The Feldspar Compositional Ternary Diagram

Minerals of the feldspar group consist of three compositional end members. This can be illustrated in the feldspar compositional ternary diagram.

Feldspar minerals are the most abundant constituents of igneous rocks. Despite being sensitive to weathering and alteration, feldspars are abundant (second after quartz) in arenaceous sedimentary rocks, either in form of detrital grains or as secondary, authigenic phases.

Classification of the feldspar group minerals

The generalized chemical composition of feldspars is X(Al,Si)₄O₈, where X is commonly potassium (K), sodium (Na), or calcium (Ca); – rarely X can be barium (Ba), rubidium (Rb), or strontium (Sr).

Feldspar minerals can generally be classified by their chemical composition and expressed in a ternary system of KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈ (potassium, sodium, and calcium feldspar, respectively).

KAlSi3O8	K-feldspar	Or (orthoclase)
NaAlSi3O8	Na-feldspar / Na-plagioclase	Ab (albite)
CaAl2Si2O8	Ca-feldspar / Ca-plagioclase	An (anorthite)

The differentiation of the feldspar minerals is commonly expressed in reference to their NaAlSi₃O₈ (albite) or Ab mole percentage (see below).

The feldspar compositional ternary diagram

While feldspars between the compositional end-members KAlSi₃O₈ and NaAlSi₃O₈ are referred to as alkali feldspars, the series between NaAlSi₃O₈ and CaAl₂Si₂O₈ are called plagioclase feldspars.

A ternary diagram, based on these three endmembers, can therefore represent the feldspar compositions accordingly (Figure 1).

Alkali feldspars

The alkali feldspar compositions and crystallographic symmetries depend on their crystallization temperatures.

Thus, K-feldspars occur in different crystallographic symmetries, i.e., monoclinic and triclinic, depending on their formation temperatures.

Highest temperature:	sanidine	(monoclinic)
Lower temperature:	orthoclase	(monoclinic)
Lowest temperature:	microcline	(triclinic)

High temperature

At high temperatures (≥ 1000 °C), a solid solution exists between NaAlSi₃O₈ and KAlSi₃O₈. Nevertheless, a change in the symmetry between triclinic anorthoclase (Ab100 to Ab63) and monoclinic sanidine (Ab63 to Ab0) occurs. (Most volcanic rocks are typically high-temperature products.)

Anorthoclase ((Na,K)AlSi₃O₈) occurs in high-temperature sodium-rich volcanic and shallow intrusive igneous rocks. Slow cooling allows the separation of Na- and K-rich feldspars within the same specimen (see below).

Low temperature

At lower crystallization temperatures (≤ 650 °C), however, there is no solid solution. The two feldspars (K- and Na-rich) are therefore separated (miscibility gap; see dashed line in Figure 1).

Interestingly, this separation can result in the intergrowth of two feldspars often in form of laminae. Perthite is the term for K-feldspar that is intergrown by Na-feldspar, while antiperthite is the name for Na-feldspar intergrown by K-feldspar. Mesoperthite is the label for ± equal proportions of K- and Na-feldspar (Le Maitre et al. 2005). (Low-temperature or slow cooling is common for plutonic rocks.)

Plagioclase feldspars

A solid solution exists for the Na- and Ca-rich plagioclase feldspars (albite and anorthite respectively). The plagioclase series is differentiated into six compositional ranges (Figure 1), which is expressed through their Ab mole percentage:

All members of the plagioclase series, however, crystallize in the triclinic symmetry.

References

Le Maitre, R.W.; Streckeisen, A.; Zanettin, B.; Le Bas, M.J.; Bonin, B.; and Bateman, P. (2002). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks (2 ed.). Cambridge University Press. p. 20. [Link]