K-feldspar is the final feldspar to crystallize from the magma. ![]() This defines the continuous Bowen's reaction series. Calcium-rich plagioclase is the first feldspar to crystallize from a cooling magma, but the plagioclase becomes increasingly sodium-rich as crystallization continues. The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of quartz, is the basis for the QAPF classification of igneous rock. Albite is considered both a plagioclase and alkali feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the Earth. Solid solutions between albite and anorthite are called plagioclase, or, more properly, plagioclase feldspar. ![]() ![]() Solid solutions between K-feldspar and albite are called alkali feldspar. potassium feldspar (K-spar) endmember K Al Si 3 O 8.Compositions of major elements in common feldspars can be expressed in terms of three endmembers: ![]() The feldspar group of minerals consists of tectosilicates, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network.
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