Ultramafic Rocks

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Ultramafic Rock Classification Diagram

Ultramafic ternary
Ternary diagram for ultramafic rocks: click the image for the larger size. Diagram (c) 2013 Andrew Alden, licensed to About.com (fair use policy)

This diagram is for the fraction of plutonic rocks that don't qualify for the QAP classification diagram because they have less than 10 percent in total of quartz (Q), alkali feldspar (A) and plagioclase feldspar (P). They are 90 percent or more mafic (iron- and magnesium-rich) or "dark minerals," which in most cases means some combination of olivine, pyroxene minerals and amphibole minerals (conventionally called hornblende, by far the most common of this group). Biotite (black mica), if present in significant amounts, is customarily added to the hornblende content.

Peridotite includes a wide range of rock compositions that are dominated by olivine. Peridot, the jeweler's name for gem-quality olivine, gives rise to the rock name. Dunite is almost entirely olivine. The other two rock names, pyroxenite and hornblendite, are self-explanatory.

Pyroxenite and peridotite may be further divided according to the particular pyroxene minerals making them up. That diagram is on the next page. You can picture that diagram as taking the left edge of this diagram and fanning it out into a new triangle or ternary diagram. What that means for professional purposes is that "pyroxenite" and "peridotite" are considered field names—names you can assign just by inspecting the rock by eye—rather than precise rock names, but often that kind of name is very useful.

Ultramafic rocks are commonly altered to serpentinite.

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Peridotites and Pyroxenites Classification Diagram

Peridotite-pyroxenite ternary
Ternary diagram for peridotites and pyroxenites: click the image for the larger size. Diagram (c) 2013 Andrew Alden, licensed to About.com (fair use policy)

This diagram is a subset of the general ultramafic rock diagram on the previous page that assumes a low percentage of amphibole minerals ("hornblende"). It corresponds to the left edge of that diagram and lays out the named subdivisions of peridotite and pyroxenite.

Most of the Earth's upper mantle is made of ultramafic (low-silica, high-iron, high-magnesium) rock, mainly composed of olivine and pyroxene minerals. Petrologists modeling the mantle care a lot about which specific pyroxenes are involved: orthopyroxene or clinopyroxene.

Orthopyroxene (abbreviated Opx) is an iron-magnesium silicate without any calcium, whereas clinopyroxene (Cpx) also contains calcium, which changes the mineral's molecular structure to one with an inclined crystallographic axis. This is determined in the lab and is not something that amateurs or even professionals can tell by just looking at rocks in the field.

Peridotite (per-RID-otite) is dominated by olivine (40 percent or more Ol) while pyroxenite (per-ROX-inite) is dominantly pyroxene. Peridotite can be given one of three names: harzburgite (HARTS-burgite) in which the pyroxene is Opx, wehrlite (WEAR-lite) in which the pyroxene is Cpx, or lherzolite (LERT-solite) in which the pyroxenes are mixed.

Mantle petrologists toss these names around a lot because peridotite gives rise to basalt by partial melting, making it fundamental to plate-tectonic processes. Harzburgite is the hardest to melt of the peridotites because clinopyroxene melts before orthopyroxene, which melts before olivine. Lherzolite contains a good share of clinopyroxene as well as garnet, and is commonly called "fertile mantle" because it readily yields a melt of basaltic composition. Harzburgite, by contrast, is referred to as "depleted mantle" because it represents the residue after melt has been extracted. Dunite is the extreme result of repeated partial melting. Wehrlite is not especially common.

Pyroxenite is usually found in relatively small masses related to bodies of peridotite (which also contains olivine) or gabbro (which also contains plagioclase). They appear to form as magmas rising through bodies of peridotite or gabbro react with these rocks. Websterite is the name for a pyroxenite of mixed Opx and Cpx.

If hornblende is present to a notable extent in any of these rocks, "hornblende" can be added in front of the rock name.

All of these rocks are commonly altered to serpentinite.