Rapakivi granite

Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic rocks. Rapakivi is a Finnish compound of "rapa" (meaning "mud" or "sand", while rapautua means "to erode") and "kivi" (meaning "rock"),^[1] because the different heat expansion coefficients of the component minerals make exposed rapakivi crumble easily into sand.^[2]

Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891.^[3] Since then, southern Finland's rapakivi granite intrusions have been the type locality of this variety of granite.^[4]