Pyroxenes (M2)(M1)T206
Pyroxenes are inosilicates composed of singles of silica tetrahedra held together by divalent and trivalent cations.
M2 = Na, Ca, Fe, Mg (larger cations site)
M1 = Fe, Mg, Al, Ti (smaller)
T = Al, Si (even smaller)
Pyroxene crystal structures are found to be Orthorhombic "Orthopyroxene" and Monoclinic "Clinopyroxene"
Solid Solutions (substitutions) occur between
Fe2+ – Mg2+
Common constituent of mafic to ultramafic igneous rocks such as basalt, diabase, eclogite as well as their metamorphic equivalents
Compositional Variations in Pyroxene
Optical Properties in Pyroxenes
Both orthopyroxene clinopyroxene are biaxial with varieties displaying a larger 2V angle are (-) and small 2V are (+). Orthopyroxene 2V (50-130) and Clinopyroxene 2V (0-30).
Orthopyroxenes can be distinguished from clinopyroxenes based on extinction interference color and a very slight appearance of pleochroism
Orthopyroxenes displace parallel or straight extinction and first - order interference colors
Clinopyroxene displays inclined extinction and second-order interference colors
Extinction in Pyroxene
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Parallel Extinction in Orthopyroxene
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Inclined Extinction in Clinopyroxene
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Orthopyroxene: show slight pleochroism
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