Poikilitic

From Wikipedia, the free encyclopedia - View original article

 
Jump to: navigation, search

Poikilitic texture refers to crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals. The 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.

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.

A poikioblast is a larger crystal that contains smaller crystals of other minerals.[1] Poikioblasts are a texture of metamorphic rocks.

Ophitic

A variety of this, known as ophitic is very characteristic of many diabases, in which large plates 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). [2]

References

  1. ^ Blatt, Harvey: "Petrology Igneous, Sedimentary and Metamorphic", page 510. WH Freeman and Company, 2006
  2. ^  This article incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). "Petrology". Encyclopædia Britannica (11th ed.). Cambridge University Press.