Cummingtonite is a member of the cummingtonite-grunerite solid solution series which ranges from Mg 7Si 8O 22(OH) 2 for magnesiocummingtonite to the iron rich grunerite endmember Fe 7Si 8O 22(OH) 2. Cummingtonite is used to describe minerals of this formula with between 30 and 70 per cent Fe 7Si 8O 22(OH) 2. Thus, cummingtonite is the series intermediate.