An amphibole (a double-chain silicate mineral) with the idealized chemical formula Mg₇Si₈O₂₂(OH)₂ that crystallizes with monoclinic symmetry. Naturally occurring samples generally are solid solutions between Mg₇Si₈O₂₂(OH)₂ and the corresponding iron (Fe)-end member with the general formula (Mg,Fe)₇Si₈O₂₂(OH)₂. The name cummingtonite (derived from the location Cummington, Massachusetts) is applied to all solid solutions with Mg/(Mg + Fe²⁺) ≥ 0.5, whereas those with Mg/(Mg + Fe²⁺) <0.5 are termed grunerite. Up to a total of 1.0 (Ca + Na) atom per formula unit may also be present in cummingtonite.