Pyroxene (PDF)

Pyroxene (Px) Pyroxene Group Occurrence: § Orthopyroxene (opx) series: mafic and ultramafic rocks. basalt, gabbro, norite, peridotite § Clinopyroxene (cpx) series: § Pigeonite: basalt § Augite: basalt, gabbro, peridotite. metamorphic rocks – amphibolites and granulites § Diopside –...

Pyroxene (Px) Pyroxene Group Occurrence: § Orthopyroxene (opx) series: mafic and ultramafic rocks. basalt, gabbro, norite, peridotite § Clinopyroxene (cpx) series: § Pigeonite: basalt § Augite: basalt, gabbro, peridotite. metamorphic rocks – amphibolites and granulites § Diopside – metamorphic rocks, carbonate rich http://www.pitt.edu/~cejones/GeoImages/1Minerals/1IgneousMineralz /Pyroxene/Pyroxene.jpg http://www.fabreminerals.com/specimens/s_imagesM4/Diopside- http://www.mineralminers.com/images/diopside/crystals/diom135. AR47M4fm.jpg jpg Structural formula Pyroxene: General Composition Skarn groups, § Ca-Mg-Fe ternary diagram look up this § Pyroxene “quadrilateral” mineral!! § OPX vs.CPX White space = no natural occurring minerals form in this compositional space Pyroxene: Structure § Single chain silicate, also called inosilicates § Crystal viewer – Bridging O? – Linked cations: § 2 different spots in structure – OPX vs. CPX § Orthopyroxene’s are orthorhombic § Clinopyroxene’s are monoclinic CPX - Diopside OPX - Enstatite Pyroxene: Cleavage Draw in I-beam structure and cleavage, think ev Perkins, 2011 Pyroxene: General Composition Skarn groups, § Ca-Mg-Fe ternary diagram look up this § Pyroxene “quadrilateral” mineral!! § OPX vs.CPX White space = no natural occurring minerals form in this compositional space Pyroxene: T dependent composition § This compositional ternary diagram was constructed at a specific temperature – At different temperatures, different Px form § More than one Px can form at a given temperature – Both OPX and CPX can form at the same temperature § BRS?!?!?! § Use a tie line (!!) on these diagrams to express which phases of Px co- exist with each other Pyroxene: T dependent composition § Can construct different compositional diagrams for different temperatures § Miscibility Gap (the grey area in this diagram, white area in colour diagrams): – Compositional gap: Composition range in which a single homogeneous mineral is not stable (sound familiar?) § Compositional range where un- mixing occurs Perkins, 2011 Exsolution § Slow cooling causes exsolution lamellae that have a distinctly different composition than the rest of the solid (sometimes called the host mineral). § What we have seen in the Alkali feldspars in the lab § Helps to determine cooling history of a mineral/rock http://upload.wikimedia.org/wikipedia/com Field of view = 4.4 mm mons/5/50/Perthitic_feldspar_Dan_Patch_ SD.jpg Solid immiscibility: another phase diagram!!!! § No liquid in this phase diagram!!! § Refer to lectures on temperature dependence of substitution § Solvus: § Outlines a miscibility gap § Both diagrams have a MG! § Huh….. § Below this a solid solution become Sol unstable and splits into separate solid vus phases § When crossing a solvus, unmixing occurs (called solid immiscibility) § unmixing of one, homogeneous solid phase by solid state diffusion into 2 solids of different compositions Pyroxene § How are these diagrams related? OPX