Magnesiohastingsite is a calcium-containing amphibole and a member of the hornblende group. It is an inosilicate (chain silicate) with the formula NaCa₂(Mg₄Fe³⁺)Si₆Al₂O₂₂(OH)₂ and molar mass 864.69 g. In synthetic magnesiohastingsite it appears that iron occurs both as ferrous iron Fe²⁺ and as ferric iron Fe³⁺, but the ideal formula features only ferric iron. It was named in 1928 by Marland P. Billings. The name is for its relationship to hastingsite and its magnesium content. Hastingsite was named for the locality in Dungannon Township, Hastings County, Ontario, Canada.