Exsolution

At high temperatures, crystal structures tolerate a wider range of cation sizes substituting for each other than they do at low temperatures. When a mineral that formed as a homogeneous solid solution at high temperature cools below a critical temperature called the solvus, that solid solution becomes unstable - the crystal can no longer accommodate both cation types in a single homogeneous phase, and the mineral spontaneously separates into two distinct coexisting phases. The process by which this separation occurs within an already-crystallised solid is exsolution. ^[1] Exsolution does not involve melting or large-scale atomic migration - it proceeds entirely in the solid state, driven by the thermodynamic instability of the original single-phase composition below the solvus. ^[1]

The Alkali Feldspar Example

The alkali feldspars - K-feldspar (KAlSi₃O₈) and albite (NaAlSi₃O₈) - provide the best-documented example of exsolution in mineralogy. At high temperatures these two end members display extensive solid solution: Na⁺ and K⁺ substitute freely for each other and a single homogeneous alkali feldspar can span the entire compositional range from pure albite to pure K-feldspar. At lower temperatures a solvus appears because only limited amounts of K⁺ can substitute for Na⁺ in the albite structure, and vice versa - the size difference between the two cations, which is tolerable at high temperature, becomes too large for the structure to accommodate at low temperature. ^[1]

A Worked Example at 1 kbar

Consider an alkali feldspar of composition Ab₄₀Ks₆₀ (40% albite, 60% K-feldspar component) that crystallised from a granitic magma and is now cooling through 700°C. At this temperature the composition lies outside the solvus - a single homogeneous feldspar is the stable configuration, and the crystal is intact. ^[1]

Continued cooling brings the feldspar to 590°C - the temperature at which the solvus is reached for this composition at 1 kbar water pressure. Below this temperature the K-feldspar structure can no longer hold all its Na⁺. Provided sufficient time is available for nucleation, small domains of albite begin to crystallise and grow within the K-feldspar host, concentrating the excess Na⁺ into those domains. Because albite and K-feldspar have essentially the same crystal structure framework, forming albite domains does not require breaking up the existing structure - it requires only diffusion of Na⁺ into the nascent domains and migration of K⁺ out of them in the other direction. The albite typically forms as irregular lamellae within the K-feldspar. A tie-line across the solvus at 590°C gives the equilibrium composition of the exsolving albite as Ab₈₅Ks₁₅. ^[1]

As cooling progresses further, the solvus continues to narrow the range of Na⁺ tolerated in the K-feldspar host and the range of K⁺ tolerated in the albite lamellae. Both phases therefore become progressively purer as temperature drops. By 500°C, the K-feldspar host has a composition of Ab₂₈Ks₇₂ and the albite lamellae have become Ab₉₃Ks₇. The relative proportions of the two phases at this point - 81% K-feldspar host and 19% albite lamellae - are given by the lever rule applied to the solvus tie-line at that temperature. ^[1]

Rate Controls: Temperature and Time

Exsolution requires ions to diffuse through the crystal structure, so it is inherently dependent on both temperature and the time available. At high temperatures diffusion is fast; at low temperatures it becomes very slow. The practical consequence is that volcanic rocks and shallow intrusives - where cooling is rapid - rarely show significant exsolution because there is not enough time for the diffusion to proceed to a measurable degree. Deep-seated intrusives and regionally metamorphosed rocks - where cooling is extremely slow over millions of years - typically show well-developed exsolution lamellae that may be visible in thin section or even in hand specimen in coarse pegmatites. ^[1]

Perthite and Antiperthite

K-feldspar that contains microscopic or larger lamellae of albite produced by exsolution is called perthitic or is referred to as perthite. In some K-feldspar, particularly from granitic pegmatites, the exsolution lamellae grow large enough during very slow cooling to be visible to the naked eye in a hand specimen. ^[1]

The reverse texture - albite host with K-feldspar lamellae exsolved from it - is called antiperthitic or antiperthite. Antiperthite is less common than perthite because most alkali feldspars crystallise with K-feldspar as the dominant phase in silica-rich magmas, but it is found where albite-rich compositions crystallised and then cooled slowly through the solvus from the albite side. ^[1]

Exsolution in Other Mineral Groups

Exsolution is not confined to the alkali feldspars. It is a common feature of the pyroxenes, where Ca-rich and Ca-poor pyroxenes unmix on cooling, producing lamellae of one pyroxene variety within a host of another. The Fe-Ti oxides - particularly ilmenite and titanomagnetite - also commonly display exsolution textures as they cool, with ilmenite exsolving from magnetite or vice versa depending on the bulk composition and cooling history. ^[1]

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