An uncommon borosilicate mineral with variable composition, generally given as (Ca,Mn,Fe2+)Al2(BO3)Si4O12(OH); sp. gr. 3.26–3.36; hardness 7; violet-brown; crystals normally triclinic; occurs mainly as a product of contact metamorphism of calcium-rich sedimentary rocks (e.g. contact zones of Cornish granites).