X-ray powder, optical, infrared, and chemical data suggest that kirwanite is an amphibole of composition (Na,K) 0.10 C a 1.80 (F e 2+ ,Mn) 3.10 M g 0.79 F e 1.14 3+ A l 0.78 S i 7.18 O 22 (OH) 2 and thus corresponds to ferri-ferro-hornblende (Leake, 1978). The type locality, originally given as the NE coast of Ireland, is most likely Dunmore Head, Co. Down. Kirwanite occurs in the groundmass and the vesicles of variolitic andesite dykes of the Mourne dyke swarm. Thomson's original analysis of kirwanite was probably made on a mixture and the name should not be used for an amphibole species.