Orthopyroxenite

Orthopyroxenite is an ultramafic rock that contains more than 90 % orthopyroxene. Orthopyroxenites are usually found as dikes in peridotite massifs in the mantle sections of ophiolites and in orogenic peridotites. They are also observed as xenoliths or part of composite xenoliths entrained in diorites and basalt. In general, orthopyroxenites are separated by sharp boundaries from accompanying peridotites, and some orthopyroxenites contain resorbed olivine and hydrous minerals such as phlogopite and amphibole.
Field studies and laboratory experiments invoke melt-peridotite or fluid-peridotite interaction to explain the formation of orthopyroxenites in ultramafic massifs and xenoliths, although the natures of reacting melt, peridotite protolith, and detailed reaction regime remain enigmatic. However orthopyroxenites, can also be generated through fractional crystallization processes in layered igneous complexes.

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Orthopyroxenite from the Bushveld Complex. Driekop, South Africa. Keele collection.



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Cumulate orthopyroxenite with intracumulus plagioclase (small white grains) and poikilitic augite (large dark-green patches with orthopyroxene inclusions). Stillwater Complex, Montana, USA. From Union College.

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Cumulate orthopyroxenite with intracumulus plagioclase (small white grains) and poikilitic augite (large dark-green patches with orthopyroxene inclusions). Stillwater Complex, Montana, USA. From Union College.



Bibliography



• Wang, C., Liang, Y., Dygert, N., & Xu, W. (2016). Formation of orthopyroxenite by reaction between peridotite and hydrous basaltic melt: an experimental study. Contributions to Mineralogy and Petrology, 171(8-9), 77.

Photo
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 2x (Field of view = 7mm)
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 10x (Field of view = 2mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 10x (Field of view = 2mm)
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 10x (Field of view = 2mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 10x (Field of view = 2mm)
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Orthopyroxene crystals in a orthopyroxenite. PPL image. 10x (Field of view = 2mm)
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Orthopyroxene crystals in a orthopyroxenite. XPL image. 10x (Field of view = 2mm)