cronstedtite (cron'-stedt-ite). A trioctahedral Fe-rich phyllosilicate based on a 1:1 T-O layer approximately 7 Å thick. Composition is (Fe²⁺,Fe³⁺)₃OH₄(Si,Fe³⁺)₂O₅. Cronstedtite occurs as jet-black crystals in a wide variety of layer stacking sequences, primarily in low-temperature hydrothermal sulfide veins. A member of the serpentine group.