Perthite

A perthite is an intimate intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution (unmixing of two minerals). Strictly speaking a perthite has blebs or irregular lamellae of sodic feldspar within potassic alkali feldspar, however, the term perthite is often used to describe all types of exsolution in the feldspars.

An Antiperthite is an intergrowth arising due to exsolution where potassic feldspar is present as blebs or lamellae within a sodic feldspar. The term mesoperthite is used when sodic and potassic feldspars are in broadly equal anbundance.

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.

aborsystrem.jpg

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)



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 70% orthoclase and 30% albite (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 CA and one with a composition of point D.

antiperthitic.jpg

An antiperthitic intergrowth of K-feldspar (the veinlets) in the Na-feldspar. From wikipedia.





Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• K.G.Cox, J.D.Bell & R.J Pankhurst (1979): The interpretetion of igneous rocks. George Allen&Unwin editori.
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori


Photo
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
antipertite(5).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
antipertite(6).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
antipertite(7).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note the difference in relief) in a Charnokite, Rogaland Province, Norway. PPL image. 10x (Field of view = 2mm)
antipertite(16).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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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)
antipertite(23).jpg

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)
antipertite(35).jpg

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)
antipertite(37).jpg

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)
antipertite(42).jpg

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)
antipertite(46).jpg

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)
antipertite(49).jpg

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)
antipertite(50).jpg

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)
antipertite(51).jpg

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)
antipertite(47).jpg

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)
antipertite(52).jpg

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)
antipertite(13).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(18).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(19).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(20).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(21).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(22).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(24).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(25).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(28).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(29).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(33).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(34).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(36).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(39).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(41).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(43).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(53).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)