Summary: An examination of available chemical analyses shows that the anthophyllite and cummingtonite series of amphibole minerals are not isodimorphous. Whilst the anthophyllites are truly lime-free amphiboles, small amounts of calcium are essential to stabilize the monoclinic lattice of cummingtonites, which should be regarded as lime-poor amphiboles, analogous to the lime-poor pyroxenes. The existence of synthetic clino-anthophyllites is considered.