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 forms. For example, melting of
orthoclase
Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
(KAlSi
3O
8) produces
leucite
Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in ...
(KAlSi
2O
6) in addition to a melt. The melt produced is richer in
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
(SiO
2). The proportions of leucite and melt created can be recombined to yield the bulk composition of the starting
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
. Another mineral that can melt incongruently is
enstatite (Mg
2Si
2O
6), which produces
forsterite
Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthorh ...
(Mg
2SiO
4) in addition to a melt richer in SiO
2 when melting at low pressure. Enstatite melts congruently at higher pressures between 2.5 and 5.5
kilobar
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea leve ...
s.
See also
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Congruent melting
Congruent melting occurs during melting of a compound when the composition of the liquid that forms is the same as the composition of the solid. It can be contrasted with incongruent melting. This generally happens in two-component systems. To take ...
*
Incongruent transition
Incongruent transition, in chemistry, is a mass transition between two phases which involves a change in chemical composition. This is contrasted with congruent transition, for which the composition remains the same.
The transition may be that ...
*
Phase diagram
References
Incongruent meltingfrom Eric Weisstein's World of Physics
Materials science
Phase transitions
Geochemistry
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