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The Pyroxene Minerals

The Pyroxene Minerals


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Pyroxenes are abundant primary minerals in basalt, peridotite, and other mafic igneous rocks. Some also are metamorphic minerals in high-grade rocks. Their basic structure is chains of silica tetrahedra with metal ions (cations) in two different sites between the chains. The general pyroxene formula is XYSi2O6, where X is Ca, Na, Fe+2 or Mg and Y is Al, Fe+3 or Mg. The calcium-magnesium-iron pyroxenes balance Ca, Mg and Fe in the X and Y roles, and the sodium pyroxenes balance Na with Al or Fe+3. The pyroxenoid minerals are also single-chain silicates, but the chains are linked to fit more difficult cation blends.

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Aegirine

DEA/R.APPIANI / Getty Images

Pyroxenes are usually identified in the field by their nearly square, 87/93-degree cleavage, as opposed to the similar amphiboles with their 56/124-degree cleavage.

Geologists with lab equipment find the pyroxenes rich in information about a rock's history. In the field, usually, the most you can do is note dark-green or black minerals with Mohs hardness of 5 or 6 and two good cleavages at right angles and call it "pyroxene." The square cleavage is the main way to tell pyroxenes from amphiboles; pyroxenes also form stubbier crystals.

Aegirine is a green or brown pyroxene with the formula NaFe3+Si2O6. It is no longer called acmite or aegirite.

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Augite

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Augite is the most common pyroxene, and its formula is (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. Augite is usually black, with stubby crystals. It is a common primary mineral in basalt, gabbro and peridotite and a high-temperature metamorphic mineral in gneiss and schist.

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Babingtonite

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Babingtonite is a rare black pyroxenoid with the formula Ca2(Fe2+,Mn)Fe3+Si5O14(OH), and it is the state mineral of Massachusetts.

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Bronzite

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Iron-bearing pyroxene in the enstatite-ferrosilite series is commonly called hypersthene. When it displays a striking red-brown schiller and glassy or silky luster, its field name is bronzite.

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Diopside

Eduardo Estéllez / Getty Images

Diopside is a light-green mineral with the formula CaMgSi2O6 typically found in marble or contact-metamorphosed limestone. It forms a series with the brown pyroxene hedenbergite, CaFeSi2O6.

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Enstatite

Harry Taylor / Getty Images

Enstatite is a common greenish or brown pyroxene with the formula MgSiO3. With increasing iron content it turns dark brown and may be called hypersthene or bronzite; the rare all-iron version is ferrosilite.

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Jadeite

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Jadeite is a rare pyroxene with the formula Na(Al,Fe3+)Si2O6, one of the two minerals (with the amphibole nephrite) called Jade. It forms by high-pressure metamorphism.

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Neptunite

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Neptunite is a very rare pyroxenoid with the formula KNa2Li(Fe2+,Mn2+,Mg)2Ti2Si8O24, shown here with blue benitoite on natrolite.

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Omphacite

Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)

Omphacite is a rare grass-green pyroxene with the formula (Ca,Na)(Fe2+,Al)Si2O6. It is reminiscent of the high-pressure metamorphic rock eclogite.

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Rhodonite

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Rhodonite is an uncommon pyroxenoid with the formula (Mn,Fe,Mg,Ca)SiO3. It's the state gem of Massachusetts.

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Spodumene

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Spodumene is an uncommon light-colored pyroxene with the formula LiAlSi2O6. You'll find it with colored tourmaline and lepidolite in pegmatites.

Spodumene is found almost entirely in pegmatite bodies, where it usually accompanies the lithium mineral lepidolite as well as colored tourmaline, which has a small fraction of lithium. This is a typical appearance: Opaque, light-colored, with excellent pyroxene-style cleavage and strongly striated crystal faces. It is hardness 6.5 to 7 on the Mohs scale and is fluorescent under long wave UV with an orange color. Colors range from lavender and greenish to buff. The mineral alters easily to mica and clay minerals, and even the best gemmy crystals are pitted.

Spodumene is fading in importance as a lithium ore as various salt lakes are being developed that refine lithium from chloride brines.

Transparent spodumene is known as a gemstone under various names. Green spodumene is called Hiddenite, and lilac or pink spodumene is kunzite.

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Wollastonite

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Wollastonite (WALL-istonite or wo-LASS-tonite) is a white pyroxenoid with the formula Ca2Si2O6. It typically is found in contact-metamorphosed limestones. This specimen is from Willsboro, New York.

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Mg-Fe-Ca Pyroxene Classification Diagram

Pyroxene Minerals Click the image for a larger version. Diagram (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Most occurrences of pyroxene have a chemical makeup that falls on the magnesium-iron-calcium diagram; the abbreviations En-Fs-Wo for enstatite-ferrosilite-wollastonite may also be used.

Enstatite and ferrosilite are called the orthopyroxenes because their crystals belong to the orthorhombic class. But at high temperatures, the favored crystal structure becomes monoclinic, like all of the other common pyroxenes, which are called the clinopyroxenes. (In these cases they are called clinoenstatite and clinoferrosilite.) The terms bronzite and hypersthene are commonly used as field names or generic terms for orthopyroxenes in the middle, that is, iron-rich enstatite. The iron-rich pyroxenes are quite uncommon compared to the magnesium-rich species.

Most augite and pigeonite compositions lie far from the 20-percent line between the two, and there is a narrow but pretty distinct gap between pigeonite and the orthopyroxenes. When calcium exceeds 50 percent, the result is the pyroxenoid wollastonite rather than a true pyroxene, and compositions cluster very near the top point of the graph. Thus this graph is called the pyroxene quadrilateral rather than a ternary (triangular) diagram.

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Sodium Pyroxene Classification Diagram

Pyroxene Minerals Click the image for a larger version. Diagram (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

The sodium pyroxenes are much less common than the Mg-Fe-Ca pyroxenes. They differ from the dominant group in having at least 20 percent Na. Note that the upper peak of this diagram corresponds to the whole Mg-Fe-Ca pyroxene diagram.

Because Na's valence is +1 instead of +2 like Mg, Fe and Ca, it must be paired with a trivalent cation like ferric iron (Fe+3) or Al. The chemistry of the Na-pyroxenes thus is significantly different from that of the Mg-Fe-Ca pyroxenes.

Aegirine historically was also called acmite, a name that is no longer recognized.