EARTH232 Lecture Notes - Lecture 8: Forsterite, Enstatite, Eutectic System
26 Aug 2016
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Lecture 8: phase relationships discussed thus far pertain to ca-poor pyroxenes, binary 2 chemical components. Mg2sio4 and sio2: binary eutectic diagram two phases for minerals, binary peritectic system for mg2sio4 sio2: 3 phases. Forsterite, quartz, enstatite: *disregard clinoenstatite in the diagram. Equilibrium crystallization slow cooling: a) above t1 all liquid. Liquid 2 is depleted in fo and enriched in silica. At t2 (point g), we hit the peritectic point. Chemical rxn b/w forsterite and silica and liquid to produce enstatite. Bulk composition look at bulk comp. , the reactant in bulk will not be used up first. For the rxn above; silica (melt) will be used up first. Final mineral assemblage = fo + en: b) bulk composition is slightly to the right of en. T3 to t2: continuous rxn l1 fo + l2. At g: discontinuous rxn at t3: fo + l en. This time fo is consumed we do not use up all the liquid (sio2)
