In subsolvus or ''two feldspar''
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 undergro ...
crystallisation occurs at high water pressures resulting in the formation of two types of
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 ...
as opposed to
hypersolvus granites in which crystallization at relatively low water pressures results in the formation of a single feldspar variety.
Quoting Tuttle and Bowen in 1958 (abstract, page 3): ″''A classification of salic rocks based on the nature of the alkali feldspar is proposed. The classification has two major divisions: (1) subsolidus, and (2)
hypersolvus, depending on the whereabouts of the soda feldspar. In the
hypersolvus rocks all the soda feldspar is or was in solid solution in the potash feldspar whereas in the subsolvus rocks the plagioclase is present as discrete grains. The two major divisions are further subdivided according to the nature of the alkali feldspar modification.''″ Note that here the word "subsolidus" unfortunately looks like a misprint and probably has to be replaced by "subsolvus".
The two types of feldspar are usually:
*
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
: a member of the
anorthite-
albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate ...
series (CaAl
2Si
2O
8-NaAlSi
3O
8)
*
alkali feldspar: a member of the
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 ...
-albite series (KAlSi
3O
8-NaAlSi
3O
8).
In fact ternary feldspars (comprising albite+orthoclase+anorthite) are believed to have been present in the high temperature state of the rock before cooling. Because Na and K are extremely mobile by solid-state diffusion, cooling gives rise to multiple forms of unmixing products, e.g. various kinds of
perthite
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, N ...
. "Due to slow reaction kinetics, feldspars usually do not attain equilibrium states and thus their thermodynamic behaviour is not yet fully understood."
[BENISEK A., DACHS E. & KROLL H. (2010): Excess heat capacity and entropy of mixing in ternary series of high-structural-state feldspars]
''European Journal of Mineralogy''
22 (3), 403-410
References
Igneous petrology
{{Petrology-stub