Earth Materials. John O'Brien
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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.
3.2.7 Two component phase diagram: forsterite–silica
Another set of mineral relationships is well illustrated by the two‐component system forsterite–silica (Figure 3.11). Forsterite (Mg2SiO4) is the magnesium end member of the olivine solid solution series, and the silica mineral is commonly quartz (SiO2). As in the nepheline–silica system discussed above, this system contains an intermediate compound, in this case the orthopyroxene mineral enstatite (MgSi2O6 ), that can be thought of as being composed of one molecular unit of each of the two end member components (MgSiO4 + SiO2 = MgSi2O6). The horizontal axis in this phase diagram is weight % silica end member component (weight % SiO2), rather than the molecular proportions used in the nepheline–silica diagram. Temperature increases on the vertical axis. Six phase stability fields are defined: (1) 100% melt, (2) melt + quartz, (3) melt + enstatite, (4) melt + forsterite, (5) forsterite + enstatite, and (6) enstatite + quartz. No solid solution exists between the three minerals in this system (forsterite, enstatite, and quartz). Instead, a discontinuous reaction occurs between forsterite and enstatite in which early formed minerals react with the melt to produce new minerals at a specific temperature. These reactions occur when the system reaches point P on the liquidus line, the peritectic point at 1585 °C and 35% silica component. There is also a eutectic point (E) located in the trough in the liquidus where it intersects the solidus at 1540 °C and 46% silica. Let us examine four selected compositions in this system during crystallization, each of which demonstrates different behaviors and/or results.For compositions of >46% silica component by weight, the system behaves as a simple eutectic system. As melts cool to the liquidus, silica (quartz) begins to separate from the melt and continues to separate as the system cools further (Figure 3.11). This causes the composition of the melt to evolve down the liquidus toward lower silica contents. Upon reaching the eutectic at 1540 °C, both quartz and enstatite crystallize simultaneously until the melt is used up. For compositions of ~35–46% silica that are richer in silica component than the peritectic (P) composition, the system also behaves as a simple eutectic. The only change is