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Norite, also known as orthopyroxene gabbro, is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. Norite may be essentially indistinguishable from gabbro without thin section study under the petrographic microscope. The principal difference between norite and gabbro, however, is the type of pyroxene of which it is composed; norite is predominately composed of orthopyroxenes, largely high magnesian enstatite or an iron bearing intermediate hypersthene, whereas the principal pyroxenes in gabbro are clinopyroxenes, generally medially iron-rich augites.
Norite occurs with gabbro and other mafic to ultramafic rocks in layered intrusions which are often associated with platinum orebodies such as in the Bushveld Igneous Complex in South Africa, the Skaergaard igneous complex of Greenland, and the Stillwater igneous complex in Montana, United States. Norite is also the basal igneous rock of the Sudbury Basin complex in Ontario, which is the site of a comet impact and the world's second-largest nickel mining region. Norite is a common rock type of the Apollo samples. On a smaller scale, norite can be found in small localized intrusions such as the Gombak norite in Bukit Batok, Singapore. It is also plentiful in the Egersund intrusion area of southwestern Norway, with titanium deposits to the east.
The name norite is derived from the Norwegian name for Norway: Norge.
- Carmichael, Turner and Verhoogen, 1974, Igneous Petrology, McGraw-Hill, pp. 603–620
- Hyndman, Donald W., 1972, Igneous and Metamorphic Petrology, McGraw-Hill, pp. 122–139, esp. references.
- Petrus, Joseph A.; Ames, Doreen E.; Kamber, Balz S. (October 18, 2014), "On the track of the elusive sudbury impact: geochemical evidence for a chondrite or comet bolide (accepted for publication)", Terra Nova, doi:10.1111/ter.12125