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A ternary hexaerbium triacontacobalt enneakaidecasilicide, ErCo5Si(3.17), crystallizes as a combination of disordered variants of the hexagonal UCo5Si3 (P6₃/m) and Yb6Co30P19 (P6) structure types and is closely related to the Sc6Co30Si19 and Ce6Rh30Si19 types. The Er, Co and three of the Si atoms occupy sites of m.. symmetry and a fourth Si atom occupies a site of -6.. symmetry. The environment of the Er atom is a 21-vertex pseudo-Frank-Kasper polyhedron. Trigonal prismatic coordination is observed for the Si atoms. The Co atoms are enclosed in heavily deformed cuboctahedra and 11-vertex polyhedra. Crystallochemistry analysis and the data from electronic structure calculations (TB-LMTO-ASA) suggest that the Er atoms form positively charged cations which compensate the negative charge of the [Co12Si9](m-) polyanions.