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Mineral compositions may offer vital clues to the conditions under which they were produced. This is well illustrated by the temperature‐dependent substitution of potassium (K⁺¹) and sodium (Na⁺¹) in the alkali feldspars (Na,K)AlSi₃O₈, as illustrated by the albite–orthoclase phase diagram (Figure 3.9). Because albite‐rich plagioclases and potassium feldspars are abundant in felsic/acidic igneous rocks, a significant component of continental crust, this diagram is useful in understanding their formation.

At high temperatures (>~620 °C at 1 atm pressures) a complete substitution solid solution series exists between the two end members. These are the potassium feldspar orthoclase (KAlSi₃O₈) and the sodium plagioclase feldspar, albite (NaAlSi₃O₈). Feldspar crystals that form at high temperatures can have any proportions of orthoclase (Or) or albite (Ab) end member. Actual proportions depend largely on the composition of the system; that is, the availability of potassium and sodium ions. Because a complete solid solution exists between the two end members, crystallization and melting in this system share many similarities with the albite–anorthite system (see Figure 3.7) discussed earlier. For systems with <40% Or, initial crystals are rich in the albite plagioclase component. As plagioclase crystals continue to separate on cooling, they react continuously with the melt so that crystal composition changes down the solidus as the remaining liquid changes composition down the liquidus, both toward increasing Or content until no melt remains.

Figure 3.9 Albite–orthoclase phase diagram at atmospheric pressure.

The result is a solid composed of sodic plagioclase, with a potassic orthoclase component in solid solution. For systems with >40% Or, initial crystals are relatively enriched in the potassium feldspar (orthoclase) component. As such crystals continue to separate on cooling, they react continuously with the melt so that crystal composition changes down the solidus as the remaining liquid changes composition down the liquidus, both toward decreasing Or content until all the melt is used up. The result is a rock composed of a feldspar solid solution. For systems with >40% Or, these crystals may be thought of as potassic orthoclase crystals with an albite component in solid solution. All solid solutions between the two end members are stable at high temperatures (and low pressures) after they begin to cool below the solidus temperature.

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 solvus temperature (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 Or₇₀ in Figure 3.9) that is a complete solid solution of composition Or₇₀ (Ab₃₀) as it cools below the solidus temperature. As this feldspar cools it eventually intersects the convex‐up solvus curve 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 exsolves into two stable, but distinctly different, feldspars whose compositions lie on the solvus line that borders the miscibility gap. In this case, plagioclase of composition Ab₇₀ (Or₃₀) 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 ~Or₈₈ and the plagioclase component (point D) is ~Or₃. 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 exsolution of 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.,Ab₈₀), 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.