A miscibility gap is a region in a
phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseou ...
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)."
A miscibility gap between isostructural phases may be described as the ''
solvus
In a physical or geochemical system, a solvus is a line (binary system) or surface (ternary system) on a phase diagram which separates a homogeneous solid solution from a field of several phases which may form by exsolution or incongruent melting ...
'', a term also used to describe the boundary on a phase diagram between a miscibility gap and other phases.
Thermodynamically, miscibility gaps indicate a maximum (''e.g.'' of
Gibbs energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and p ...
) in the composition range.
Named miscibility gaps
A number of miscibility gaps in phase systems are named, including
* The ''huttenlocher'' (found in
bytownite
Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals with composition between anorthite and labradorite. It is usually defined as having between 70 and 90% An (formula: (Ca0.7-0.9,Na0.3-0.1) l(Al,Si)Si2 ...
, anorthite composition An55-95.), ''boggild'' (in
labradorite
Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect (schiller).
Labradorite is an intermediate to calcic member of the plagioclase series. It ...
, An39-48 and An53-63.) and peristerite (in
oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite ( Na Al Si3 O8) and anorthite ( CaAl2Si2O8). The albite:ano ...
, ~An5-15.) miscibility gaps in the
plagioclase feldspars.
* A ''Nishwawa horn'', term for a miscibility gap existing when phases with different magnetic properties co-exist in the phase diagram.
* Miscibility gaps in liquid states can cause
spinodal decomposition
Spinodal decomposition is a mechanism by which a single thermodynamic phase spontaneously separates into two phases (without nucleation). Decomposition occurs when there is no thermodynamic barrier to phase separation. As a result, phase separatio ...
, commonly referred to as ''oiling out'', as commonly occurs in oil/water mixtures.
See also
*
Miscibility
Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...
*
Solid solution
A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...
*
Incongruent melting
Incongruent melting occurs when a solid substance does not melt uniformly, so that the chemical composition of the resulting liquid is not the same as that of the original solid. During incongruent melting a new solid of different composition form ...
References
{{Chemical solutions
Materials science
Phase transitions
Geochemistry