Miscibility gap

A miscibility gap is a region in a phase diagram for a mixture of components where the mixture exists as two or more phases - any region of composition of mixtures where the constituents are not completely miscible.

The IUPAC Gold Book defines miscibility gap as "Area within the coexistence curve of an isobaric phase diagram (temperature vs composition) or an isothermal phase diagram (pressure vs composition)."^[1]

A miscibility gap between isostructural phases may be described as the solvus, a term also used to describe the boundary on a phase diagram between a miscibility gap and other phases.^[2]

Thermodynamically, miscibility gaps indicate a maxima (e.g. of Gibbs energy) in the composition range.^[3][4]

Named miscibility gaps

A number of miscibility gaps in phase systems are named, including:

• The Huttenlocher (found in bytownite, Anorthite composition An55-95.), Boggild (in labradorite, An39-48 and An53-63.) and Peristerite (in oligoclase, ~An5-15.) miscibility gaps in the plagioclase feldspars.^[5][6][7]
• A Nishwawa horn, term for a miscibility gap existing when phases with different magnetic properties co-exist in the phase diagram.^[8]
• Miscibility gaps in liquid states may be referred to as oiling out, and commonly occurs in oil/water mixtures.^[9]

See also

• Solid solution
• Incongruent melting

References

1. "miscibility gap", IUPAC Gold Book
2. Bucher, Kurt; Grapes, Rodney (2011), "4.7.4 Miscibility Gaps and Solvus Thermometry", Petrogenesis of Metamorphic Rocks, Springer
3. "Miscibility Gaps", MTDATA – Phase Diagram Software from the National Physical Laboratory, 7 May 2010
4. "Phase diagrams" (PDF), www.its.caltech.edu, p. 1
5. Parsons, Ian, ed. (1994), Feldspars and their Reactions: [proceedinsg of the NATO Advanced Study Institute on Feldspars and Their Reactions, Edinburgh, United Kingdom, June 29-July 10, 1993], Springer, Subsolidus Phase Relations of the Plagioclase Feldspar Solid Solution, pp.221-2
6. Wenk, Hans-Rudolf; Bulakh, Andrei (2004), Minerals: Their Constitution and Origin, Cambridge University Press, p. 326
7. Pokines, James; Symes, Steven A. (2013), Manual of Forensic Taphonomy, CRC Press, p. 57
8. Zhao, Ji-Cheng (2011), Methods for Phase Diagram Determination, Elsevier, pp. 38, 46, 390
9. Beckmann, Wolfgang (2013), "3.3.6 Oiling-out", Crystallization: Basic Concepts and Industrial Applications, John Wiley & Sons