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Pyroxenite

Pyroxenite is an ultramafic igneous rock that contain pyroxene group minerals such as augite, diopside, hypersthene, bronzite or enstatite. This is a coarse-grained rock and that contains at least 90 percent pyroxene minerals. Also Pyroxenite contain olivine and oxide minerals, when it occurs in layered intrusions or nepheline. It is hard and heavy rock and color is light green. Single crystals may be 3 inc or more in lenght. Pyroxenites are usually found with gabbros and peridotites. There are three type of pyroxenite rock that areclinopyroxenites, orthopyroxenites, and the websterites.

Color: Light green, dark green, or black

Protolith or Parent Rock:

Hardness: 5 to 7

Grain size: Coarse-grained texture

Group: Ultramafic igneous rock

Alterations: Crystallization of a silica-poor magma in a major intrusion

Minerals: Pyroxene, Biotite, hornblende, olivine, plagioclase, nepheline

Dominant Minerals: Pyroxene

Pyroxenite Classification

There are three type of pyroxenite rock that areclinopyroxenites, orthopyroxenites, and the websterites.

Orthopyroxene

Orthopyroxene partitions nickel, cobalt, and manganese less than olivine and there are no clear correlations amongst these elements. Although low in abundance, orthopyroxene can be a significant reservoir for the trivalent cations vanadium, scandium plus tetravalent titanium, due to its high modal abundance, especially in depleted xenoliths with little or no clinopyroxene. The abundances of strontium, niobium, zirconium, and yttrium in orthopyroxenes are near or below the ppm-level and show no clear correlations. A general feature is enrichment of titanium, zirconium, and niobium relative to coexisting clinopyroxene. The few measurements of the REE patterns in orthopyroxene are typically LREE-depleted, with all REE one to two orders of magnitude below clinopyroxene. There is far more scatter and far less coherent variation amongst many incompatible trace elements in orthopyroxene, likely reflecting more heterogeneity on the micro-scale as well as temperature effects that are not well understood with the paucity of data available (D.G.Pearson.,D.Canil.,S.B.Shirey, 2003 )

Clinopyroxene

Clinopyroxene is a major host for sodium, calcium, chromium, and titanium in mantle xenoliths and shows extensive solid solution toward orthopyroxene and/or garnet at high P and T in the mantle (Boyd, 1969, 1970; Brey and Köhler, 1990). The Mg# of clinopyroxene is usually slightly greater than that of coexisting olivine, due to a KD greater than 1. The calcium content of clinopyroxene is strongly T-dependent and is between 40 mol.% and 50 mol.% wollastonite component. Subcalcic clinopyroxenes (Wo<35%) occur in cratonic suites as megacrysts or discrete nodules, and indicate very high T of equilibration, perhaps in equilibrium with melt
D.G.Pearson.,D.Canil.,S.B.Shirey (2003) .

Websterite

Websterite is ultramafic igneous rock that consists of roughly equal proportions of orthopyroxene and clinopyroxene. It is a type of pyroxenite.

Triangle diagram for peridotites. Axis are olivine-clinopyroxene-orthopyroxene. Fields include peridotite, pyroxenite, dunite, lherzolite, harzburgite, wehrlite, olivine-websterite, websterite, orthopyroxenite, clinopyroxenite.
Triangle diagram for peridotites. Axis are olivine-clinopyroxene-orthopyroxene. Fields include peridotite, pyroxenite, dunite, lherzolite, harzburgite, wehrlite, olivine-websterite, websterite, orthopyroxenite, clinopyroxenite. 

Pyroxenite Composition

Pyroxenite composition is contain at least 90 percent pyroxene group minerals, such as as augite, diopside, hypersthene, bronzite or enstatite.Pyroxenite also contain olivine and oxide minerals. Pyroxenite has less olivine than peridotites. The principal minerals usually found accompanying pyroxenites, in addition to olivine and feldspar, are chromite and other spinels, garnet, rutile, and magnetite. It has been proposed that large volumes of pyroxenite form in the upper mantle. Rare metamorphic pyroxenites are known and are described as pyroxene hornfels.

Where is the Pyroxenite Located

They frequently occur in the form of dikes or segregations in gabbro and peridotite: in Shetland, Cortland on the Hudson River, North Carolina (websterite), Baltimore, New Zealand, and in Saxony. They are also found in the Bushveld Igneous Complex in South Africa and Zimbabwe.

Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth’s mantle

The pyroxenites are often subject serpentinization under low temperature retrograde metamorphism and weathering. The rocks are often completely replaced by serpentines, which sometimes preserve the original structures of the primary minerals, such as the lamination of hypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende is developed and various types of amphibolite and hornblende-schist are produced. Occasionally rocks rich in pyroxene are found as basic facies of nepheline syenite; a good example is provided by the melanite pyroxenites associated with the borolanite variety found in the Loch Borralan igneous complex of Scotland.

Pyroxenite Uses

  • Countertops, Decorative Aggregates, Interior Decoration, Kitchens
  • As Building Stone, As Facing Stone
  • Curbing
  • As Dimension Stone, Building houses or walls, Cement Manufacture, Construction Aggregate, for Road Aggregate
  • Cemetery Markers, Commemorative Tablets, Laboratory bench tops, Jewelry, Sea Defence, Tombstones

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • D.G.Pearson.,D.Canil.,S.B.Shirey (2003) Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds, Available online 7 June 2004, https://doi.org/10.1016/B0-08-043751-6/02005-3