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analcitite An igneous rock in which the abundant plagioclases have been replaced by the mineral analcime during late-stage deuteric activity (see DEUTERIC reaction). Magma-derived, late-stage aqueous fluids penetrate along cleavages and fractures in primary plagioclase within the crystalline rock body and react with it, converting the plagioclase to analcime which is stable in the presence of a late-stage fluid.