An igneous rock containing large crystals embedded in a fine-grained, invisible-crystalline matrix.
From Greek 'aphanes' (invisible) and 'porphyre' (porphyry, a rock with large crystals in fine matrix). The term specifically describes rocks where the groundmass is too fine to see, with visible 'phenocrysts' embedded within.
An aphanophyre is like a secret: from far away it looks like an ordinary fine-grained rock, but up close under magnification you discover giant crystals hidden in a microscopic matrix! Geologists love these rocks because they reveal the two-stage cooling history written in the stone.
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