Alkali Feldspar Solvus
The alkali feldspars - K-feldspar (KAlSi3O8, abbreviated Ks) and albite (NaAlSi3O8, abbreviated Ab) - form a binary system with an additional complication not seen in the simple olivine solid solution: at low temperatures, a single homogeneous feldspar is no longer stable, and the system separates into two distinct coexisting feldspars, one K-rich and one Na-rich. The curve that marks the boundary of this two-feldspar region is the solvus. [1] The physical reason for the solvus is exactly what the solid solution page established: at high temperatures, K+ and Na+ can substitute for each other despite their ~25% size difference, but at lower temperatures the crystal structure loses this tolerance and the two cations segregate into separate phases. [1] Water also plays an important role in this system: the presence of water substantially reduces the melting temperatures of both feldspars relative to dry conditions. [1]
The System at 1.96 kbar Water Pressure
At 1.96 kbar water pressure, the diagram shows separate liquidus and solidus curves for the K-feldspar-rich and albite-rich portions of the system. Each half behaves like a simple solid solution system similar to the olivine case. The two liquidus segments meet in the middle at 770°C, which is the binary minimum - the lowest temperature at which any melt can exist in this system at this pressure. [1] Below the solidus and above the solvus, complete solid solution is possible - a single homogeneous alkali feldspar can span the entire compositional range. [1] A tie-line drawn across the solvus at any temperature within the two-feldspar field gives the equilibrium compositions of the coexisting albite and K-feldspar at that temperature.
Crystallization of a 60% Ks Melt at 1.96 kbar
Consider a melt whose composition is 60% K-feldspar component and 40% albite component at 950°C. Crystallization proceeds as follows. [1]
At some temperature below 950°C (point b), crystallization of Ks92 crystals begins - the first crystals are strongly enriched in K relative to the melt. [1] When temperature drops to 850°C (point c), the existing Ks87 solid phase coexists stably alongside a melt containing 50% Ks, yielding a bulk mixture of 28% crystalline material and 72% liquid. [1] Solidification concludes at 786°C (point d), yielding a final crystalline phase of Ks60 (identically matching the initial bulk liquid). The final vestige of liquid remaining immediately prior to total crystallization measured 33% Ks. [1]
Exsolution Below the Solvus
Continued cooling of the now fully solid Ks60 feldspar eventually brings the system to the solvus. Below this curve (point e), a single homogeneous feldspar is no longer the lowest-energy configuration - it is more stable for the system to separate into two distinct phases. The first-formed albite to exsolve from the K-feldspar has a composition of Ks14Ab86 (strongly Na-rich). As cooling continues to point f, the exsolved albite grows progressively richer in Na while the remaining K-feldspar host grows progressively richer in K, as the two phases exchange Na and K to approach their equilibrium compositions at each new temperature. [1] This exsolution produces the characteristic perthite texture seen in many igneous and metamorphic rocks - thin lamellae or blebs of albite coherently intergrown within a K-feldspar host crystal, recording the history of cooling through the solvus.
The System at 5 kbar Water Pressure
At higher pressure (5 kbar water pressure), the solvus shifts to intersect the liquidus and solidus curves - the two-feldspar stability field now intrudes into the crystal + melt region. This means that complete solid solution between albite and K-feldspar is no longer possible at any temperature, and two separate stability fields exist for albite and K-feldspar even during crystallization. [1]
Consider a melt with 50% Ks and 50% Ab at 850°C (point a). K-feldspar crystallization begins at point b with Ks91 crystals - again strongly K-enriched first crystals. At 750°C (point c), Ks85 crystals coexist with a 37% Ks melt, and the system is 28% crystals and 72% melt. Further cooling brings the system to the eutectic at 703°C (point d), where albite also begins to crystallize alongside K-feldspar. The last melt at this eutectic has composition 28.5% Ks, and the coexisting feldspar crystals have compositions of Ks19 (albite-rich) and Ks53 (K-feldspar-rich). When crystallization is complete, the rock consists of 91% K-feldspar crystals and 9% albite crystals. [1] Below the eutectic the two already-crystallised feldspars continue to exchange Na and K as they cool, progressively enriching the albite in Na and the K-feldspar in K. [1]
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References & Citations
- 1.Introduction to Mineralogy Nesse, W. D.
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