Crystallization of alkalic feldspar phenocrysts and origin of perthites in granitoids (as in certain intrusives in Northeast USSR)

Magmatic origin of large (up to 5 cm) phenocrysts of K-feldspar is indicated by mineralogical, chemical, and roentgenometric-structural analysis of the crystals. Perthites are found to be metasomatic products of replacement of plagioclase by K-feldspar. This reaction is accompanied by albitization of plagioclase and then by growth of albite after feldspar, in cleavage planes and elsewhere, or by spot-segregations of albite in places formerly occupied by plagioclase. — V.P. Sokoloff