Earth:Poikilitic
Poikilitic texture refers to igneous rocks where large component crystals contain smaller crystals of other minerals within them. In poikilitic rocks, the contained smaller crystals are termed 'chadacrysts' and the larger phenocrysts are termed 'oikocrysts'. A poikilitic texture is most easily observed in petrographic thin sections. In some rocks there seem to be little tendency for the minerals to envelop one another. This is true of many gabbros, aplites and granites. The grains then lie side by side, with the faces of the latter moulded on or adapted to the more perfect crystalline outlines of the earlier.[1]
When the smaller idiomorphic crystals of the first-formed are scattered irregularly through the larger and less perfect crystals of later origin, the structure is said to be poikilitic.[1]
A poikioblast is a larger crystal that contains smaller crystals of other minerals.[2] Poikioblasts are a texture of metamorphic rocks.
Poikilitic texture describes the occurrence of one mineral that is irregularly scattered as diversely oriented crystals within much larger host crystals of another mineral.
Ophitic
A variety of this, known as ophitic is very characteristic of many diabases, in which large crystals of augite enclose smaller laths of plagioclase feldspar. Biotite and hornblende frequently enclose feldspar ophitically; less commonly iron oxides and sphene do so. In peridotites the "lustre-mottled" structure arises from pyroxene or hornblende enveloping olivine in the same manner. In these cases no crystallographic relation exists between the two minerals (enclosing and enclosed).[1]
References
- ↑ 1.0 1.1 1.2
One or more of the preceding sentences incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Petrology". in Chisholm, Hugh. Encyclopædia Britannica. 21 (11th ed.). Cambridge University Press. p. 330.
- ↑ Blatt, Harvey: "Petrology Igneous, Sedimentary and Metamorphic", page 510. W. H. Freeman and Company, 2006
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