Harzburgite is a peridotite primarily made up of olivine, orthopyroxene, and a small amount of clinopyroxenes.
Peridotites are coarse-grained, ultrabasic, and ultramafic plutonic rocks with at least 40% olivine by volume.
Ultrabasic means rocks have less than 45% silica, while ultramafic has more than 90% mafic minerals. Mafic minerals are dark-colored and rich in iron and magnesium.
Lastly, plutonic or intrusive igneous rocks form beneath the Earth’s surface. These rocks have a slow cooling history, resulting in a coarse-grained or phaneritic texture.
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What do harzburgites look like?
Harzburgites are dense, coarse-grained rocks with a 3.1-3.1g/cm3 density and a Mohs hardness scale of 6-7.
Its color is mostly dark-greenish with black specs. The greenish is from olivine and orthopyroxene, which dominate the rock, while some mica, amphiboles, and accessory minerals give it a black tint.
However, olivine and pyroxene weathering and serpentinization can alter its color. Some can appear reddish brown, dark gray, or bluish-gray, among other colors.
Serpentinization is when hydrothermal fluids transform the iron and magnesium-rich minerals like pyroxene, olivine, and sometimes amphibole in ultramafic rocks to serpentine.
Chemical and mineral composition
Harzburgite is an ultrabasic, ultramafic rock. It has less than 45 wt.% silica, is very high in magnesium, and has considerable iron. This rock is, however, low in alkali oxides (sodium and potassium oxides).
It is dominated by olivine and orthopyroxene but may have minor clinopyroxene and sometimes micas, amphiboles, and interstitial plagioclase, often labradorite. Accessory minerals are ilmenite, granite, spinel, and chromite, among others.
The orthopyroxene present is the low calcium, Mg-rich bronzite or enstatite.
We mentioned earlier that harzburgite is a peridotite. Yes, it is. However, it is a pyroxene peridotite with lherzolite and wehrlite since they have more than 5% pyroxenite.
Specifically, this rock has 40-90% olivine, more than 5% orthopyroxene, and less than 5% clinopyroxene.
Lastly, the other peridotites are kimberlite associated with gold. The other one is dunite with nearly 100% olivine. Pyroxene-hornblende peridotites and hornblende periotites are the others.
Formation
Harzburgites are mantle rocks, i.e., they occur in the mantle. They form partial melting and removal of basalt magma from lherzolites.
Lherzolites have 40-90% olivine and considerable clinopyroxene and orthopyroxene. Partially melting them will melt most of the clinopyroxene, transforming the rock into a harzburgite.
Another way it forms is fractional crystallization at high pressure. This favors olivine, bronzite, and enstatites to crystallize together. They will then settle, forming harzburgite.
Occurrence
Harziburgite occurs in ophiolites, as xenoliths in basaltic magma or layered igneous intrusion.
Ophiolites are a part of the Earth’s crust and underlying mantle that is exposed to the Earth by obducton. Examples of ophiolites with this rock include
- Semail Ophiolite in Orman and UAE
- Trinity ophiolite and Coast Range ophiolite in the USA
- Lanzo Massif in North Italy
- Horoman Massif in Japan
- Troodos ophiolite in Cyprus,
- Lherz Massif in France
- Bay of Islands Ophiolite in Canada
On the other hand, layered igneous intrusions like Bushveld in South Africa and Stillwater in Montana, USA, have harzburgites.
Why are harzburgites important?
They help us understand the interior Earth that is not accessible, including processes and conditions that occur in the upper mantle.
Also, some harzburgites are xenoliths that host diamonds.