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cummingtonite A monoclinic, Mg-rich member (Mg,Fe)2(Mg,Fe)5[Si4O11]2(OH,F)2 of the calcium-poor amphiboles, along with grunerite (Mg < Fe) with which it forms a series, similar to anthophyllite–gedrite; sp. gr. 3.1–3.6; hardness 5.5; dark green; occurs in amphiboles and some intermediate igneous rocks.
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