Norite is a coarse-grained, dark-colored plutonic rock. It has mainly calcium-rich plagioclase (= labradorite to bytownite) and orthopyroxene (= hypersthene or enstatite) with minor hornblende, clinopyroxene, biotite, and sometimes olivine.
The name ‘norite’ comes from the word Norge, the Norwegian name for Norway, its type locality, i.e., where it was first described.
Where does it occur? This rock occurs mostly as part of layered igneous intrusions and is often associated with valuable minerals. However, it may form non-layered intrusions, especially lopoliths.
Also, norite is known to occur in lunar highlands on the moon with anorthosite and troctolite.
Here is more on norite, including its description, appearance, and composition. We will also give you its use or significance, where it is found, and how it forms.
Quick overview and properties
- Name: Norite
- Rock type: Igneous
- Origin: Intrusive
- Colors: Light to dark gray, greenish or brownish
- Texture: Coarse-grained or aphanitic
- Chemical composition: Mafic
- Minerals: Calcic plagioclase, orthopyroxenes, and small amounts of hornblende, clinopyroxene, and other mafic minerals.
- Silica content: 45-52%
- Mohs hardness scale: 6-7
What does norite look like?
If you have a hand sample, it may be hard to tell it from gabbro. Usually, norite is a massive, light gray to dark gray rock, with some having a brownish or greenish hue. It has a speckled or salt-and-pepper appearance, like gabbro, due to light and dark minerals.
The usual color index ‘M’ of noritic rocks is > 35. However, leuconorite is a paler variety with a color index of 10-35, while melanonorites are darker and have a color index of 65-90.
The texture of norite is coarse-grained or phaneritic with large visible crystals. This rock is mostly hypidiomorphic granular, characterized by some euhedral and subhedral crystals, while the rest are anhedral.
However, micronorite varieties will have a medium-grained texture. Such rocks are common on the margins of plutons.
Also, this rock may have other textures, including pegmatitic, porphyritic, or ophitic (poikilitic type). However, they are relatively uncommon.
A good example is leuconorite. It may have ophitic intergrowth of large orthopyroxenes surrounding plagioclase laths that may be centimeters long. Such rocks occur towards the upper ends of anorthosite bodies where pyroxene increases, grading them to norites, states Philpotts & Ague (2022).
Lastly, noritic rocks may have rhythmic interlayering with anorthosites and gabbro. Such may also exhibit cumulus textures.
Norite composition
What is the chemical and mineral composition of norite?
1. Chemical composition
Norite is a mafic or basic rock with 45-52% silica content. It is relatively high in ferromagnesium or dark-colored elements and lower in felsic ones.
2. Mineral composition
Norite rock has mainly calcic plagioclase and orthopyroxene with a minor amount of clinopyroxene, biotite, hornblende, and sometimes olivine or cordierite. Also, this rock may have a small amount of alkali feldspar, accessory ilmenite, and quartz.
The main felsic mineral is calcium-rich plagioclase, usually labradorite to sometimes bytownite. The other felsic minerals may be a small amount of alkali feldspar and quartz (since it forms from silica-saturated magma).
On the other hand, Gruenewaldt (1989) notes that Mg-rich orthopyroxene is the main mafic mineral. It is typically iron-rich bronzite, enstatite, or hypersphere. The other pyroxene is augite, a clinopyroxene, which occurs in minor amounts. Sometimes, pyroxenes present in this rock may show intergrowths.
However, for many norites and gabbros, hypersthene forms from the inversion of pigeonite. It doesn’t come directly from magma.
The other less often mafic minerals are olivine and cordierite. Olivine will have a cumulus nature and may occur with magnetite.
On the other hand, cordierite forms from gabbroic magma contaminated by argillaceous material. It is not a xenocryst but crystallizes from such contaminated magmas.
On the QAPF diagram, norite is a plutonic rock with plagioclase, accounting for over 90% of total feldspar by volume, and quartz up to 5% of QAPF volume.
Therefore, on the QAPF diagram, norite takes the same spot as gabbro. However, it differs from gabbro by having orthopyroxene as the dominating mafic mineral. In contrast, clinopyroxenes dominate gabbro.
Varieties and related rocks
Some of the rocks related to norite are:
1. Gabbronorite
Gabbronorite is a mafic plutonic rock with mainly calcic plagioclase and roughly equal amounts of orthopyroxene and clinopyroxene. This gabbroic rock may have less than 10% alkali feldspar and 0-5% quartz of QAPF volume. Also, it has less than 5% olivine and other minor mafic minerals.
2. Monzonorite
Monzonorite is a plutonic rock between monzonite and norite. It has predominantly calcic plagioclase (oligoclase to labradorite), hypersthene, and a lesser but essential amount of alkali feldspar (orthoclase).
Also, it may have a small amount of accessory quartz and some mafic minerals.
3. Clinopyroxene norite
It is a noritic rock with substantial clinopyroxene, i.e., > 5% but not more than orthopyroxenes. This plutonic rock has calcic-plagioclase and orthopyroxene. It may also have minor alkali feldspar, quartz, and other mafic minerals.
How does norite form?
Norite rock mostly forms from the slow cooling of crustal contaminated and progressively fractional crystalized basaltic magma deep inside the Earth’s crust. The slow cooling produces coarse-grained texture, while progressive fractionation creates a layered intrusion.
However, it can also occur as independent plutons, i.e., not part of layered intrusions.
Blatt & Tracy (2006) state that noritic magma formation is evidence of assimilation since primary igneous magma hardly has such composition. Furthermore, strontium and oxygen isotope ratios indicate this magma contamination by aluminum-rich crustal material.
To further support the hypothesis, Philpotts & Ague (2022) say that abundant aluminous metamorphic rocks xenoliths in the norite border zone at Haddo House Complex of Aberdeenshire, Scotland, is evidence of contamination.
Lastly, small granitic magma mixing with a large amount of basaltic magma will form noritic magma. This adds Al2O3 and SiO2. The CaO available will form calcic plagioclase, with the remaining mafic constituents forming orthopyroxene and other minerals.
Where are norite rocks found?
Norite rock occurs mostly as part of layered intrusions. It occurs with pyroxenites, peridotites (dunite or harzburgite), gabbros, diorite, anorthosite, and some granitoids.
Also, it can occur as intrusions, especially lopoliths and sometimes dike swarms. Such will have small amounts of peridotites, anorthosites, and pyroxenites.
Notable places where norite is found in layered intrusions are Bushveld (South Africa), Stillwater (Montana, USA), and Skaergaard (Greenland) igneous complexes. Others are Sukinda and Nausahi (India), Gombak (Singapore), Great Dyke (Zimbabwe), and Rogaland igneous complex (Norway).
Sometimes, norite may have rhythmic layering, such as seen in the Haddo House Complex of Aberdeenshire, Scotland, and Bjerkreim-Sokndal, Norway.
Lastly, the Sudbury Igneous Complex in Canada has norite basement rock. However, unlike layered intrusions, it formed from a meteorite impact.
What is norite used for?
Norite is a hard and durable rock. The construction industry sells it as black granite with diorite, anorthosite, basalt, diabase, and gabbro since their hardness is comparable to granite.
Some of the uses of norite include crushed to make aggregate for concrete, roads, railroad ballast, etc., and as a dimension stone for building, facing, paving, curbing, etc. Also, you can use it for landscaping, making cemetery markers (headstones), etc.
Cut and polished norite can make tiles, kitchen countertops, and windowsills, among many other uses.
Finally, norite is associated with valuable mineral ores like chromite, nickel, titanium magnetite, ilmenite, and platinum group minerals. For instance, Bushveld and Stillwater have platinum deposits, Sukinda and Nausahi chromite, Sudbury Basin nickel, and Egersund intrusion titanium.
FAQs
Norite and gabbro hand specimens are indistinguishable. Their difference lies in the type of pyroxene present, only identifiable on thin sections using a petrological microscope.
Norite will have orthopyroxene (orthorhombic pyroxenes) as the dominant mafic mineral. In contrast, gabbro will have clinopyroxene (monoclinic) as the dominant mafic mineral. For these reasons, some authors will describe norite as a gabbro type, with predominantly orthorhombic pyroxene, i.e., hypersthene.
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.
- Philpotts, A. R., & Ague, J. J. (2022). Principles of igneous and Metamorphic Petrology (3rd ed.). Cambridge University Press.
- Gruenewaldt, G. V. (1989). Gabbro and norite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 175-177). New York: Van Nostrand Reinhold.
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.