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The nepheline–silica phase diagram (Figure 3.10) illustrates a type of two‐component system in which there is an intermediate compound whose composition can be produced by combining the compositions of the two end member components. In this case silica (SiO₂) and nepheline (NaAlSiO₄) are the two end member components. The intermediate compound formed by combining one molecular unit of nepheline and two of silica [NaAlSiO₄ + 2(SiO₂)] is the plagioclase mineral albite (NaAlSi₃O₈). No solid solution exists between nepheline, albite, and silica minerals. Compositions are expressed on the horizontal axis in terms of molecular percent silica (SiO₂) component, so that the percentage of nepheline component is %Ne = 100% − %SiO₂ component. The composition of the intermediate compound albite is two‐thirds SiO₂ component. Temperature increases on the vertical axis; pressure is 1 atm. The polymorphs of silica (see Figure 3.6) that crystallize in this system are the high‐temperature, low‐pressure minerals cristobalite and tridymite, but the more common polymorph of silica at slightly higher pressures is quartz. All these silica minerals have the same chemical composition, but different crystal structures.

Several important concepts are illustrated and reinforced by the nepheline–silica phase diagram (Figure 3.10). The most important is the notion of silica saturation, which is fundamental to igneous rock classification (Chapter 7). When there is sufficient silica component (more than two‐thirds) so that each molecular unit of nepheline component can be converted into albite by adding a molecular unit of silica with an additional silica component remaining, the system is said to be oversaturated with respect to silica. Evidence for silica oversaturation is the presence of a silica mineral, such as tridymite or quartz formed from the excess silica, along with plagioclase feldspar in the final rock. When there is insufficient silica component (less than two‐thirds) to convert each molecular unit of nepheline into albite by adding a molecular unit of silica, the system is said to be undersaturated with respect to silica. Evidence for silica undersaturation is the presence of a low silica feldspathoid mineral such as nepheline in the final rock. Only when the silica component is exactly two‐thirds is there precisely the amount of silica component required to convert each molecular unit of nepheline into albite. Such systems are said to be exactly saturated with respect to silica. Evidence for exact silica saturation is the presence of feldspar and the absence of both silica and feldspathoids from the final rock, which in the system nepheline–silica consists of 100% albite. The International Union of Geological Sciences (IUGS) classification of igneous rocks (Chapter 7) is largely based on the concept of silica saturation; rocks in the upper triangle contain quartz and feldspar, whereas those in the lower triangle contain feldspathoids and feldspar. Rocks that lie on the line or join between the two triangles contain feldspar but neither quartz nor feldspathoids and are ideally saturated with respect to silica.

The nepheline–silica phase diagram shows some similarity to the diopside–anorthite phase diagram. The most significant difference is the presence of two eutectic points where troughs in the liquidus intersect the solidus. For many purposes, this diagram may be interpreted as two side‐by‐side eutectic diagrams: one diagram for undersaturated compositions (less than two‐thirds silica component), with a eutectic point at 1070 °C and ~62% silica component, and a second diagram for oversaturated compositions (over two‐thirds silica component), with a eutectic point at 1060 °C and ~77% silica component. A brief discussion of the crystallization and melting behaviors for these two compositional ranges follows.

Let us first examine the behavior of silica oversaturated systems with between 78 and 100% silica component. If a cooling melt with silica content between 89 and 100% intersects the liquidus, the first crystals to separate are composed of the high temperature silica polymorph called cristobalite. With continued cooling (Figure 3.10), more cristobalite separates from the melt, causing its composition to evolve down the liquidus toward lower percentages of silica as the proportion of melt decreases. As the system reaches a temperature of 1470 °C, cristobalite becomes unstable and inverts isothermally to the stable, low temperature polymorph of silica called tridymite. This ideal inversion temperature is shown by the phase boundary between cristobalite and tridymite in the silica plus melt field. With continued cooling below 1470 °C, more tridymite separates from the melt, and melt compositions continue to evolve (with progressively lower silica concentrations) down the liquidus toward the eutectic (E₁) at 1060 °C. Upon reaching the eutectic, both albite and tridymite crystallize simultaneously until the melt is used up and the system enters the solid albite plus tridymite field. For compositions between 78 and 89% silica component, the behavior is similar except that the first crystals to form are tridymite. Final rock compositions can be calculated using the lever rule.

For compositions between ~67% and 78% SiO₂ (Figure 3.10), albite crystallizes when the system cools to the liquidus temperature. Continued separation of albite on cooling causes the liquid composition to move down the liquidus toward increasing silica content. As the system cools to the eutectic temperature of 1060 °C, albite and tridymite crystallize simultaneously and isothermally until the melt is used up. The final rock contains both albite and a silica mineral in proportions that can be determined by the lever rule.

Let us now examine the behavior of so‐called silica undersaturated systems with between 0 and 67% silica component. For compositions between 62 and 67% silica, cooling of the system to the liquidus temperature causes albite crystals to separate from the melt (Figure 3.10). Continued cooling below the liquidus temperature causes further separation of albite from the melt which causes melt compositions to change down the liquidus to the left toward decreasing silica content. As the eutectic temperature (E₂) is reached at 1070 °C, both albite and nepheline crystallize isothermally until the melt is used up. The final rock contains percentages of both albite and nepheline that can be determined by the lever rule. Lastly, for those compositions with 50–62% silica component addressed in the diagram (additional complexities, not shown, exist for systems with lower amounts of silica component), the first crystals to separate are nepheline crystals. Continued separation of silica‐poor nepheline causes melt compositions to change down the liquidus toward the eutectic at 1070 °C. At the eutectic, both albite and nepheline crystallize isothermally until the melt is used up. Once again the final rock is composed of albite and nepheline, and their percentages can be calculated using the lever rule.