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Monoclinic and Orthorhombic Pyroxene

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.

Monoclinic Pyroxenes (Clinopyroxenes)
AegirineNaFe+++Si2O6
Augite(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
BurnettiteCaVAlSiO6
ClinoenstatiteMgSiO3
ClinoferrosiliteFe++SiO3
DavisiteCaScAlSiO6
DiopsideCaMgSi2O6
EsseneiteCaFe+++[AlSiO6]
GrossmaniteCaTi+++ AlSiO6
HedenbergiteCaFe++Si2O6
JadeiteNa(Al,Fe+++)Si2O6
Jervisite(Na,Ca,Fe++)(Sc,Mg,Fe++)Si2O6
JohannseniteCaMn++Si2O6
KanoiteMn++(Mg,Mn++)Si2O6
KosmochlorNaCrSi2O6
KushiroiteCaAl[AlSiO6]
NamansiliteNaMn+++Si2O6
NatalyiteNaV+++Si2O6
Omphacite(Ca,Na)(Mg,Fe++,Al)Si2O6
PetedunniteCa(Zn,Mn++,Mg,Fe++)Si2O6
Pigeonite(Ca,Mg,Fe)(Mg,Fe)Si2O6
SpodumeneLiAl(SiO3)2
Single chain silicates
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.
Orthorhombic Pyroxenes (Orthopyroxenes)
Hypersthene(Mg,Fe)SiO3
Donpeacorite(MgMn)MgSi2O6
EnstatiteMg2Si2O6
FerrosiliteFe2Si2O6
NchwaningiteMn2+2SiO3(OH)2•(H2O)
ProtoenstatiteMg2Si2O6
Bonding of silica chains
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 XYZ2O6 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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