Subsolvus
Encyclopedia
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 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:
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
. "Due to slow reaction kinetics, feldspars usually do not attain equilibrium states and thus their thermodynamic behaviour is not yet fully understood."
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
as opposed to hypersolvus
Hypersolvus
In hypersolvus granites, as used by Tuttle and Bowen in 1958, crystallization at relatively low water pressures results in the formation of a single feldspar as opposed to subsolvus granites in which two distinct types of feldspar are present....
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
Hypersolvus
In hypersolvus granites, as used by Tuttle and Bowen in 1958, crystallization at relatively low water pressures results in the formation of a single feldspar as opposed to subsolvus granites in which two distinct types of feldspar are present....
, depending on the whereabouts of the soda feldspar. In the hypersolvus
Hypersolvus
In hypersolvus granites, as used by Tuttle and Bowen in 1958, crystallization at relatively low water pressures results in the formation of a single feldspar as opposed to subsolvus granites in which two distinct types of feldspar are present....
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:
- plagioclasePlagioclasePlagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
: a member of the anorthiteAnorthiteAnorthite is the calcium endmember of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl2Si2O8.-Mineralogy :...
-albiteAlbiteAlbite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
series (CaAl2Si2O8-NaAlSi3O8) - alkali feldsparAlkali feldsparThe alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
: a member of the orthoclaseOrthoclaseOrthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
-albite series (KAlSi3O8-NaAlSi3O8).
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
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...
. "Due to slow reaction kinetics, feldspars usually do not attain equilibrium states and thus their thermodynamic behaviour is not yet fully understood."