# Miscibility gap

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{{short description|Property of chemical mixtures}}
A '''miscibility gap''' is a region in a [phase diagram](/source/phase_diagram) for a [mixture](/source/mixture) of components where the mixture exists as two or more [phases](/source/Phase_(matter)) – any region of composition of mixtures where the constituents are not completely miscible.

The [IUPAC Gold Book](/source/IUPAC_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)."<ref>{{cite encyclopedia| title = miscibility gap| publisher = International Union of Pure and Applied Chemistry|version=Online version 3.0.1| doi = 10.1351/goldbook.MT07270|doi-access=free| encyclopedia = IUPAC Compendium of Chemical Terminology| year = 2019}}</ref>

A miscibility gap between isostructural phases may be described as the ''[solvus](/source/solvus)'', a term also used to describe the boundary on a phase diagram between a miscibility gap and other phases.<ref>{{citation| title = Petrogenesis of Metamorphic Rocks| first = Kurt |last = Bucher| first2 =  Rodney |last2= Grapes| publisher = Springer| year = 2011|chapter = 4.7.4 Miscibility Gaps and Solvus Thermometry}}</ref>

Thermodynamically, miscibility gaps indicate a maximum (''e.g.'' of [Gibbs energy](/source/Gibbs_energy)) in the composition range.<ref>{{citation| url = http://resource.npl.co.uk/mtdata/winmt_miscibility_gaps.html| title = Miscibility Gaps| work = MTDATA – Phase Diagram Software from the National Physical Laboratory| date = 7 May 2010}}</ref><ref>{{citation| url =http://www.its.caltech.edu/~me260/class%20notes/Phase_diagrams.pdf| title = Phase diagrams|page = 1| work = www.its.caltech.edu }}</ref>

The miscibility gap condition is a candidate for [thermal storage](/source/thermal_storage).<ref>{{cite web |title=Green steam and thermal storage startup reboots pilot plant after “tough lesson” |url=https://reneweconomy.com.au/green-steam-and-thermal-storage-startup-reboots-pilot-plant-after-tough-lesson/ |website=RenewEconomy |language=en-AU |date=28 April 2025}}</ref>

==Named miscibility gaps==
A number of miscibility gaps in phase systems are named, including
* The ''huttenlocher'' (found in [bytownite](/source/bytownite), anorthite composition An55-95), ''boggild'' (in [labradorite](/source/labradorite), An39-48 and An53-63) and peristerite (in [oligoclase](/source/oligoclase), ~An5-15) miscibility gaps in the [plagioclase](/source/plagioclase) feldspars.<ref>{{citation| title = Feldspars and their Reactions: [proceedinsg of the NATO Advanced Study Institute on Feldspars and Their Reactions, Edinburgh, United Kingdom, June 29-July 10, 1993]| editor-first = Ian |editor-last = Parsons| publisher = Springer| year =  1994|at = Subsolidus Phase Relations of the Plagioclase Feldspar Solid Solution, pp.221-2}}</ref><ref>{{citation| title = Minerals: Their Constitution and Origin| first = Hans-Rudolf| last = Wenk| first2= Andrei| last2= Bulakh| year = 2004| publisher = Cambridge University Press|page = 326}}</ref><ref>{{citation| title = Manual of Forensic Taphonomy| first = James|last =  Pokines| first2 = Steven A.|last2= Symes| publisher = CRC Press| year = 2013|page = 57 }}</ref>
* A ''Nishwawa horn'', term for a miscibility gap existing when phases with different magnetic properties co-exist in the phase diagram.<ref>{{citation| title = Methods for Phase Diagram Determination | first = Ji-Cheng| last = Zhao|publisher = Elsevier| year =  2011| pages = 38, 46, 390}}</ref>
* Miscibility gaps in liquid states can cause [spinodal decomposition](/source/spinodal_decomposition), commonly referred to as ''oiling out'', as commonly occurs in oil/water mixtures.<ref>{{citation| title = Crystallization: Basic Concepts and Industrial Applications | first = Wolfgang|last =  Beckmann|publisher = John Wiley & Sons| year =  2013| chapter = 3.3.6 Oiling-out}}</ref>

==See also==
* [Miscibility](/source/Miscibility)
* [Solid solution](/source/Solid_solution)
* [Incongruent melting](/source/Incongruent_melting)

==References==
{{Reflist}}

{{Chemical solutions}}
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Category:Materials science
Category:Phase transitions
Category:Geochemistry

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