Alkali feldspar

Alkali feldspars are feldspar minerals whose major cations are potassium and sodium. Their compositions and crystal structures vary with temperature, cooling history, and exsolution.

Feldspar: A group of rock-forming tectosilicate minerals containing aluminum and calcium, sodium, or potassium. Alkali feldspar is one of feldspar’s major compositional branches.
Solid solution: A crystalline phase in which one or more elements substitute for others within a shared structure. Potassium and sodium substitute across much of the alkali-feldspar composition range.
Granite: A coarse-grained intrusive igneous rock composed mainly of quartz, feldspar, and minor minerals. Alkali feldspar is a common major constituent of granite.
Plagioclase: A feldspar series ranging from sodium-rich albite to calcium-rich anorthite. Unlike alkali feldspar, plagioclase spans sodium–calcium compositions.
Tectosilicate: A silicate mineral whose silicon-oxygen tetrahedra form a three-dimensional framework. Feldspar’s framework structure underlies its cleavage and broad chemical substitutions.
Alkali feldspar series: The compositional series between potassium-rich and sodium-rich feldspar end-members. It describes the chemical range occupied by alkali feldspars.
Rhyolite: A fine-grained, silica-rich volcanic rock with a composition comparable to granite. Its alkali feldspar phenocrysts can reveal crystallization before eruption.
Orthoclase–microcline distinction: The distinction between monoclinic orthoclase and triclinic microcline, both potassium feldspars. Their shared chemistry can obscure their different crystal symmetries.
Orthoclase: A potassium-rich monoclinic feldspar, commonly found in granites and other felsic rocks. It is a familiar potassium-rich member of the alkali-feldspar group.
Perthite: An intergrowth of potassium-rich feldspar and albite formed by unmixing within an alkali feldspar. It is a visible record of sodium–potassium separation during cooling.

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