Melting Points of Rocks

Igneous rocks form through the crystallization of magma. There is a considerable range of melting temperatures for different compositions of magma. All the silicates are molten at about 1200°C (when a part of rock) and all are solid when cooled to about 600°C. Often the silicates are grouped as high, medium and low-melting point solids.

ApproximateTemperature (°C)
Minerals which are molten
All molten
Olivine, pyroxene, Ca-rich plagioclase
Amphibole, Ca/Na- plagioclase
Quartz, K-feldspar, Na-plagioclase, micas.

The pattern shown above where different kinds of minerals crystallize at different temperatures is further developed in the Bowen reaction series. The crystallization temperatures play a large role in the development of the different kinds of igneous rocks upon the cooling of magma.

Different minerals, when together in the same rock, melt at much lower temperatures than individual minerals. The crystallization temperatures in this table are characteristic of the environment of rock that contains these minerals, as in magma beneath the Earth's surface. The melting temperatures of pure minerals may be quite different. For example, in the Bowen reaction series, quartz is crystallizing around 650°C, but pure quartz at one atmosphere pressure does not melt until about 1700°C. (See Quartz Wiki, 1670°C for β-tridymite and 1713°C for β-cristobalite)

Thanks to Dr. Dexter Perkins, Professor of Geology and Geological Engineering at the University of North Dakota, for comments on the dry melting temperatures of minerals:

  1. It is hard to generalize about the relation of melting temperatures of mafic vs felsic minerals in pure form relative to those in the Bowen series. For examples, see the discussion in Stack Exchange.
  2. It is impossible to determine melting temps for micas, amphiboles and any other mineral that contains water - because hydrous minerals break down via dehydration reactions before they melt.
  3. And, we really cannot determine melting temperatures for minerals like quartz and Kspar because they change into different minerals when heated, before they melt, even though they do not contain water. (Melting temps for these minerals listed in tables are really the melting temps of high-temperature polymorphs.)
  4. And, many minerals melt incongruently, so what is really the melting temperature? The eutectic, or the temp at which everything is molten?
    • "A eutectic system is a homogeneous, solid mixture of two or more substances that form a superlattice; the mixture either melts or solidifies at a lower temperature than the melting point of any of the individual substances."ThoughtCo
  5. And, many melting temps in the literature are eutectic temps. For example if you google melting of muscovite, you will find that it always refers to melting of muscovite and quartz together.

Igneous rock concepts

Lutgens & Tarbuck
Ch 3

Sec 4.4
HyperPhysics***** Geophysics R Nave
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