| Igneous rock | |
 | |
| Composition | |
|---|---|
| Primary | Alkali feldspar; Quartz |
| Secondary | Biotite; Plagioclase; Hornblende |
Rapakivi granite is an igneous intrusive rock and variant of alkali feldspar granite. It is characterized by large, rounded crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). Common mineral components include hornblende and biotite. The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic (intrusive) 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]
- ^ "Definition of RAPAKIVI".
- ^ Tietoaineistot – maaperäkartan käyttöopas – rapautuminen – GTK
- ^ "Ueber die finnländischen Rapakiwigesteine
- ^ "3000 miljoonaa vuotta, Suomen Kallioperä" Finnish geological society, 1998, chapter 9, ISBN 952-90-9260-1 . Language: Finnish.