Pyroxene

Jade ornaments and figurines fashioned by ancient Aztec and Mayan artisans are pyroxene’s most familiar face. An important rock-forming mineral of igneous and metamorphic rocks, pyroxene is not a specific mineral, but an informal name used for a number of group of related minerals. These minerals share a similar crystal structure, but contain different proportions of sodium (Na), calcium (Ca), iron (Fe) and magnesium (Mg), which substitute for one another in that structure. The pyroxene minerals are so similar in appearance and mineral properties that they are usually only identified as ‘pyroxene’ in the field. Pyroxene minerals are significant components of many intermediate, and most mafic, igneous rocks. They also occur in many medium-to-high grade metamorphic rocks. Only one pyroxene mineral, a sodium-rich pyroxene called Spodumene, occurs in felsic igneous rocks.

Description and Identifying Characteristics

The pyroxenes most commonly occur in intermediate to ultra-mafic igneous rocks, although they are also common in some medium-grade to high-grade metamorphic rocks. Pyroxene minerals share a similar crystal structure and their physical properties are so similar they are often only identified as ‘pyroxene’ in the field. Typically dark green to black in color, some pyroxene varieties range to light green or white. All of them are harder than glass, and exhibit two well-developed cleavage directions. Because of their dark color and glassy luster, broken cleavage surfaces in pyroxene samples are shiny and beginning students often mistake pyroxene samples as having a metallic luster.

The pyroxenes can be divided up into four subgroups, depending on whether they are relatively rich in calcium, magnesium, sodium or (more rarely) lithium ions.

• Calcium-rich pyroxenes are augite or diopside. Augite, the most common pyroxene, is a dark green to black iron- and calcium-rich pyroxene that is common in mafic and ultramafic igneous rocks, along with some intermediate igneous rocks. Diopside is a white to light green iron-free, calcium pyroxene that occurs in medium- to high-grade metamorphosed carbonate rocks.

• Magnesium-rich pyroxenes form a continuous replacement series between enstatite, an iron-free magnesium pyroxene, and hypersthene, an iron-bearing magnesium pyroxene. These minerals share a similar appearance to augite and occur in relatively calcium-free mafic to ultramafic igneous rocks and meteorites.

• Sodium-rich pyroxenes include a dark green to black iron-bearing form known as aegirine and a green iron-free variety known as jadeite, one of the two varieties of the gemstone jade. Aegirine looks like augite, but occurs in felsic igneous rocks. Jadeite is a relatively rare mineral that forms in metamorphic rocks and is often associated with serpentine.

• Spodumene is the only lithium pyroxene. It is a white or pale-colored mineral that forms in some pegmatites.

Their typical dark color, hardness and well-developed cleavage usually serve to distinguish these minerals from most other common rock-forming minerals, except for those of the amphibole (hornblende) group. These two mineral groups are easily confused for one another, as both groups share a similar hardness, dark coloration, and two dominant cleavage directions. Often the two can only be distinguished by the angle at which their cleavage directions meet, which is a reflection of the difference between pyroxene’s single chain structure and hornblende’s double chain amphibole structure. Pyroxene cleavage fragments have square shaped cross-sections, with cleavage faces meeting at nearly right angles (87^o and 93^o). In contrast, amphibole cleavage fragments (including hornblende minerals) exhibit a wedge-shaped cross-section, with cleavage faces meeting at 56^o and 124^o. Unfortunately, it is much easier to describe this difference than to recognize it in many samples.

To make matters worse, in some igneous rocks, hornblende minerals form as an alteration of pyroxenes and mimic the original pyroxene’s crystal shape, making it even more difficult to distinguish the two. Often the easiest way to tell whether you are dealing with a pyroxene or hornblende mineral is to use the other minerals present to identify the host rock. Pyroxene minerals are more common in mafic igneous rocks, while hornblende minerals are more typical of intermediate to felsic igneous rocks.

The name pyroxene comes from the Greek words for ‘fire’ and ‘stranger’, and arose as crystals of pyroxene occur in volcanic glass that early naturalists mistakenly thought were impurities caught up during a volcanic eruption.  In reality, these pyroxene crystals were simply the first crystals to form as the magma began to cool. When the remaining magma erupted and froze to form volcanic glass, these early formed crystals were distinct from the surrounding glass.

No strangers to the ‘fire’ of volcanic activity or magma, pyroxene minerals usually occur as important accessory components of intermediate to ultramafic igneous rocks. These minerals have very high melting points and crystallization points, so they form very early during a magma’s cooling and are only preceded by the olivine minerals. Pyroxene also occurs in some medium-grade and high-grade metamorphic rocks, including those formed in hydrothermal and contact metamorphic settings. These anhydrous (non-water-bearing) minerals are less common in metamorphic rocks, however, than the similar looking hydrous (H2O)-bearing and hydroxide (OH)-bearing hornblende (amphibole) minerals. Diopside, which forms in metamorphosed carbonates, is the primary metamorphic pyroxene. The two mineral groups are related to one another, as during metamorphism water reacts with pyroxene minerals to convert them to amphibole. So pyroxenes are more common in rocks that formed in water-free settings, while amphiboles are more common in water-wet rocks or during late water-rich stages of magma cooling. It is not surprising then that pyroxene minerals occur in many extraterrestrial rocks, such as stony meteorites and lunar samples that formed in water-free settings.

Pyroxene minerals in igneous rocks are commonly associated with olivine, plagioclase, biotite and amphibole minerals (especially hornblende). In metamorphic rocks, the minerals associated with pyroxene depend on the rock’s setting and original compostition, but may include serpentine, barite, quartz, dolomite, calcite, garnet, beryl, and tourmaline, as well as metallic ore minerals like magnetite and galena.

Although they are relatively common minerals, most of the pyroxenes have little economic value on their own. They are an important component of many decorative building stones, where their dark green to black colors contribute to the stones’ decorative pattern. Spodumene is mined as an important source of lithium, used in ceramics, and is also prized as a gemstone. Jadeite is one of two minerals commonly known as jade (nephrite, an amphibole mineral, is the other jade variety). Jadeite is a gemstone that was especially valued by the Aztec and Mayan civilizations of Mexico and Central America, who considered it more valuable than gold. Although the Chinese originally used nephrite for their exquisite jade carvings, as trade routes expanded to new areas, they supplemented nephrite with newfound sources of jadeite.

Pyroxenes are important accessory components in many of mafic igneous rocks across the Upper Midwest. The gabbro and basalt complex that forms Lake Superior’s North Shore is probably the most famous regional pyroxene-bearing occurrence, although the pyroxene crystals are too small to be separately seen and distinguished in the basalt layers. These rocks date to a period of immense volcanic activity that occurred about 1.1 billion years ago. The volcanism was part of an active rift system that began to break North America apart and form new oceanic crust between the separating fragments. The rifting ended when North America collided with another large continental mass (that would eventually be called South America), but the North Shore gabbros and basalts remain as the ancient legacy of a stillborn ocean basin.

The pyroxene minerals' typical dark color, hardness and well-developed cleavage usually serve to distinguish them from most common rock-forming minerals, with the exception of the hornblende (amphibole) mineral group. These two groups can be distinguished by the angles at which their cleavage planes meet, but this is easily one of the more difficult distinctions for beginning students to master.

Hornblende (Amphibole) Group:

Chemically, the most significant differences between the pyroxene and amphibole groups are the addition of O- and OH-groups in the amphiboles, and the groups' different silicate structures. The pyroxenes are single chain silicates, while the amphiboles are double chain silicates. In general, pyroxene crystals tend to be stubbier than the more elongated amphibole crystals, but the crystal shapes may be very similar in those amphiboles that formed from the alteration of pyroxenes. As a result, the best way to distinguish the two groups, short of performing a chemical analysis, is by determining the angle between cleavage faces of broken crystal fragments. On fragments of pyroxene, the cleavage faces tend to meet at nearly right angles. In contrast, hornblende and amphibole cleavage fragments have cleavage faces that meet at angles of nearly 60 degrees and 120 degrees. If you look down the long axis of a cleavage fragment, pyroxenes will tend to have rectangular cross-sections, while hornblende cleavage fragments will exhibit a diamond- or wedge-shaped pattern.

Magnetite:

Samples of dark, vitreous pyroxene may be mistaken as having a metallic luster. This may cause them to be confused with magnetite or other dark metallic minerals. However, none of the latter minerals will exhibit the two well-developed cleavage directions present in pyroxene minerals. Magnetite's magnetic character also serves to easily distinguish it from the pyroxene minerals.

Tourmaline:

The tourmaline group is a common accessory in many metamorphic rocks that may mimic pyroxene's color and hardness. However, tourmaline minerals have a distinctive triangular cross-section and lack the pyroxene minerals' well-developed cleavage.