Monzonite is a coarse-grained, intermediate intrusive igneous rock. Its composition lies between diorite and syenite and has roughly equal amounts of plagioclase and alkali feldspar. Also, this rock has minor mafic minerals like biotite, hornblende, or pyroxenes and may have a small amount of quartz.
Monzonite got its name from the Monzoni range in Italy, where the rock is widespread. However, it is no longer associated with this place. Some authors may refer to it as synenodiorite, an outdated synonym.
Since it doesn’t form plutons alone, monzonite is a less common plutonic rock. However, it is not so rare. It often occurs as small masses, part of acidic or granitic (granodiorite, granite, or tonalite) intrusion. Sometimes, it is associated with diorites and mafic intrusions like gabbro.
Although uncommon, many rocks use the ‘monzo’ name, including monzodiorite, monzogranite, foid monzosyenite, monzogabbro, etc. Its use implies that such a rock has a significant amount of plagioclase and feldspar.
Let us look at monzonite rock in detail, including its texture, color, chemical, and mineral composition. We will also discuss its varieties, its formation, where it is found, and some of its uses.
Quick overview and properties
- Name: Monzonite
- Rock type: Igneous
- Origin: plutonic or intrusive
- Colors: Whitish, gray (light to dark), pink, bronze, brown, reddish-brown or greenish
- Texture: Phaneritic or coarse-grained
- Chemical composition: Intermediate
- Silica content: 52-63 wt. %
- Mohs hardness scale: 6 to 7
- Volcanic equivalent: Latite
- Tectonic settings: Convergent plate tectonic boundaries, especially on continental arc margins above subduction zones. However, it can also occur in other tectonic settings.
What does monzonite look like?
In hand specimens, color, texture, and general appearance should help you easily identify this rock. It is a massive, coarse-grained, off-white, gray, pink, reddish, or bronze-hued rock. However, some may be porphyritic.
You cannot fail to see the predominantly light-colored grains and fewer dark ones. It looks slightly like granite but is darker, and the salt-and-pepper appearance isn’t as prominent.
Let us discuss more about the monzonite color and texture.
1. Colors
Monzonite is leucocratic to mesocratic rock with typical colors: off-white, gray, pink, reddish brown, and bronze, with some having a greenish hue. This rock has light-colored minerals (felsic) and dark (mafic), giving it a speckled appearance.
Minerals present and their colors determine the overall appearance of monzonites. The lighter colors, colorless, white, grayish, pink, reddish, or yellowish, come from felsic minerals, especially felspars like andesine, orthoclase, and oligoclase.
On the other hand, the opaques and mafic minerals give this rock darker-colored speckles. For instance, biotite is brown to black, hornblende greenish, brown or black, and augite dark green to black.
2. Texture
Monzonite is usually a massive rock with a coarse-grained or phaneritic texture. Its grains are mostly hypidiomorphic-granular. However, some can be porphyritic, and those with orbicular texture are less common.
Porphyritic monzonite will often have plagioclase (chemically zoned) and less often orthoclase or perthitic potassic felspar phenocryst, notes Blatt (2006) in a finer but phaneritic groundmass or matrix. Groundmass includes feldspars, mafic, and accessory minerals.
Monzonite composition
Monzonite has an intermediate composition. Here is its chemical and mineral composition.
Chemical composition
The chemical composition of monzonite is intermediate, with 52-63 wt % silica. It is moderate in alkali oxides (Na2O and K2O) and relatively lower in ferromagnesium (iron and magnesium oxide).
The average percentage weight chemical composition of monzonite from 336 samples, according to Le Maitre (1976), is SiO2: 62.60%, TiO2: 0.78%. Al2O3: 15.67%, Fe2O3: 1.92%, FeO: 3.08%, MnO: 0.10%, MgO: 2.02%, CaO: 4.17%, Na2O: 3.73%, K2O: 4.06% and P2O5: 0.25%
Monzonite mineral composition
Monzonite mineral composition is mainly a nearly equal amount of plagioclase and alkali feldspar. Also, it has little to no quartz, minor hornblende, biotite, and sometimes pyroxenes and olivine.
Accessory minerals include apatite, zircon, titanite (sphene), magnetite, ilmenite-hematite, allanite, and, less often, kaersutite.
The plagioclase present is sodic, usually oligoclase to andesine. It usually forms euhedral to subhedral crystals. On the other hand, alkali feldspar is orthoclase, less often microcline.
These feldspars may show perthitic (plagioclase exsolved in alkali feldspar) or antiperthitic (alkali feldspar in plagioclase) textures. Also, plagioclase phenocrysts may show chemical zoning while poikilitic (potassic feldspar enclosing plagioclase) texture does occur, but rarely.
Mafic content is mainly biotite (mica), and hornblende (amphibole), while pyroxenes are augite, diopside, or both. Sometimes, this rock may have olivine.
Secondary minerals from alteration include chlorite, epidote, kaolinite, calcite, serpentine, sericite, etc.
On the QAPF (Quartz, Alkali, Feldspar, and Feldspathoids) diagram for plutonic rocks, monzonite is a rock in which alkali feldspar is 35% -65% of total feldspar and quartz is less than 5% QAPF content by volume.
If quartz is 5-20% of the QAPF by volume, you will have a quartz monzonite, while foid-bearing monzonite will have up to 10% feldspathoids of the QAPF content by volume. Those with more than 10% feldspathoids are either foid monzosyenite or monzodiorite.
On the other hand, an increase in alkali grades monzonite into syenite, while a decrease makes it a monzodiorite.
How to name a monzonite rock
Monzonite names may have textural, minerals, chemical, and other generic prefixes. For instance, those with considerable olivine, hornblende, biotite, olivine, etc., or a porphyritic texture will have these prefixes as part of their name.
However, for the foid-bearing, you can use the name of the foid, such as leucite- or nepheline-bearing monzonite, if you know the exact foid.
Last but not least. Name those with more than two minerals in the increasing order of abundance, with a hyphen between them.
Varieties and related rocks
Some monzonite varieties and closely associated rocks we will briefly look at include larvikite, kentallenite, akerite, nepheline monzonite, and mangerite.
Here is what each is:
1. Larvikite
Larvikite is a variety of monzonite named after Larvik Batholith in Oslo, Norway, where it commonly occurs. It has thumb-sized, rhomb-shaped ternary feldspars. These feldspars’ polished surfaces have a silver-blue Schiller effect known as labradorescence.
The Schiller effect or labradorescence comes from microperthitic (micro-scale immiscible feldspars) texture with alternating plagioclase and alkali feldspar layers.
Compositionally, larvikite rock mainly has plagioclase and feldspar (nearly equal). Also, it has minor amounts of olivine, quartz, titanaugite, biotite, and amphibole.
Lastly, the common accessory minerals in this rock are apatite, zircon, and Fe-Ti-oxides (magnetite and ilmenite),
2. Kentallenite
Kentallenite is a local term for a melanocratic or dark variety of monzonite named Kentallen Quarry (UK). It has zoned plagioclase, interstitial alkali feldspar, and minor amounts of augite, olivine, biotite, and green mica.
3. Akerite
Akerite is a shared term for medium-grained monzonite and syenite (micromonzonite and microsyenite).
`These rocks have nearly equal alkali feldspar and rectangular oligoclase (plagioclase), with minor amounts of biotite, pyroxene, and quartz.
4. Nepheline monzonite
It refers to an alkalic plutonic rock with > 10% nepheline and approximately equal amounts of plagioclase and alkali feldspar. Also, this rock has minor amounts of mafic minerals.
5. Mangerite
Magnerite is a hypersthene-bearing monzonite named after Manger, Norway, where it occurs.
It has roughly equal amounts of plagioclase and alkali feldspar. Also, hypersthene, an orthorhombic or orthopyroxenes pyroxene, is the dominant mafic mineral that may be up to 10% of QAPF volume.
Lastly, this plutonic rock occurs with norite, charnockite, rapakivi granite, and anorthosite, especially in Proterozoic metamorphic belts.
How is monzonite formed?
Monzonite rock forms when magma of intermediate composition intrudes and slowly cools inside the Earth’s crust to form plutons.
The slow cooling allows ample time for forming large crystals visible with the unaided eye, i.e., coarse-grained texture.
Later, erosion and uplift are what expose some of these plutons.
Monzonite magma originates from fractional crystallization of mantle-derived magma. Also, it may form from anatexis lower crustal alkali gabbroic/basaltic rocks.
However, contamination and magma mixing may contribute to the formation of these intermediate magmas.
Where is monzonite found?
Monzonite rock occurs in batholiths, stocks, and sometimes other plutons like dikes, sills, lopoliths, etc.
It is commonly found as small masses in granitic rocks, norite, quartz-diorite, and diorite. Also, it may sometimes be part of mafic rocks like gabbro as minor components or in smaller masses.
This rock is mainly found on continents at convergent plate boundaries, especially on active continental arc margins. However, smaller amounts may occur in other tectonic settings, including extensional crustal zones and islands.
Also, most of the large orogenic belt batholiths have calc-alkaline suites, which include granite, tonalite, granodiorite, gabbro, and monzonitic rocks.
Some places with monzonitic rocks in the US include Coastal Maine Magmatic Province, Bingham Copper Mine (Utah), Pikes Peak batholith (Colorado), and Red Mountain pluton (Alaska).
In Canada, this rock occurs in the White Mountains—Monteregian Hills belt, Central Gneiss Belt, and Nain Plutonic Suite.
More places with this rock include the Alborz Mountains (Iran), the Paleoproterozoic North China craton, and the Larvik plutonic complex (Norway).
Some islands with this rock include Fiji, which has monzonite Tuvatu Mine. Also, the Islands of the Koro Sea have tholeiitic, calc-alkaline to alkali basalt, and shoshonites, which have mainly basalt. However, some have olivine monzonite, tonalite, hornblende, andesite.
Furthermore, according to (Gill 2010, p. 275), “granites (and associated monzonites and syenites) may intrude into the continental crust thinned significantly by passive-margin extensions, such as the Paleogene granites of E Greenland and NW Scotland.”
Lastly, monzonite fragments occur on the moon, where they formed most likely via silicate liquid immiscibility.
Monzonite uses
In the stone market, vendors refer to monzonite and related rocks like granite, syenite, granodiorite, larvikite, anorthosite, and gneiss as commercial granite. Its durability and hardness are comparable to granite. However, it isn’t as readily available or widespread as the latter.
Common uses of monzonite include making aggregate for construction, subbase and base fill, and dimensional stones for building houses, monuments, retaining walls, paving, curbing, etc. Also, cut and polished stones can make countertops, slabs, paving tiles, windowsills, stair treads, etc.
More monzonite uses include making sculptures and carvings. You can also use it in landscaping, riprap, or gravel on unpaved pathways, walkways, patios, etc.
Besides the above uses, in rare cases, monzonite may host valuable ores like copper, gold, zinc, and silver deposits, including copper porphyry.
These hydrothermal-formed ores are associated with monzonite or diorite intrusions. For example, some of these intrusions will have closely packed quartz veinlets, chalcopyrite, pyrite, bornite, and less amounts of sphalerite and molybdenite.
Chalcopyrite and bornite are copper ores copper, pyrite gold, sphalerite zinc, and molybdenite molybdenum. Kennecott Copper Mine in Utah, USA, is an example of porphyry copper deposits.
Frequently asked questions
Monzonite is a mesocratic, intermediate rock with nearly equal plagioclase and alkali feldspar. Also, it has little to little quartz (< 5 of QAPF vol. %) and more mafic minerals than granite. In contrast, granite is a light-colored or felsic rock with more alkali feldspar than plagioclase, more quartz (20-60 of QAPF vol %.), and fewer dark minerals.
Chemically, monzonite has less silica (52-63 wt. %), lower alkali oxides (Na2O and K2O), and more mafic oxides (MgO and FeO), while granite has more silica (> 69%) and alkali oxides and less mafic components.
References
- Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Parsons, I. (1989). Monzonite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 389-390). New York: Van Nostrand Reinhold.
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press
- Haldar, S. K., & Tisľjar, J. (2014). Introduction to Minerology and petrology (1st ed.). Elsevier.
- Klein, C., & Philpotts, A. R. (2017). Earth materials: Introduction to mineralogy and petrology (2nd ed.). Cambridge University Press.
- Le Maitre, R. W. (1976). The chemical variability of some common igneous rocks. Journal of Petrology, 17(4), 589–598. https://doi.org/10.1093/petrology/17.4.589