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Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. Sign In or Create an Account. User Tools. Sign In. Advanced Search. Skip Nav Destination Article Navigation. Close mobile search navigation Article navigation. Volume 98, Number 7. Previous Article Next Article. Article Navigation. They are found in almost every variety of igneous rock and occur in rocks of widely different compositions formed under conditions of regional and contact metamorphism.
The range of possible chemical substitutions in pyroxene is constrained by the sizes of the available sites in the structure and the charge of the substituting cations. The X-cation sites in general are larger than the Y-cation sites. Extensive atomic substitution occurs between the ideal end-member compositions. Most pyroxenes have only limited substitution of aluminum for silicon in the Z tetrahedral site. When a substituting ion differs in charge, electrical neutrality is maintained by coupled substitutions.
Since no true pyroxenes exist with calcium contents greater than that of the diopside-hedenbergite join, the part of this system below this join is known as the pyroxene quadrilateral. Augite, subcalcic augite, and pigeonite lie within the interior of the pyroxene quadrilateral. Augites with substantial aluminum or sodium cannot be strictly represented in the quadrilateral plane.
Monoclinic pigeonite encompasses a field of magnesium-iron solid solution with a slightly higher calcium content than the orthorhombic enstatite-orthoferrosilite series. Other less common pyroxenes with compositions outside the pyroxene quadrilateral include johannsenite [CaMnSi 2 O 6 ], and kosmochlor ureyite [NaCrSi 2 O 6 ].
Johannsenite involves the substitution of manganese for iron in hedenbergite. Kosmochlor has chromium Cr in place of iron or aluminum in a sodic pyroxene. At high temperatures, pyroxenes have more extensive fields of solid solution than they do at lower ones. Consequently, as temperatures decrease, the pyroxene adjusts its composition in the solid state by exsolving a separate phase in the form of lamellae within the host pyroxene grain.
Unfortunately, natural diopside is rarely found in deposits that simultaneously have a size, purity, and location that allows economic mining. This makes synthetic diopside cost-competitive with diopside produced by mining.
Rare specimens of hypersthene and enstatite sometimes display a colorful sheen known as "schiller". Rare specimens of enstatite will display asterism in the form of four-ray or six-ray stars. These specimens of gem-quality enstatite and hypersthene are so rare that they are "collector's stones" instead of gems frequently seen in jewlery.
Crushed Trap Rock: "Trap rock" is a name used in the construction industry for any dark-colored igneous rock that is used to make crushed stone. These are usually diabase, gabbro, basalt, or peridotite that contain pyroxene minerals as an important part of their composition.
Pyroxene minerals are primary or secondary constituents of many rocks used to make crushed stone , dimension stone, and architectural stone.
Pyroxene is in the trap rock of the construction industry and in the " black granites " that are popular for making decorative tile and facing stone. Generalized Composition Ranges of Common Igneous Rocks: This chart shows that pyroxenes are found in several types of igneous rocks.
In Earth's crust, pyroxenes are found in a wide range of igneous and metamorphic rocks. They are most abundant in the dark-colored igneous rocks, such as basalt and gabbro , that comprise most of the oceanic crust.
Pyroxenes and plagioclase feldspar are the most abundand minerals in these rocks. Pyroxenes are also important minerals in many peridotites. Rocks of the upper mantle are composed mainly of olivine and pyroxenes. In the continental crust, pyroxenes are often accessory minerals in granite , rhyolite , diorite , and andesite.
See the accompanying chart for a graphical representation of pyroxene presence in various igneous rocks. Pyroxenes are not abundant in sedimentary rocks because pyroxenes rarely form at low temperatures and are very susceptible to chemical and physical weathering. Pyroxene minerals are known beyond Earth.
They are abundant in many stone meteorites and present in some stony-iron meteorites. They have also been identified in the rocks and regolith of the Moon and Mars. When compared to terrestrial rocks, rocks of the Moon and Mars have simple mineral compositions. This is because they have not been significantly altered by chemical weathering. Their composition is dominated by plagioclase feldspar and pyroxene minerals. Single Chain Silicates: As silicate minerals, pyroxenes have the silica tetrahedron as their basic structural unit.
Each silica tetrahedron consists of four oxygen ions that surround a single, and much smaller, silicon ion in the center of the tetrahedron. Pyroxene minerals are single-chain silicates. That means their silica tetrahedra are arranged in long single chains. Adjacent tetrahedra in the chain share a single oxygen atom at their connection points. The illustration above is a view looking down on two chains comprised of silica tetrahedra.
Metal Ions Between the Chains: This diagram is a view looking down the long axis of a pyroxene mineral. Each pair of green triangles is an end-view of a single chain of silica tetrahedra. You may remember that XYZ 2 O 6 is the generalized chemical composition of a pyroxene mineral.
The X and Y ions occupy positions between adjacent chains of tetrahedra, as shown in the diagram above.
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