Norite is a coarse-grained, dark-colored plutonic rock of mafic composition. Mafic refers to minerals or rocks rich in iron and magnesium.
On the other hand, plutonic or intrusive igneous rocks form from the slow cooling of magma deep inside the Earth’s crust. The slow cooling will result in a coarse-grained texture. However, they can have other textures too.
Norite is a gabbro subtype with mostly calcic plagioclase and orthopyroxene with clinopyroxene less than 5%. It can have other minor minerals, too.
On the other hand, gabbro has calcic plagioclase and clinopyroxene with minor orthopyroxenes and constituents of other minerals.
Thus, the difference is that gabbro has abundant clinopyroxene, and norite has abundant orthopyroxene.
Lastly, this rock occurs in layered igneous intrusion and sometimes in independent plutons.
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Description and appearance
Norite is a layered, sometimes massive, coarse-grained rock with a Mohs hardness scale of 6-7 and a 2.7-3.0g/cm3 density.
This rock is medium to dark gray but can have a black, brownish, or greenish hue. Its color index M is greater than 35.
However, some are paler, i.e., leuconorites have a color index of M is less than 35, while melanonorites‘ color index is 65-90. Rocks with M over 90 are called ultramafic.
Norite’s texture is phaneritic, with hypidiomorphic equigranular (nearly equal) interlocking grains that are mostly dark and a few lighter.
A hypidiomorphic fabric has mostly subhedral crystals, i.e., intermediate crystals that are not perfectly formed.
However, some norite rocks also show other textures like porphyritic, pegmatitic, ophitic, or cumulus.
Porphyritic has large crystals called phenocrysts in a finer but phaneritic groundmass or matrix, while pegmatitic is characterized by giant crystals over 1 cm up to tens of meters. It formed from the last water-rich melt to crystallize.
On the other hand, ophitic describes a texture where pyroxene, usually orthopyroxenes or sometimes olivine, surrounds plagioclase laths.
Lastly, layered norite shows cumulus texture. This texture is characteristic of crystals that separate and form rocks form during fractional crystallization. Their faces touch and will have intercumulus liquid in between the crystals.
Chemical composition
Norite is a silica-poor or basic rock with 45-52 wt.% silica. It is relatively high in calcium, magnesium, and iron oxides and lower in alkali oxides, i.e., sodium and potassium oxide.
Norite mineral composition
Norite is mafic rock. It primarily has calcic plagioclase and orthopyroxene. Also, it will have minor clinopyroxene, hornblende, biotite, and, less often, cordierite or olivine.
Accessory minerals include ilmenite, magnetite, and sometimes quartz, as well as alkali feldspar.
The calcium-rich plagioclase has more than 50 mol.% anorthite and is usually labradorite to bytownite.
On the other hand, orthopyroxene is usually iron-rich enstatite or bronzite, while clinopyroxenite is augite.
On the QAPF classification, norite is a coarse-grained igneous rock in which quartz is less than 5% of the QAPF content by volume, and plagioclase accounts for over 90% of the total feldspars.
Lastly, norite is a subtype of gabbro in which orthopyroxene is the dominant mafic mineral, with clinopyroxene less than 5%. Those in which clinopyroxene is more than 5% are called clinopyroxene norite.
How does norite form?
Norite forms deep inside the Earth’s crust from fractional crystallization of magma when in layered intrusions or from solidifying crustal contaminated magma in plutons.
Layered norite forms from the fractional crystallization of basaltic magmas. As the magma fractionally crystallizes, crystals in the resulting magma mushes will separate from the melt and settle or float to form a cumulate rock.
Therefore, it will occur in layered igneous intrusion with other cumulate rocks like pyroxenites, peridotites, gabbros, anorthosites, diorites, and granitic rocks. Sometimes, this rock is often interlayered with gabbro and anorthosites.
On the other hand, basaltic magma contaminated with subcrustal rocks can cool, forming noritic plutons like lopoliths and dikes. Otherwise, no primary mafic or igneous magmas have a composition like norite.
These plutons may have a small amount of other rocks like pyroxenite, peridotites, and anorthosites.
Occurrence
Norite occurs as plutons like lopoliths and dike swarms or layered igneous intrusions.
Examples of these layered intrusions with norite include Bushveld in South Africa, Skaergaard in Greenland, Stillwater in Montana, USA, Rogaland igneous complex in Norway, and Great Dyke in Zimbabwe.
More examples include the Haddo House Complex of Aberdeenshire, which shows rhythmic layering, Gombak in Singapore, and Sukinda and Nausahi in India. Also, Canada’s Sudbury Igneous Complex has norite as a basement rock.
Norite uses
Norite is dense, hard, and durable, making it an excellent dimensional stone sold as a black granite rock. Also, it makes aggregate rock and has other uses.
As a dimensional stone, cut and polished norite can make tiles, pavers, window sill, and countertops, while unpolished can serve as masonry stone.
Crushed norite rock makes aggregate for making concrete, asphalt, railway ballast, roading industry, and other construction projects.
More uses include making monuments, statues, and cemetery markers. Also, you can use it for landscaping, riprap, or gravel on unpaved walkways, patios, or driveways.
Lastly, some layered intrusions with norite may have valuable ores and metals like chromite, nickel, ilmenite, Ti-magnetite, platinum, etc. Examples are platinum in Stillwater and Bushveld and chromite at Sukinda and Nausahi. Also, Sudbury has nickel and titanium, which occurs in Egersund intrusion in Norway.