Figure 3.11 Phase diagram for the system forsterite–silica with the intermediate compound enstatite, at atmospheric pressure.

      Systems of between 0 and 35% silica component behave somewhat differently because they pass through the peritectic point where reactions occur between forsterite, enstatite, and melt. During cooling and crystallization, three fundamentally different situations can be recognized. For example, with a composition of 12% silica component (dashed line A, Figure 3.11), the system cools to the liquidus at 1810 °C, where forsterite begins to separate. Continued cooling causes additional forsterite to separate from the melt, which causes the melt composition to evolve down the liquidus toward the peritectic. At 1700 °C, the system consists of 50% forsterite crystals (line segment x) and 50% melt (line segment w) with a composition of ~24% silica component as inferred from the lever rule. Further cooling and separation of forsterite crystals cause the melt composition to approach the peritectic point at 1585 °C, where the lever rule shows that the system contains ~66% forsterite and ~34% melt of the peritectic composition 35% silica component. Below this temperature the system enters the 100% solid forsterite plus enstatite field with ~60% forsterite olivine line, as shown by tie‐line segment z) and ~40% enstatite, as shown by tie‐line segment y). So what happens when the melt reaches the peritectic? The percentage of solids increases as the melt is used up, and the percentage of solid forsterite decreases while the percentage of solid enstatite increases dramatically. The percentage of forsterite decreases because some of the forsterite reacts with some of the remaining melt to produce enstatite. Simultaneously the percentage of enstatite increases dramatically because as some olivine is converted to enstatite, new enstatite crystallizes simultaneously from the remaining melt until it is used up. More generally, for all compositions of <30% silica component (enstatite composition), the equilibrium behavior is (1) forsterite crystallization as the melt cools below the liquidus; (2) increasing proportions of olivine and decreasing proportions of melt as the melt cools; (3) evolution of the remaining silica‐enriched melt down the liquidus toward the peritectic; and (4) isothermal conversion of some forsterite to enstatite by discontinuous reaction with the remaining melt at the peritectic accompanied by additional isothermal crystallization of enstatite until the melt is used up. Some forsterite always remains because there is insufficient silica component to convert all of it into the intermediate compound enstatite. This indicates that the system was undersaturated with respect to silica. The peritectic reaction that converts the olivine mineral forsterite to the pyroxene mineral enstatite, as note previously, is called a discontinuous reaction. This phase diagram provides an excellent example of how early formed crystals can react with remaining melt to produce an entirely different mineral. These reactions are characteristic of the minerals in the discontinuous reaction series of Bowen's reaction series (Chapter 8).

Systems with exactly 30% silica component have the composition of the intermediate compound enstatite. They are precisely saturated with respect to silica. Line B (Figure 3.11) indicates the cooling behavior of such a system. Initial crystallization at the liquidus produces forsterite crystals. Continued cooling, accompanied by addition, separation, and growth of forsterite crystals, causes the remaining melt to evolve down the liquidus toward the peritectic with forsterite and melt percentages and melt compositions given by the lever rule. When this system reaches the peritectic, it contains 14% forsterite crystals and 86% melt with 35% silica component. Discontinuous reaction between the remaining melt and the forsterite crystals converts all the forsterite crystals to enstatite, while simultaneous crystallization of enstatite causes the melt to be used up. The final rock is 100% enstatite and is neither undersaturated (lacks forsterite) nor oversaturated (lacks quartz), but is 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 (Figure 3.11). 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.

      Compositions in this system between 30 and 35% silica component clearly show that minerals, such as forsterite, that are undersaturated with respect to silica can crystallize from magmas that are oversaturated with respect to silica. If equilibrium conditions between these crystals and the melt are maintained, they will eventually be converted to the intermediate compound and therefore will not be preserved. However, if disequilibrium conditions exist, of the kinds that commonly occur during fractional crystallization, early formed crystals may well be preserved in the final rock. In addition, because silica in the remaining melt was not used to convert forsterite to enstatite, the melt will be more enriched in silica than would otherwise be the case. As discussed in Chapter 8, such concepts are very important in understanding the evolution of magma composition.

      The system forsterite–silica (Figure 3.11) clearly illustrates the concept of silica saturation. Compositions of >30% silica (SiO₂) end member component by weight are oversaturated with respect to silica, so that there is sufficient silica to convert all the forsterite into enstatite, with additional silica remaining. Equilibrium crystallization in such silica‐rich systems produces the intermediate compound enstatite with excess silica to form quartz. As discussed in connection with the nepheline–silica diagram (see Figure 3.10), quartz forms by equilibrium crystallization of melts that are oversaturated with respect to silica. On the other hand, compositions of <30% silica component by weight are undersaturated with respect to silica, so that there is insufficient silica to convert all the forsterite into enstatite. Equilibrium crystallization in such silica‐poor systems produces forsterite plus as much of the intermediate compound enstatite as can be formed at the peritectic. Forsterite forms by equilibrium crystallization of melts that are undersaturated with respect to silica. As detailed in Chapter 7, both forsterite‐rich olivine and feldspathoids suggest crystallization from systems undersaturated with respect to silica. Systems with exactly 30% silica component are exactly saturated with respect to silica because they possess precisely the amount of silica component required to convert forsterite into enstatite without excess silica remaining.

      One can also investigate melting behaviors