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Classification
Crystal System: Monoclinic
Class: Silicate
Sub Class: Inosilicate (single chain)
Group: Pyroxene group
Structure and Composition
Composition: (Ca,Mg,Fe)2Si2O6
Structure: Pyroxenes have a single-chain silicate structure, where silica tetrahedra are linked linearly by sharing two oxygen atoms, forming long chains. These chains are bonded to metal cations, magnesium, and iron, which occupy octahedral or sites between the chains. Also between the chains are larger cations, calcium, for charge balancing.
In the structure on the right, the red represents oxygen, the tan represents silicon, the orange represents magnesium or iron, and the purple represents calcium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Black or dark green
Luster: Vitreous to dull
Hardness: 5-6
Streak: White to grey
Specific gravity: 3.2 to 3.5
Common Habit: Stubby prismatic crystals
Cleavage: Two directions that meet at nearly right angles at 87 degrees and 93 degrees, uneven fracture
Identification In Thin Section
Orthopyroxene PPL (photo by me)
Orthopyroxene XPL (photo by me)
Orthopyroxene
PPL Properties
Relief: Moderate to high
Habit/Form: May appear as blocky four or eight-sided crystals, rectangular lathes, anhedral crystals, and masses
Color: Commonly light green
Cleavage: Basal sections show a cleavage angle near 90 degrees. Longitudinal sections show one cleavage
Pleochroism: Some orthopyroxene shows marked pleochroism to pink
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Usually ranges to Order I yellow
Extinction angle: Parallel to cleavage planes
Twins: May show fine twinning
Uniaxial/Biaxial: Biaxial (+/-)
Clinopyroxene
PPL Properties
Relief: Moderate to high
Habit/Form: May appear as blocky four- or eight-sided crystals, stubby prisms or football-shaped grains, or anhedral crystals and masses.
Color: Generally light green, maybe clear, varieties may be light brown or yellow.
Cleavage: Basal sections show a cleavage angle near 90 degrees. Longitudinal sections show one cleavage
Pleochroism: Weak or absent
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I-II; bright
Extinction angle: Maximum extinction angle of 35° to 45°
Twins: Twins with {100} as twin-plane are common; these often appear as twin seams. Polysynthetic twins with {001} as twin-plane are occasionally found. Combined {100} twins with {001} polysynthetic twins give what is known as herringbone structure
Uniaxial/Biaxial: Biaxial (+)
Clinopyroxene PPL (photo by me)
Clinopyroxene XPL (photo by me)
Geologic Occurrence
Orthopyroxene is common in mafic igneous rocks, including gabbro, basalt, and norite, and is commonly associated with plagioclase and clinopyroxene. It is also found in some high-grade metamorphic rocks. Augite, the most common clinopyroxene, is found in mafic to intermediate igneous rocks and their metamorphic equivalents. Diopside, a Ca-Mg clinopyroxene is found mostly in marbles.
Significance
The pyroxene mineral jadeite has been used by ancient Aztec and Mayan artisans to make jade ornaments. o this day pyroxenes are still an important part of many decorative stones.
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