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aplite A light-coloured, fine-grained, equigranular igneous rock composed of subhedral to anhedral grains of quartz and alkali feldspar, and found as late-stage veins in granite bodies. The quartz–alkali feldspar composition corresponds to the lowest temperature melts in granite magma systems, suggesting that they are residual melts formed by the differentiation of granite magma (see MAGMATIC DIFFERENTIATION). The lack of any hydrous minerals and the fine grain size points to the aplites crystallizing from dry residual melts.