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In hypersolvus
granites Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergr ...
, as used by Tuttle and Bowen in 1958,TUTTLE O.F. & BOWEN N.L. (1958): Origin of granite in the light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O, ''Geological Society of America Memoirs'', 74, 153 p crystallization at relatively low water pressures results in the formation of a single
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) feldspa ...
as opposed to
subsolvus In subsolvus or ''two feldspar'' granites crystallisation occurs at high water pressures resulting in the formation of two types of feldspar as opposed to hypersolvus granites in which crystallization at relatively low water pressures results in ...
granites in which two distinct types of feldspar are present. The distinctive character of feldspar in hypersolvus granite is to present
exsolution 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 wo ...
textures. That is because the high temperature feldspar was ternary (i.e. contained comparable parts of the Ca, Na, K components) and was later dissociated during the cooling phase into K-rich parts and Na-Ca-rich parts, within the initial crystal. The resulting texture is referred to as perthitic.


References

Igneous petrology {{Petrology-stub