= Feldspar
{wiki}
<Feldspar> is a group of framework <silicate minerals> important in planetary <crust>. Common ideal endmembers include <albite>, $\mathrm{NaAlSi_3O_8}$, 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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