Antiperthite

An Antiperthite is an intergrowth an intergrowth of orthoclase (often as lamellae) enclosed in a plagioclase host.

Many minerals that show complete solid solution at higher temperatures do not show such solid solution at lower temperatures. When this is the case, the phenomenon of exsolution occurs. Since solid solutions are really one mineral phase dissolved in another mineral phase to form a single mineral phase, exsolution implies that one or the other of the mineral phases in the solution must "exsolve" or come out of solution with the other mineral phase.

Orthoclase (KAlSi₃O₈) - Albite (NaAlSi₃O₈) system

Consider the X point: We will start at a temperature of 750°C in the region where alkali feldspar solid solutions are stable. At 750°C the composition of the alkali feldspar solid solution is Or₇₀Ab₃₀. This solid solution remains stable with lowering of temperature until the temperature of the solvus is obtained at point A (a temperature of about 590°C). At this temperature the solid solution is no longer stable and begins to exsolve. The composition of coexisting exsolved phases can be found by drawing an isotherm until it intersects the solvus. Such an isotherm at 590°C shows that at this temperature a solid solution having the composition of point B Or₃₂Ab₆₈ coexists with an alkali feldspar solid solution with the composition of point A Or₇₀Ab₃₀. With further lowering of temperature further exsolution occurs. At a temperature of 300°C our original composition X has exsolved into two alkali feldspar solid solutions, one with the composition of point C and one with a composition of point D.

aborsystem(2019).png

Phase diagram of the alkali feldspar system which exhibits such exsolution behavior at low temperatures. At high temperatures the diagram shows that albite (Ab) or NaAlSi₃O₈ and orthoclase (Or) or KAlSi₃O₈ form a complete solid solution series. At temperatures just below the solidus, alkali feldspar solid solutions are stable. At lower temperatures, along the curve labeled "solvus" the solid solution is no longer stable. In this case the exsolution phenomena occurs below the solidus and so is a "sub-solidus" reaction. From smith & Brown (1981)

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (note the difference in relief) in a charnokite, Rogaland Province, Norway. PPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)
Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)	Antiperthite unmixing in plagioclase (note that the anthiperthite have within them perthite unmixing) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)	Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
Antiperthite unmixing in plagioclase (the darker spots) in a charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)