Poikilitic

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[edit] Definition

Refers to crystals in rock which contain small grains of other minerals.

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. More commonly some closer relationship exists between them.

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 metamorphic mineral texture, which is where they differe specifically with Poikilitic, as poikilitic refers to a rock containing many poikioblasts.

[edit] Ophitic

A variety of this, known as ophitic is very characteristic of many dolerites and diabases, in which large plates of augite enclose many small laths of plagioclase felspar. Biotite and hornblende frequently enclose felspar 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]

[edit] References

1. ^ Blatt, Harvey: "Petrology Igneous, Sedimentary and Metamorphic", page 510. WH Freeman and Company, 2006
2. ^ This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Petrology", a publication now in the public domain.