Poikilitic Texture

Poikilitic texture refers to crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals. In igneous rocks Poikilitic texture is widely used to determine order of crystallization; if one mineral is enclosed by another then the enclosed grain must have been the first to crystallize. This may sometime be true , but it is certainly not always so.

McBirney and Noyes (1979) show that this texture may be originated as a result of differing nucleation and growth rates, so that a single crystal (like pyroxene) nucleate and growth to a large size (low nucleation rate) in contrast to several other minerals (like feldspars) with higher nucleation rate which necessarily remain relatively small and became successively entrapped in the pyroxene.

In many Poikilitic texture the enclosed crystals are randomly arranged, in other may be concentrated In zone and in some case crystals have a specific crystallographic relationship to the host. One of the best known is the arrangement of plagioclase and mica enclosed in K-feldspar. Most commonly the (010) faces of plagioclase and the (001) faces of mica are aligned parallel to the principal faces of K-feldspar.

The smaller enclosed crystals are known as chadacrysts, whilst the larger crystals are known as oikocrysts. The most common poikilitic texture involves plagioclase laths enclosed by augite and is known as an ophitic texture (elongate crystals enclosed by another mineral). It is often found in dolerites and gabbros.

The term subophitic is sometimes used to descibe an ophitic texture where chadacrysts are not entirely enclosed by the oikocryst.

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Devolopment of Poikilitic texture by simultaneous growth but different nucleation rates of Pyroxene (green) and Plagioclase (pale-blue).




Bibliography



• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.

Photo
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Poikilitic phlogopite crystals, dioposide and Priderite (the small brown crystals in the groundmass) in a Madupite from Leucite hills. PPL image, 2x (Field of view = 7mm)
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Poikilitic phlogopite crystals and dioposide in a Madupite from Leucite hills. XPL image, 2x (Field of view = 7mm)
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Poikilitic phlogopite crystals, dioposide and Priderite (the small brown crystals in the groundmass) in a Madupite from Leucite hills. PPL image, 2x (Field of view = 7mm)
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Poikilitic phlogopite crystals and dioposide in a Madupite from Leucite hills. XPL image, 2x (Field of view = 7mm)
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Poikilitic phlogopite crystal, dioposide and Priderite (the small brown crystals in the groundmass) in a Madupite from Leucite hills. PPL image, 10x (Field of view = 2mm)
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Poikilitic phlogopite crystal and dioposide in a Madupite from Leucite hills. XPL image, 10x (Field of view = 2mm)
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Augite Crystals with plagioclase inclusions. Patagonia. XPL image , 10x (Field of view = 2mm)
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Augite Crystals with plagioclase inclusions. Patagonia. XPL image , 10x (Field of view = 2mm)
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Augite Crystals with plagioclase inclusions. Patagonia. XPL image , 10x (Field of view = 2mm)
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Augite Crystals with plagioclase inclusions. Patagonia. XPL image , 10x (Field of view = 2mm)
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Augite Crystals with plagioclase inclusions. Patagonia. PPL image , 10x (Field of view = 2mm)
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Augite Crystals with plagioclase inclusions. Patagonia. XPL image , 10x (Field of view = 2mm)