Incongruent melting occurs when a substance does not melt

Melt

Melt can refer to:* Melting, in physics, the process of heating a solid substance to a liquid*Melt , the semi-liquid material used in steelmaking and glassblowing*Melt inclusions, a feature of igneous rock...

 uniformly and decomposes into another substance. For example, potassium feldspar (KAlSi₃O₈) decomposes to leucite

Leucite

Leucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric measurements made by...

 (KAlSi₂O₆) when it melts. The decomposition is not complete, however. Most of the feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 does melt, a portion of it decomposes to leucite and some quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 (SiO₂) is left over, since the chemical formula

Chemical formula

A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

s of potassium feldspar and leucite differ by SiO₂. Another mineral that melts incongruently is enstatite

Enstatite

Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...

 (MgSiO₃), which decomposes to forsterite (Mg₂SiO₄). Enstatite does melt congruently between pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

s of 2.5 and 5.5 kilobar

Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

s.

External links

• Incongruent melting from Eric Weisstein's World of Physics