Feldspar is a group of framework silicate minerals important in planetary crust. Common ideal endmembers include albite, , and calcium- and potassium-bearing compositions. Coupled substitutions preserve electrical charge while changing elemental proportions, so a natural feldspar need not have one composition shared by every specimen.
Plagioclase denotes the sodium-calcium solid-solution series. Composition and crystal structure influence a rock's physical properties and the interpretation of mineral observations. In chemical equilibrium calculations, an endmember formula is a useful component definition; it should not automatically be substituted for the full composition of a natural mixed crystal.
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