Popp, Robert K. Department of Geology, Texas A&M University, College Station, Texas.
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An amphibole (double-chain silicate) 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.
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