Why do pyroxenes have 90° cleavage

Hypersthene - is commonly found in both plutonic and volcanic igneous rocks and in meta-igneous rocks as well. Hypersthene is sometimes pleochroic, showing light pink to light green colors. The chemical composition of hypersthene can be estimated using 2V see p. Compositions close to Enstatite are optically positive with a 2V of 60 to 90 o , whereas intermediate compositions are optically negative with a 2V of 50 to 90 o. Pigeonite - is generally only found in volcanic igneous rocks, although, as mentioned above, it can occur as exsolution lamellae in augites of more slowly cooled igneous rocks.

Aegerine acmite - Aegerine Augite - are sodic pyroxenes and thus are found in alkalic igneous rocks associated with sodic amphiboles, alkali feldspars, and nepheline. The mineral is common in alkali granites, quartz syenites, and nepheline syenites all alkalic plutonic rocks , and are also found in sodic volcanic rocks like peralkaline rhyolites.

Aegerine is also optically negative with a 2V of 60 to 70 o , whereas Aegerine-augite has a higher 2V and can be optically positive or negative. It is distinguished from the pleochroic sodic amphiboles by its nearly 90 o pyroxene cleavage angle. Jadeite - is a sodium aluminum pyroxene that is characterized by its presence in metamorphic rocks formed at relatively high pressure. It can form by a reaction of Albite to produce :.

Jadeite has a lower refractive index than all other pyroxenes, and has low birefringence, showing low order 1 st and 2 nd order interference colors. It is monoclinic with an extinction angle of 33 to 40 o , and can thus be easily distinguished form hypersthene. It is usually colorless in thin section, helping to distinguish it from augite and aegerine, and has lower birefringence than augite and aegerine.

I nosilicates Double Chain Silicates - The Amphiboles The amphibole group of minerals is based on the double-chain silicate structure as shown here. The basic structural unit is Si 4 O 11 The composition of the common non-sodic amphiboles are shown in the diagram here. Note the similarity to the pyroxene compositional diagram, above.

Hornblende is the most common amphibole and has more in common with the Tremolite - Ferroactinolite series, with Al substituting into the Y sites and the tetrahedral site. The angles between the cleavages, however are 56 o and o making all amphiboles easy to distinguish from the pyroxenes. Looking at faces that show only a single cleavage trace would show inclined extinction, except in Anthophyllite.

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Skip to content Science. 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. Pyroxene minerals crystallize in both the monoclinic and orthorhombic crystal systems. Monoclinic pyroxenes are known as "clinopyroxenes" and orthorhombic pyroxenes are known as "orthopyroxenes". Pyroxenes can also be classified on the basis of their chemical composition.

The classes include: magnesium-iron pyroxenes, calcium pyroxenes, calcium-sodium pyroxenes, sodium pyroxenes, and lithium pyroxenes. Two lists of pyroxene minerals and their chemical compositions are presented below. One list contains clinopyroxenes, the other contains orthopyroxenes. Some, such as jadeite and spodumene, occur in a range of colors.

Streak White Luster Vitreous to dull Diaphaneity Semi-translucent to opaque Cleavage Most pyroxene minerals exhibit two directions of cleavage that intersect at approximately 87 o and 93 o. Cleavage fragments often have a nearly square cross-section. Angle of cleavage plane intersection. Crystal System Monoclinic or orthorhombic Uses Most pyroxenes have no regular use as industrial minerals.

Pyroxene minerals are minor to secondary components of some rocks that are used as crushed stone and dimension stone. A few pyroxene minerals are used as gem materials, but only in rare instances when they have attractive color and clarity.

Jadeite, diopside, and spodumene are the most important pyroxene gem materials. Spodumene was once an important source of lithium. Find Other Topics on Geology. Maps Volcanoes World Maps. Usually dark green to dark brown or black. Most pyroxene minerals exhibit two directions of cleavage that intersect at approximately 87 o and 93 o. A generalized composition of XYZ 2 O 6 , as described in the accompanying sections.

Most pyroxenes have no regular use as industrial minerals. Mohs Hardness Scale. What Is Luster? A repeat distance of approximately 5. The Si 2 O 6 chains are bonded to a layer of octahedrally coordinated cation bands which also extend parallel to the c-axis. From Wikipedia The octahedral strips consist of M1 and M2 octahedrons sandwiched between two oppositely pointing tetrahedral chains.

The M1 sites are occupied by smaller cations such as magnesium, iron, aluminum, and manganese, which are coordinated to six oxygen atoms to form a regular octahedron. In monoclinic pyroxenes, the M2 site is a large irregular polyhedron occupied by the larger calcium and sodium cations which are in eightfold coordination. In the low-calcium orthorhombic pyroxenes, M2 contains magnesium and iron, and the polyhedron takes on a more regular octahedral shape.

The M1 cation strip is bonded to oxygen atoms of two oppositely pointing tetrahedral chains Fig. Together, these form a tetrahedral-octahedral-tetrahedral t-o-t strip. A schematic projection of the pyroxene structure perpendicular to the c-axis and the relationship of the pyroxene cleavage to the t-o-t strips or I-beams is shown in Fig.

Orthorhombic pyroxenes differ from monoclinic pyroxenes in that they have parallel extinction. An introduction to the rock-forming minerals p.