Monzonite
Intermediate intrusive igneous rock (roughly equal alkali feldspar and plagioclase, little to no quartz)
An intermediate plutonic rock with nearly equal alkali and plagioclase feldspar and very little quartz, sitting between diorite and syenite.
- Mohs hardness
- 6-7
- Color
- Grey to pinkish-grey, often speckled black and white
- Type
- igneous
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Overview
Monzonite is a coarse-grained, intermediate intrusive igneous rock named after Monzoni in the Italian Dolomites. It is defined by roughly equal proportions of alkali feldspar and plagioclase feldspar, with quartz making up less than about 5% of the felsic minerals.
Where quartz rises to 5-20%, the rock is called quartz monzonite (older texts call this adamellite). Mafic minerals such as biotite, hornblende, and pyroxene give the rock its characteristic salt-and-pepper appearance.
Monzonite is the plutonic equivalent of the volcanic rock latite, and grades into diorite as plagioclase dominates and into syenite as alkali feldspar dominates.
Formation & geology
Monzonite crystallizes slowly from intermediate magma deep within the crust, allowing the interlocking, visible crystals typical of plutonic rocks. It commonly forms in continental-margin and subduction-related settings as part of large batholiths and stocks, often associated with diorite, granodiorite, and syenite.
Its intermediate chemistry reflects magmas that have differentiated from more mafic parents or assimilated crustal material. Many monzonite bodies are economically important because related fluids can carry copper, gold, and molybdenum, producing porphyry-style ore deposits.
How to identify it
Look for a coarse, evenly crystalline rock with a speckled grey-to-pink color and abundant feldspar but little or no glassy quartz. The two feldspars (white-to-grey plagioclase and pink-to-cream alkali feldspar) occur in roughly equal amounts.
Hardness is dominated by feldspar (about 6) with any quartz at 7. Distinguish from granite by its scarcity of quartz, from syenite by the presence of abundant plagioclase, and from diorite by the significant alkali feldspar content. A hand lens and a quartz-versus-feldspar check are the key field tests.
Uses & significance
Monzonite is quarried as a durable construction and dimension stone for aggregate, road base, facing stone, and countertops. Its main economic significance, however, is geological: monzonite and quartz monzonite intrusions host many of the world's porphyry copper-molybdenum-gold deposits, making them prime exploration targets.
It carries no traditional gemstone or metaphysical role, but polished slabs are valued for their attractive speckled texture.
Frequently asked questions
What is the difference between monzonite and granite?
Granite is quartz-rich (over 20% quartz), while monzonite contains little to no quartz and has roughly equal alkali and plagioclase feldspar.
Is monzonite the same as latite?
Chemically yes; latite is the fine-grained volcanic equivalent of monzonite, formed when the same magma erupts and cools quickly.
What is quartz monzonite?
It is monzonite with 5-20% quartz, transitional toward granite; older literature called it adamellite.
Why is monzonite important to mining?
Monzonite and quartz monzonite intrusions frequently host porphyry copper, molybdenum, and gold ore bodies.
Monzonite guides
In-depth guides for identifying, valuing, and understanding Monzonite.
Monzonite identified by the community
Real specimens identified with Rock Identifier.
