A Na-rich host with K-rich lamellae is an antiperthite. Thin section shows lamellae bands with twinning. If sodic feldspar is the dominant phase, the result is an antiperthite and where the feldspars are in roughly equal proportions the result is a mesoperthite. Microcline and albite both exsolved (unmixed) from the homogeneous solid solution during cooling. Mircoperthite is only seen microscopically, cryptoperthite is submicroscopic and needs x-ray diffraction to see extremely fine lamellae. 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. Bands lose their appearance as strips and seem less uniform when the axis' are not well aligned to the plane. Perthite in plane polarized light, click to go to rollover The lamellae are K- and Na-fledspars. Fick's Laws of diffusion) that provide multiple representations of the process of exsolution. If sodic feldspar is the dominant phase, the result is an antiperthite and where the feldspars are in roughly equal proportions the result is a mesoperthite.[1]. Perthite in thin section Originally uploaded in Cutting Edge:Develop Program-Wide Abilities:Complex Systems:Workshop 2010:Participant Workspace. This was probably a solid solution that cooled; the two minerals separated and formed a perthitic intergrowth. Thin section in crossed polarized light of perthite. with albite intergrowth. Originally described from North Burgess, Drummond Township, Lanark Co., Ontario, Canada. 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). Perthite from a metaluminous biotite granite. link, so you can view it in plane polarized light and crossed polarized light. Orthoclase perthite thin section under microscope - YouTube Typically the host grain is orthoclase or microcline, and the lamellae are albite. The less abundant phase forms into irregular, crystallographically-oriented planar bands within the more abundant host during cooling. Thin section in crossed polarized light of microcline The lamellae are K- and Na-fledspars. View Original Image at Full Size. Indices between those of microcline and albite. Perthite in thin section. The gem varieties of potassium feldspar, amazonite and moonstone are variant colored perthites. The exsolution lamellae of Na- and If cooling is sufficiently slow, the alkali feldspar may exsolve to form separate grains with near-endmember albite and K-feldspar compositions. Perthite is an unmixing texture of an originally homogeneous feldspar grain. Common in granite pegmatites. Perthite in sample K-21 showing coarse intergrowth. Perthite, any member of a class of alkali feldspars in which tiny crystals of sodium-rich feldspar (albite; NaAlSi 3 O 8) are intimately intergrown with, but distinct from, tiny crystals of potassium-rich feldspar (orthoclase or, less commonly, microcline; KAlSi 3 O 8). albite will have a higher refractive index. The microcline has tartan twinning, while Typically the host grain is orthoclase or microcline, and the lamellae are albite. Exsolution lamellae visible in thin section and sometimes in handsection. intergrowth. An intergrowth of albiteor oligoclase with a microclinehost, occasionally also with a orthoclasehost. The microcline has tartan twinning, while the albite is the globular gray areas. Strictly speaking a perthite has blebs or irregular lamellae of sodic feldspar within potassic alkali feldspar, however, the term perthite is often used to … the albite is the globular gray areas. K-rich feldspar are created in the bulk composition of the K-feldspar. In the presence of water, the process occurs quickly. The orthoclase(K-feldspar) will have the lower refractive index, and the Maria Rowley (00), Sharon Mason (05), Danielle Schmandt (07). Appear in thin section as felsic thin parallel wispy layers on many cleavages embedded within a mafic host. If an alkali feldspar grain with an intermediate composition cools slowly enough, K-rich and more Na-rich feldspar domains separate from one another. https://en.wikipedia.org/w/index.php?title=Perthite&oldid=981184645, Creative Commons Attribution-ShareAlike License, This page was last edited on 30 September 2020, at 19:59. Image 23498 is a 525 by 700 pixel JPEG Uploaded: Apr30 10. This was probably a solid 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). solution that cooled; the two minerals separated and formed a perthitic The intergrowths in perthite have a great variety of shapes. When megascopically developed, the texture may consist of distinct pink and white lamellae representing exsolved white albite (NaAlSi3O8) in pink microcline. Thin section in crossed polarized light of microcline with albite intergrowth. The intergrowth forms by exsolution due to cooling of a grain of alkali feldspar with a composition intermediate between K-feldspar and albite. Natural occurrences of perthite (observed in hand sample, thin section and TEM images) are related to physical, computer, and mathematical models (e.g. To recognize complex-system behavior in the relatively common feature of perthitic alkali feldspar. Perthite in thin section: --small a 525 by 700 pixel JPEG. The intergrowths or exsolution lamellae of the albite are visible as white lines coursing through the K feldspar groundmass. The largest documented single crystal of perthite was found in Hugo Mine in South Dakota and measured about 10.7 m (35 ft) x 4.6 m (15 ft) x 1.8 m (5 ft 11 in).[2]. Note the faint, irregular stripes that run from the upper right to lower left. A perthite is an intimate intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution (unmixing of two minerals). perfect cleavage {001) and good cleavage {010}, Gray and black in cross polarized light, colorless in plain polarized light, dark gray in hand sample. 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Multiple representations of the albite are visible as white lines coursing through the K feldspar groundmass, Ontario Canada...

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