Norite
Mafic intrusive igneous rock (plagioclase + orthopyroxene)
A coarse-grained mafic plutonic rock similar to gabbro but with orthopyroxene as the dominant pyroxene instead of clinopyroxene.
- Mohs hardness
- 6-7
- Color
- Dark grey to greenish-black, speckled
- Type
- igneous
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Overview
Norite is a dark, coarse-grained mafic intrusive rock closely related to gabbro. The key distinction is mineralogical: in norite the dominant pyroxene is orthopyroxene (such as hypersthene or enstatite), whereas in true gabbro it is clinopyroxene (augite). Both are built chiefly of calcium-rich plagioclase feldspar plus pyroxene.
Because the difference requires microscope or careful mineral study, norite and gabbro are often hard to tell apart in hand specimen and are sometimes grouped as the gabbroic rocks.
Norite is famous as a host rock for major nickel-copper-platinum ore deposits, most notably at Sudbury, Canada.
Formation & geology
Norite crystallizes slowly from cooling mafic magma in deep intrusions, commonly forming layers within large layered complexes such as the Bushveld Complex in South Africa and the Stillwater Complex. It also occurs in dikes, sills, and stocks.
The Sudbury structure in Ontario, an impact-related igneous complex, contains norite that hosts world-class nickel and copper sulfide ores. Norite frequently forms cumulate layers alongside gabbro, anorthosite, and pyroxenite as dense crystals settle and accumulate in the magma chamber.
How to identify it
Norite is a heavy, dark grey-to-greenish-black rock with visible interlocking crystals of pale plagioclase and dark pyroxene, giving a speckled look much like gabbro. Hardness is about 6-7.
It cannot be reliably separated from gabbro in the field without identifying the pyroxene type; orthopyroxene tends to show a bronzy or coppery schiller and a single direction of good cleavage. Distinguish norite from diorite by its more calcic plagioclase and darker, more mafic overall composition, and from basalt by its coarse grain.
Uses & significance
Norite is widely used as a durable construction and dimension stone, frequently sold commercially as "black granite" for countertops, monuments, and facing slabs. Its toughness makes it valuable for aggregate as well.
Its greatest significance is as an ore host: noritic rocks at Sudbury and within layered intrusions supply major quantities of nickel, copper, and platinum-group metals. It has no gemstone or metaphysical tradition.
Frequently asked questions
What is the difference between norite and gabbro?
Norite's dominant pyroxene is orthopyroxene, while gabbro's is clinopyroxene; otherwise they are very similar dark mafic plutonic rocks.
Is norite the same as 'black granite'?
Commercially, some 'black granite' is actually norite or gabbro; geologically it is not granite, which is light-colored and quartz-rich.
Why is norite important for mining?
Noritic rocks, especially at Sudbury, Canada, host major nickel, copper, and platinum-group metal sulfide deposits.
How can I tell norite from basalt?
Norite is coarse-grained with crystals visible to the eye, while basalt of the same composition is fine-grained from rapid cooling.
Norite guides
In-depth guides for identifying, valuing, and understanding Norite.
