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glimmerite An ultrabasic igneous rock, consisting almost wholly of essential dark mica, either phlogopite or biotite. These rocks are rather rare, being found among ultramafic xenoliths in kimberlite pipes and within old basement gneisses. These occurrences testify to the deep-seated origin of glimmerites, which might be considered as metamorphic (see METAMORPHISM) rather than igneous.