The role of constituent intermetallic particles in the pitting corrosion of aluminium (Al) alloys is well recognised. A definitive quantification of the role of unique constituent particles has contributed towards an enhanced understanding of Al-alloy corrosion, however the complexity of Al-alloy microstructures warrants further attention. In the present work we identify a unique intermetallic type in contemporary versions of AA2024-T3, which has a two-phase structure, defined by two distinct crystal types, and distinct compositions. Detailed characterisation is used to unambiguously define this constituent, along with its role in localised corrosion.