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forsterite–silica242intermediate compoundenstatite (MgSi2O642262discontinuous reactionperitectic point

ssss1 Phase diagram for the system forsterite–silica with the intermediate compound enstatite, at atmospheric pressure.

discontinuous reaction

saturated with respect to silica

Systems with silica component contents of 30–35% silica are oversaturated with respect to silica and thus exhibit another set of behaviors. Line C (33% silica component) is representative of these behaviors (ssss1). The system cools to the liquidus at 1650 °C where forsterite crystals begin to separate. Continued separation of forsterite causes melt composition to evolve down the liquidus toward the peritectic. As the melt composition reaches the peritectic, the lever rule shows that the system contains ~6% (2/35) forsterite crystals and ~94% (33/35) melt with ~35% silica component. At the peritectic, all the forsterite is converted to enstatite with the remaining melt and additional enstatite crystallizes, but additional melt remains. The lever rule shows that as the system leaves the peritectic and enters the enstatite plus liquid stability field it contains ~40% (2/5) enstatite and ~60% (3/5) melt. Further cooling leads to additional crystallization of enstatite (30% silica component), which causes the remaining melt to evolve down the liquidus toward the eutectic (at 46% silica component). As the system reaches the eutectic, it contains ~81% enstatite (13/16) and ~19% (3/16) melt with 46% silica component. At the eutectic, enstatite and quartz crystallize simultaneously until the melt is used up. The final rock contains 96% (67/70) enstatite and 4% (3/70) quartz and records a melt that was oversaturated with respect to silica.