Exsolution

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Solid solutions frequently are not as compatible at low temperature as they are at high temperature. Differences in ionic radius that can easily be accommodated when atoms are highly thermally agitated result in unsustainable lattice deformations as the mineral cools. Thus we often find that minerals segregate into discrete phases when they cool, just as oil and water separate. This phenomenon is called exsolution.

exsol00.gif (3099 bytes) The system K-feldspar-albite is a binary solid solution series with a minimum.
Exsol01.GIF (2148 bytes) A melt initially at point a will begin to crystallize when it cools to point b. The resulting solid will be of composition c. As the system cools further the melt slides down the liquidus from b to d, and the solid slides down the solidus from c to e. When the solid composition reaches e, the system is entirely solidified. As it cools further, it drops down into the solid field, for example, to f.

The system might cool indefinitely without doing anything else. However, solid solution systems that are compatible at high temperatures are not always compatible at low temperatures.

Exsol02.GIF (2278 bytes) The system K-feldspar-albite is such a system. The thermal agitation at high temperatures is great enough that the Na ion (radius 0.97 Angstroms) and the K ion (radius 1.33 Angstroms) can substitute. As the feldspar cools and contracts, the size mismatch becomes too great to accommodate. Suddenly, the feldspar begins to separate into two distinct phases, one Na-rich, one K-rich. As the system cools, the two phases tend to become more distinct.
exsol04.gif (2528 bytes) The complete diagram looks something like this. There is a field in the solid state where the solid separates into two solids. This phenomenon is called exsolution and is very common. Most solid solutions show it to some extent.
Exsol03.GIF (3661 bytes) The composition of the two separating phases is given by the two points on the boundary of the two-solids field. The relative amounts of each solid are found by comparing the two solids to the overall system as shown.

In the case shown here, S2 (the K-rich phase) is by far the most abundant. The resulting solid will be mostly K-feldspar with irregular blobs of plagioclase. This material is called perthite. A feldspar that is mostly plagioclase with inclusions of K-feldspar is termed antiperthite.

The segregation of alkali feldspar and K-feldspar into perthite is perhaps the most important geological manifestation of exsolution, but there are a lot of other examples:

A More Complex System

exsol04.gif (2528 bytes) The diagram for the system K-feldspar-albite is actually more complex. At 5 kb in a water-saturated system, it looks more like this. Most of the diagram can be interpreted as we did above, but what happens if a system is in the range where the liquid+solid field lies directly above the two-solid field?

The way to sort out this enigma is to read the fields. When you're on the boundary of two fields, all the phases in both fields have to be present. In other words, you will have liquid plus two solids. Crossing the boundary, you go from a state with one solid plus liquid to two solids only. That means crossing the boundary entails disappearance of the melt plus appearance of a second solid phase.

Just before the system reaches the boundary, there will be liquid (whose composition is given by the liquidus) and solid (given by the solidus). When the system reaches the boundary, two feldspars will begin to form, one K-rich, one Na-rich. As Na-rich feldspar forms, the overall composition of the solid shifts along the line to the left. When it equals the original melt composition, crystallization is complete.

This will get complicated! Here's a closeup of the boundary region with fields labeled. For clarity fields may not be labeled in the diagrams below.
Just before the system reaches the boundary. M=melt, X=the overall system and S=solid. Note that the system is still 50% liquid.
The system is on the boundary. We can see from the fields that liquid and two solids must be present. The already existing solid partitions into two phases (1 and 2). S represents the overall composition of the solid, and we can see that here it is still 100% Solid 2 in composition.

If we compare the states immediately before and after reaching the boundary, we see that both before and after we have the system, the melt, and a solid of composition 2. What's new is the appearance of Solid 1.

If this were a conventional solid solution, the solid composition would slide along the liquidus until it reached the original system composition. It still does that here. As Solid 1 forms, the overall composition of the solid shifts toward Solid 1 and toward the initial system composition.

We have five points to track! The melt, the initial system, the bulk solid composition, and Solid 1 and Solid 2. But only one (the bulk solid composition) can move as long as the elt is at the eutectic.

Here the the bulk solid composition has nearly reached the initial system composition, implying that the system is almost entirely solidified. The distances S1 and S2 give the relative amounts of solids 1 and 2.
The system is entirely solidified and the solid composition equals the system composition. As temperature drops, X,S,1 and 2 drop. 1 become a bit richer in Albite and 2 becomes richer in K-spar.

What will happen in a real rock depends on how rapidly solidification can take place. Volcanic feldspars that are about half albite and half K-spar cool rapidly enough that the phases don't have time to separate. Monoclinic feldspars (more K-rich) are termed sanidine, triclinic (albite-rich) feldspars are termed anorthoclase.

If the cooling is rather slow, there may be enough time for the solid to react thoroughly with the melt and remain in equilibrium. The two solid phases will separate to form a perthite or antiperthite. Perthite texture on a microscopic scale is termed microperthite, perthite on a submicroscopic scale (detectable by X-ray study, for example) is called cryptoperthite.

In some cases the Na-rich feldspar can form rapidly enough that it surrounds an earlier-formed core of K-feldspar, resulting in so-called rapakivi texture, named for a place in Finland where it is common. The Wolf River Batholith contains a great deal of rapakivi granite.


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Created November 16, 1999, Last Update 27 January 2012

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