Photographed by Michael P. Klimetz

[var. Larvikite]

Oslo Graben

Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals. Quartz a minor constituent or is absent, although with greater than 10% quartz the rock is termed a quartz monzonite. If the rock has more orthoclase or potassium feldspar it grades into a syenite. With an increase of calcic plagioclase and mafic minerals the rock type becomes a diorite. The volcanic equivalent is a latite.