Monzonite specimen from Rock Library (NASA JPL)
|Mostly plagioclase and alkali feldspar|
Monzonite is an igneous intrusive rock. It is composed of approximately equal amounts of plagioclase and alkali feldspar, with less than 5% quartz by weight. It may contain minor amounts of hornblende, biotite and other minerals. If quartz constitutes greater than 5%, the rock is termed a quartz monzonite.
If the rock has a greater percentage of alkali feldspar, it grades into a syenite. With an increase in calcic plagioclase and mafic minerals the rock type becomes a diorite. The volcanic equivalent is the latite.
Monzonite was originally named after the Monzoni range in Val di Fassa (Trento Province - Italy) where it is abundant. As rock definitions have been systematized and codified, this association has lost any relevance to the rock's definition.