Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass or matrix. The large inclusions are called phenocrysts.
They consist essentially of plagioclase feldspar, with hornblende or augite, and may contain also biotite, quartz and a limited amount of orthoclase. Although a few authorities still recognize the aphanites as a distinct class, most systematic petrologists have discarded it, and regard these rocks as merely textural varieties of other species. The term is used as a field name pending laboratory identification. Hence, although typically an igneous term, any rock (sedimentary, igneous, or metamorphic) which is compact, crystalline and too fine-grained for its constituents to be identifiable in hand sample, is frequently said to be aphanitic, without implying exactly to which of the principal rock groups it really belongs.
Common aphanites
References
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Last updated on Sunday February 18, 2007 at 23:56:59 PST (GMT -0800)
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