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 ...
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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") and "kivi" (meaning "rock"), because the different heat expansion coefficients of the component minerals make exposed rapakivi crumble easily into sand.
Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891. Since then, southern Finland´s rapakivi granite intrusions have been the type locality of this variety of granite.
