any of a type of microscopic body occurring in such glassy igneous rocks as obsidian and pitchstone. Crystallites are regarded as incipient or embryonic crystals, though they often have no recognizable crystallographic form and are too small to polarize light. They occur when magma (molten rock material) congeals so rapidly that crystallization remains incomplete. Crystallites are distinguished from microlites, which are slightly larger forms recognizable as mineral species
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