Pyroxenes (Mg, Fe 2+ ) 2 Si 2 O 6 (monoclinic) and 20 MgSiO 3 FeSiO 3 (orthorhombic) Structure (Figure 2 of handout) Chain silicate eg Diopside Mg and Fe ions link SiO 3 chains The chain runs up and down parallel to the c-axis (2 cleavage directions. Orthorhombic series MgSiO 3 (Enstatite) Hypersthene FeSiO 3 (Orthoferrosilite) Monoclinic group - Wide range of components (see table on handout).
21 Chemistry Again, a relatively simple isomorphous series from MgSiO 3 to FeSiO 3 exists (Enstatite - Hypersthene - Orthoferrosilite) Substitution: Unlike the olivine series which was restricted to a few divalent ions, this series accepts a wide range of ions. Eg: Al, Ca, [Mn, Fe 3+, Ti, Cr, Ni] can enter the lattice in varying amounts. The latter 5 seldom exceed 10% of the composition. Similar to olivine, the Ni and Cr are found mainly in basic rocks or the Mg varieties. High Al Hypersthene occurs in metamorphic rocks. As well, Al 3+ enters the structure under high pressure conditions. Therefore, the amount of Al 3+ is roughly indicative of the pressure. Eg: you can tell if the rock is a cumulative or a very deep mantle rock, if Al 3+ then Pressure.
22 Chemistry cont. Ca can substitute into the lattice up to 15% by weight. Substitution is dependent on temperature:at higher T, more Ca can enter the lattice. The lattice is usually monoclinic which is less ordered and makes Ca substitution easier. Alteration It is common for Enstatite (opx) to be altered to Antigorite (Serpentine), often pseudomorphed. Diopside and Jadeite (cpx) are sometimes altered to Tremolite-Actinolite (amphibole). Augite can alter to 1) hornblende (amphibole) 2) Tremolite-Actinolite by hydrothermal alteration. Optical and Physical Properties Orthpyroxene Series (Enstatite - Ferrosilite) Colour: Pale coloured with pinkish to green pleochroism Form: Primsatic crystals with characteristic cross section.
23 Optical and Physical Properties cont Orthpyroxene Series (Enstatite - Ferrosilite) Relief: High Birefringence: Low. Cleavage: Two directions at nearly 90 (88 and 92 ). 2V: Varies systematically with iron content between 50 and 132. Interference Figure: Opt (+) for Enstatite and Ferrosilite Opt (-) for intermediate compositions Extinction : Parallel for all orthopyroxene.
Optical and Physical Properties cont. Clinopyroxene Group Colour: Colourless to green. Form: Prismatic crystals, 4 or 8 sided (Fig 14-3). Relief : Fairly high to high. Birefringence: Moderate to Strong. Cleavage : Two directions at nearly 90. 2V: 54-75 except for Pigeonite (0 to 40 ) and Wollastonite (39 ) Interference Figure: Opt (+) for most Opt (-) for Agerine and Wollastonite Extinction : Inclined for all clinopyroxene. 24
25 Phase Relationships Mg rich Enstatite is the High T end member, Fe rich Ferrosilite. Hypersthene represents an intermediate stage. Compare Fig 44 temp to Olivine and Plag phase diagram. This is a typical Solid Solution Series, similar to what we saw for olivine, except this system has an inversion. The exact position of the inversion curve is hard to plot. Above and to the right, any pyroxenes which crystallize are monoclinic. Below and to the left any pyroxene to crystallize are orthorhombic. Note on Fig 44 that the inversion curve dies out, at lower temperature you would expect conversion of monoclinic pyroxene to orthorhombic pyroxene.
26 Phase Relationships cont. A - the melt cools, hits the liquidus and crystallizes orthorhombic crystals B - the melt cools, some crystals may be monoclinic C - At T1 crystals are monoclinic but on further cooling to T2, they cross over the inversion curve and become orthorhombic If you Quench a monoclinic pyroxene (Pigeonite) crystal quickly, you preserve the monoclinic structure.eg volcanic rocks at the surface such as pillowed basalts. Pigeonite is monoclinic and its structure is less ordered and wider apart. This allows Ca ions into the structure more easily. The available Ca is taken up into the crystal lattice.
27 Phase Relationships cont. If a monoclinic pyroxene is allowed to cool slowly (gabbro or cumulate magma chamber) and crosses the inversion curve, the structure collapses into a more ordered, orthorhombic arrangement and the Ca ions are expelled. This expelled Ca rich fluid can: 1) Form Ca rich Clinopyroxene under the right conditions (rapid cooling) or 2) Form Ca rich Orthopyroxene below the inversion line. In this case, exsolution lamellae will form within the monoclinic clinopyroxene. An example of this is Inverted Pigeonite where you have clinopyroxene lamellae in an orthopyroxene. Right of line CPX INVERSION OPX + Ca = OPX + CPX Lamellae Left of line CPX + OPX INVERSION OPX
28 Phase Relationships cont. Now lets examine the system Mg 2 SiO 4 -SiO 2 Aside: Examine a system where no solid solution occurs. 1) Start with a liquid 70% A and 30% B and cool 2) At T1 (if slow cooling and equilibrium) crystals of pure A will form. 3) At T2 the system continues to crystallize pure A while the liquid becomes more enriched in B. 4) At T3 Both pure A and pure B crystals will form. This precipitation continues until all the liquid is used up. The temperature remains constant at this point. This is a BINARY EUTECTIC POINT and the crystals will be in proportion according to the eutectic axis (in this case about 50:50). 5) Continue to cool after liquid is used up. Eventually you will end up with a solid mixture of 70% A and 30% B.
29 Phase Relationships cont. Now back to the system Mg 2 SiO 4 -SiO 2 Case [A] 1) Take a melt of composition A and cool. At T1 crystals of pure olivine (Mg rich Forsterite) start to form. As we continue down the liquidus, olivine continues to crystallize and the melt becomes more silica rich. 2) At T2 considerable olivine has crystallized out. The early formed olivine crystals are unstable in silica rich liquid and a reaction occurs: Mg 2 SiO 4 + SiO 2 2MgSiO 3 (olivine) (melt) (clinoenstatite)
30 Phase Relationships cont. Because of this reaction, the liquid composition never proceeds past point (G the PERITECTIC) and pure SiO 2 (quartz) will never precipitate. The original composition of the melt [A] was to the left of clinoenstatite, therefore there is never enough liquid to convert all of the olivine crystals to clinoenstatite. Because of where we started on the phase diagram, the end product will be Forsterite + Enstatite. Case [B] Now lets consider a new example where a liquid of composition [B] starts cooling. 1) At point point (a) (above the peritectic), crystals of forsteritic olivine will precipitate at much lower temperature T3. The remaining liquid will be SiO 2 enriched. 2) At point G at T2 the same reaction occurs converting olivine + melt to enstatite. HOWEVER,, the starting point was to the right of the enstatite compositional boundary, therefore, there is enough SiO 2 present to convert ALL the olivine to enstatite. 3) When the reaction is complete (all the olivine converted), the system moves down the liquidus to point F. Point F is a normal, Binary Eutectic. During this cooling, new clinoenstatite and quartz will precipitate.
31 Phase Relationships cont. Note: In a rapidly cooled rock, in thin section you will often see olivine crystals with enstatite rims. In this case, quartz would be reconverted (because free quartz and olivine cannot exist in a rock in equilibrium). Case [C] Olivine Orthopyroxene At T4 pure SiO 2 forms.cooling continues to point F. The melt depletes in SiO 2 and enriches during cooling. At Point Fa solid composed of enstatite and quartz of eutectic composition is formed. Reaction continues until all liquid is used up. Orthpyroxene Paragenesis 1) Mg rich varieties are common in ultra basic rocks. Eg: enstatite associations Ol-Di-Augite-Spinel 2) Ultra Mafics Hartzburgites (Opx + Ol) Pyroxenites (Opx +Cpx) large crystals Lherzolites (Opx + Cpx + Ol + Plag)
Orthpyroxene Paragenesis cont. 3) Orthopyroxenites - Gabbro (cumulates, EastBull Lake) The Opx crystals accumulate as layers on the bottom of the magma chamber (often interlayered with Ol). It occurs in well defined layers as opposed to isotropic gabbros (not well defined). 4) Volcanics. Eg andesites often contain Mg rich phenocrysts. 5) Norites. Opx is an essential constituent of norite (Sudbury). Some come from uncontaminated basic magmas. However, many show assimilation of Al rich sediments. Ca(Mg,Fe)Si 2 O 6 + Al 2 SiO 5 (Mg,Fe)SiO 3 + CaAl 2 Si 2 O 6 (augite) (mica or clay) (hypersthene) (anorthite) 32