Pyroxene Group Orthorhombic Minerals (Opx) compositional range: MgSiO 3 - FeSiO 3 Monoclinic Minerals (Cpx) compositional range: CaMgSi 2 O 6 -CaFeSi 2 O 6 -MgSiO 3 -FeSiO 3 General Formula: X 1-p Y 1+p Z 2 O 6 X=Ca,Na, Y=Mg, Fe +2 , Fe +3 , Mn, Li, Al, Cr, Ti Z= Si, Al In Opx, p≈1 In Cpx, p varies from 0 to 1
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Pyroxene Group
Orthorhombic Minerals (Opx)compositional range: MgSiO3-FeSiO3
Monoclinic Minerals (Cpx)compositional range:
CaMgSi2O6-CaFeSi2O6-MgSiO3-FeSiO3
General Formula: X1-pY1+p Z2 O6
X=Ca,Na, Y=Mg, Fe+2, Fe+3, Mn, Li, Al, Cr, TiZ= Si, Al
In Opx, p≈1In Cpx, p varies from 0 to 1
Orthorhombic Pyroxene• Enstatite – Orthoferrosilite (Mg, Fe+2)2Si2O6
Monoclinic Pyroxene• Diopside-Hedenbergite-Johannsenite Ca(Mg, Fe+2, Mn)
Si2O6
• Augite-Ferroaugite (Ca, Na, Mg, Fe+2, Mn, Fe+3, Al, Ti)2 (Si, Al)2O6
• Pigeonite (Mg, Fe+2, Ca)(Mg, Fe+2) Si2O6
• Aegirine-Aegirine augite Na Fe+3 Si2O6 – (Na, Ca)(Fe+3
Fe+2, Mg) Si2O6
• Spodumene LiAlSi2O6
• Jadeite NaAlSi2O6
Mg SiO3 Enstatite (001)
Ca SiO3 Wollastonite (010)
• SiO4 tetrahedra shares 2 of the 4 oxygens to form continuous chain
• Unit length of the chain is 5.3Å• Lateral link is by cations (Ca, Fe, Mg, Na, Al)
Enstatite - Orthoferrosilite
• α 1.650-1.768• β 1.653-1.770• 1.658-1.788• 0.007-0.020• Optic axial angle (2V) 50o - 125o
• RI : =y, O.A.P (100)• Dispersion: red > violet, weak to strong, D: 3.21-3.96, H: 5-6• Cleavages: {201} good ,• Colour: Green, grey, yellow, brown, Mg-rich members: colourless• Iron rich members – green or dark brown, reddish or greenish in thin
section• Pleochroism: Enstatite - none, other compositional members weak to
strong, = pink, pale reddish brown, purple-violet, smoky brown, β=yellow, pale greenish brown, pale reddish yellow, pale brown, =green, pale green, smoky green
Chemistry
• Mg and Fe+2 are invariably present in the OPX• Common ions – Ca, Mn, Ni, Fe+3, Cr, Al, Ti (not
exceeds 10 mol %)• Enstatite – low temp. enstatite, high temp.
protoenstatite, (orthorhombic) and clinoenstatite
Paragenesis• Mg OPX common in ultrabasic rocks –
pyroxenites, hyrzburgites, Iherzolites, picrites• Associated with Fo – olivine, diopsidic augite,
Mg-spinel• Opx – in layered complex, • Ca(Mg,Fe)Si2O6+Al2SiO5 (Mg,Fe)SiO3+CaAl2Si2O8
Salite
• Chlorite+quartz Opx+cordierite+water• Biotite+quartz Opx+orthoclase+water
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