The Mg—Fe orthoamphibole series □ (Mg, Fe)7Si8O22(OH)2 — Na(Mg, Fe)5- Al2Si5Al3O22(OH)2 has an open end in the sense that the Si5 end member has not been found in nature; tiny domains of near end–member composition in intricately zoned crystals (Schumacher, 1980; Robinson et al. 1982), appeared to be intergrowths of two different double-chain silicates (Schumacher, pers. comm., 1994).