Pyroxenite is coarse-grained ultramafic, mostly plutonic rock. Ultramafic means they have at least 90% dark-colored minerals rich in iron and magnesium. These minerals are known as mafic minerals.
On the other hand, plutonic or intrusive means they formed deep inside the Earth’s crust. Such rocks cool slowly, allowing larger mineral crystals. Therefore, they will have a coarse-grained texture.
Compositionally, pyroxenites have more than 60% pyroxene. Also, they may have up to 40% olivine and a smaller amount of feldspar, biotite, hornblende, and other minerals.
This rock closely relates to hornblendite. Hornblendite is, however, dominated by amphibole, not pyroxene.
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What do pyroxenites look like?
Pyroxenite is a dense, coarse-grained, dark-colored rock. Its color is primarily dark green or black, with mostly stubby prismatic pyroxene crystals.
However, it may be greenish-gray, dark gray, chalky white, or other colors. Also, it can have a lighter and darker speckled appearance.
The color of a given specimen depends on the minerals present. For instance, abundant plagioclase megacrysts can give it a speckled, dark, chalky white appearance.
On the other hand, pyrope can give a reddish-brown appearance.
Pyroxenite texture is holocrystalline and unusually coarse-grained, with grains that are several inches in size. Holocrystalline means it has crystals or is entirely crystalline.
However, some are pegmatitic, which is characterized by giant crystals.
What about its density and hardness? Pyroxenite has a density of about 3-3.5 g/cm3 and an estimated Mohs hardness of 5 to 6.
What is its composition?
Chemically, pyroxenite is a silica-poor or ultrabasic rock with less than 45 wt. % silica (SiO2). It is relatively high in iron, magnesium, and calcium oxides and low in sodium and potassium oxides.
Considering its mineralogy, this rock is ultramafic. It has over 60% pyroxene. Accessory minerals include olivine, biotite, feldspar/feldspathoids, hornblende, spinels, magnetite, ilmenite, scapolite, chromite, and garnets.
Also, this rock may have less common accessory minerals like conundrum, sapphire, zircon, rutile, apatite, quartz/coesite, and sometimes graphite or diamond.
Pyroxenes in this rock are iron and magnesium-rich. It often has both orthopyroxenes with orthorhombic crystals and clinopyroxenes with monoclinic crystals.
Orthopyroxenes in this rock include enstatite, ferrosilite, bronzite, and hypersthene, while clinopyroxenes are augite, diopside, and diallage.
Pyroxenite classification
Depending on their orthopyroxene, clinopyroxene, and olivine content, pyroxenite rocks are further divided into these subtypes:
- Orthopyroxenite: It has over 90% orthopyroxene and less than 10% olivine by volume. This pyroxenite, also known as diopsidite, occurs in the Pyrenees in Europe.
- Olivine orthopyroxene: An orthopyroxene with 10-40% olivine by volume, i.e., has >90 pyroxene and 10-40% olivine.
- Clinopyroxenite: Clinopyroxenite has over 90% clinopyroxene and less than 10% olivine. An example is the A 2m layer at Mooimeisjesfontein in Bushveld Complex, South Africa.
- Olivine clinopyroxene: It has over 90% clinopyroxene and 10-40% olivine
- Websterite: Websterite has significant (>10 vol. %) or nearly equal clinopyroxene and orthopyroxene and less than 10% olivine
- Olivine websterite: This rock has significant (>10 vol. %) of both clinopyroxene and orthopyroxene and 10-40% olivine.
Naming
When naming pyroxenite, you can use a prefix of dominant accessory minerals. If the mineral is less than 5% by volume, use mineral name + a hyphen and word bearing. For instance, you can have hornblende-, garnet-, feldspar-, biotite-, or garnet-bearing pyroxenite.
Furthermore, include the mineral name in the name if the dominant mineral(s) are more than 5% by volume. If more than two, start with the less dominant.
Examples include biotite pyroxenite in Newry, County Down, Ireland, and hornblende pyroxenite in Quebec, Canada.
Another example is melanite pyroxenite in Loch Borralan igneous complex, Assynt (Ben More), Scotland. It occurs with nepheline syenite and is associated with borolanite.
Lastly, a garnet pyroxenite has at least 5 vo.% garnet. Most are colorful and make excellent gemstones.
Origin and formation?
Pyroxenites are mostly of igneous origin. Such form from the solidification of magma. However, some have a metamorphic origin.
Metamorphic pyroxenite forms under high temperatures and pressure in rocks with at least 75% pyroxenes. An example is the metamorphic Lewisian complex in Scotland. It formed from contact metamorphism.
Pyroxenites of igneous origin likely form in layered mafic to ultramafic intrusions via crystal accumulation. Another way is through reactions that replace peridotites. This happens via a pyroxene-forming melt–rock reaction.
On the other hand, according to two studies, one by Karimov (2019) and another by Acken et al. (2009), mantle pyroxenite in several ways, including:
- Cumulates formation or crystal accumulation from primitive melts
- A product of metamorphosis of recycled oceanic crust, i.e., remnants of the subducted oceanic crust
- In situ metamorphic segregations from host peridotites or partial melts from peridotite wall rock crystallization.
- Melt (rising magma) and fluid interaction or reaction with host peridotites. For instance, boninite and peridotite interaction can yield these magmas in supra-subduction environments.
How do they occur?
Pyroxenites exist in discrete layers or sheets in mafic to ultramafic intrusions like sills and lopoliths. Also, they can exist as branching veins, narrow dikes, or on the silica-poor pluton edges.
Some places they are found include mantle sequences of orogenic massifs and ophiolites as veins or dikes. One study approximates that pyroxenites account for 2-5% of the upper mantle based on their proportion on orogenic massifs. Locally, they can be up to 10%.
Secondly, pyroxenites occur as cumulates in ultramafic intrusion. Here, they are associated with gabbro and norite. Also, they can occur with layers of magnetite and ilmenite and rarely with chromite cumulates.
Thirdly, they can occur in but segregated from gabbro and peridotite masses, forming cm to meter-thick layers. These likely form from upper mantle peridotites and rising magma.
Still, this rock can occur xenoliths in kimberlites and basalts. Xenoliths refer to a foreign rock fragment trapped inside an igneous rock body.
Lastly, pyroxenites occur on lower oceanic crusts. Here, they are associated with depleted peridotites.
Where are pyroxenites found?
Pyroxenites are uncommon rocks. Some places they occur include Canada, the US, South Africa, Zimbabwe, New Zealand, and Germany.
Specific locations include Baltimore in Maryland and Cortland in the Hudson River in North Carolina in the US. Also, they occur in Saxony in Germany, the Bushveld Igneous Complex in South Africa, and the Great Dike in Zimbabwe.
Alteration and weathering
Pyroxenites originate deep inside the crust or mantle where pressure and temperatures are high. Therefore, these rocks are unstable on Earth’s surface with low temperatures and pressures.
The low temperature and pressure will make them undergo metasomatism, metamorphism, and weathering.
Metasomatism happens when hydrothermal and other fluids alter rocks chemically.
On the other hand, metamorphism is retrograde, i.e., due to the low pressure and cooling of rocks under high pressure and temperature.
Retrograde metamorphism will convert ultramafic minerals to serpentine group minerals. This happens while minerals retain their primary mineral structures.
Uses and significance
Pyroxenite is a flux in iron making. However, the chromite’s presence limits this application.
Also, it is a magnesium oxide source for the sintering of hematite pellets. It helps improve metal properties.
Another significance is it contributes directly or indirectly to basalt magma generation in mid-ocean ridges and intraplate settings.
Lastly, some pyroxenite emplacement is associated with valuable metals like platinum, chromium, and nickel osmium.
Frequently asked questions
Although closely related, pyroxenes differ from peridotites as they have less than 40% olivine by volume than peridotites have. Also, pyroxenes dominate them, while olivine dominates peridotites.
Yes. It is rare. Nonetheless, it occurs in sills, thick flows, and lava tubes in Archaean greenstone belts, characterized by a spinifex texture. Examples are the Gullewa Greenstone Belt and the Duketon Belt in Australia.