John O'Brien - Earth Materials

However, at lower temperatures (<~620 °C), the solid solution between orthoclase and albite becomes limited and a miscibility gap exists in which the solid solution between the two end members is unstable. The lower the temperature, the more limited the solid solution and the larger the miscibility gap becomes. As high temperature potassium‐sodium feldspar solid solutions cool, they eventually reach the solvustemperature ( Figure 3.9), a phase stability boundary that separates the conditions under which a complete solid solution is stable from conditions under which solid solutions are unstable. The solvus temperature is generally highest for compositions with large amounts of both end members. Below the solvus temperature, the original complete solid solution becomes unstable and begins to unmix or exsolve into an intergrowth of two distinct feldspars, one enriched in Ab component, the other in Or component.

Let us examine a potassium‐rich feldspar (line Or 70in Figure 3.9) that is a complete solid solution of composition Or 70(Ab 30) as it cools below the solidus temperature. As this feldspar cools it eventually intersects the convex‐up solvuscurve at point A, at a temperature of 520 °C, below which the solid solution becomes unstable. At temperatures below the solvus, the original solid solution unmixes or exsolvesinto two stable, but distinctly different, feldspars whose compositions lie on the solvus line that borders the miscibility gap. In this case, plagioclase of composition Ab 70(Or 30) begins to unmix (exsolve) from the potassic feldspar as the solid solution becomes limited and a miscibility gap is created. Because the solid solution becomes increasingly limited and the miscibility gap widens as the temperature decreases, more plagioclase exsolves from the potassic feldspar and becomes increasingly sodic (Ab rich) as the crystals cool. As a result, the composition of the exsolved plagioclase evolves down the solvus to the left toward increasing Ab enrichment. Because albite component is exsolving from the potassic feldspar, the latter's orthoclase content progressively increases as its composition evolves down the solvus to the right. The lever rule can be used to trace the proportions and the composition of the exsolved plagioclase and the potassic feldspar at any temperature. Tie line C–D (~85 Or units long) between the two feldspar compositions on the solvus can be used for this purpose. On cooling to 300 °C, the potassic feldspar component (point C) is ~Or 88and the plagioclase component (point D) is ~Or 3. The percentage of exsolved Ab‐rich plagioclase, given by line segment C–F, is ~21% (18/85), and the percentage of potash feldspar, given by line segment D–F, is ~79% (67/85). Progressive unmixing (exsolution) produces one feldspar increasingly enriched in potassium (Or) and another feldspar increasingly enriched in sodium (Ab). For initially potassium‐rich feldspar solid solutions, the result is a specimen of potassium‐rich feldspar that contains sodium‐rich feldspar blebs, stringers or patches. A potassium feldspar crystal that contains sodium feldspar blebs, stringers or patches produced by the exsolutionof two distinct feldspars is called perthite. Look closely at most potash feldspar crystals (e.g., orthoclase, microcline or sanidine) and you will see the generally less transparent blebs and stringers of plagioclase produced by exsolution. For initially albite‐rich compositions (e.g.,Ab 80), the result of exsolution can be plagioclase crystals that contain blebs, patches and/or stringers of exsolved orthoclase in albite and are called antiperthite. Antiperthite is less common than perthite because calcium‐rich plagioclase does not form a solid solution series with orthoclase or any other potassic feldspar.

Figure 3.10 Phase diagram for the system nepheline–silica with the intermediate compound albite, at atmospheric pressure.

The nepheline–silica phase diagram( Figure 3.10) illustrates a type of two‐component system in which there is an intermediate compoundwhose composition can be produced by combining the compositions of the two end member components. In this case silica (SiO 2 )and nepheline (NaAlSiO 4 )are the two end member components. The intermediate compound formed by combining one molecular unit of nepheline and two of silica [NaAlSiO 4+ 2(SiO 2)] is the plagioclase mineral albite (NaAlSi 3 O 8 ). No solid solution exists between nepheline, albite, and silica minerals. Compositions are expressed on the horizontal axis in terms of molecular percent silica (SiO 2) component, so that the percentage of nepheline component is %Ne = 100% − %SiO 2component. The composition of the intermediate compound albite is two‐thirds SiO 2component. 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.