(geology) A parallel to subparallel intergrowth of potassium and sodium feldspar; the potassium-rich phase is usually the host from which the sodium-rich phase evolves.
Perthite
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(′pər′thīt)
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Any of the oriented intergrowths of potassium- and sodium-rich feldspars, (K,Na)AlSi3O8, whose proportions are determined in part by the initial composition of the alkali feldspar from which they exsolved and whose physical properties are thus somewhat variable. The early stages of perthite formation from homogeneous, usually monoclinic (K,Na)AlSi3O8 may be observed experimentally by high-magnification electron microscopy.
If the final K- and Na-rich lamellae or particles are submicroscopic, the composite feldspar is called cryptoperthite. If the particles are small enough and the feldspar relatively clear, Rayleigh-type scattering of light may occur, giving rise to the beautiful blue-to-whitish luster of the semiprecious gem called moonstone. If coarsening has progressed to the micrometer scale and can be seen on a polarizing microscope, the composite is called microperthite; and if the two feldspars are visible to the eye in hand specimen, it is called perthite or macroperthite. Often the albite phase will appear as white veins or blotchy patches against a colored K-rich phase, which may be green to blue microcline (amazonite) or dull pink to orange-brown orthoclase. See also Albite; Anorthite; Anorthoclase; Feldspar; Microcline; Orthoclase.
| Wikipedia: 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, NaAlSi3O8, in composition). Typically the host grain is orthoclase or microcline, and the lamellae are albite. If sodic feldspar is the dominant phase, the result is an antiperthite.
The intergrowth forms by exsolution due to cooling of a grain of alkali feldspar with a composition intermediate between K-feldspar and albite. There is complete solid solution between albite and K-feldspar at temperatures near 700°C and pressures like those within the crust of the Earth, but a miscibility gap is present at lower temperatures. If an alkali feldspar grain with an intermediate composition cools slowly enough, K-rich and more Na-rich feldspar domains separate from one another. In the presence of water, the process occurs quickly.
When megascopically developed, the texture may consist of distinct pink and white lamellae representing exsolved white albite (NaAlSi3O8) in pink microcline K-feldspar. The intergrowths in perthite have a great variety of shapes. If cooling is sufficiently slow, the alkali feldspar may exsolve to form separate grains with near-endmember albite and K-feldspar compositions. The largest documented single crystal of perthite was found in Hugo Mine, South Dakota, US and measured ~10.7x4.6x1.8 m3.[1] The gem varieties of potassium feldspar, amazonite and moonstone are variant colored perthites.
External links
References
- ^ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885-907. http://www.minsocam.org/ammin/AM66/AM66_885.pdf.
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Mentioned in
- microperthite (mineralogy)
- perthitic (geology)
- perthosite (petrology)
- cryptoperthite (mineralogy)
- antiperthite (geology)
- Microcline (mineralogy and petrology)
- Orthoclase (mineralogy and petrology)
- Albite (mineralogy and petrology)
- Aplite (mineralogy and petrology)
- Solvus
- Enderbite
- Callander Bay
- Feldspar
- Bostonite
