁺) from water, yielding Al₂Si₂O₅(OH)₄. Kaolin is found as bedded masses in soils and sedimentary rocks. It is soft (Mohs 2.5 or less) and has an earthy odor, particularly if moistened.