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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 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). 
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