A polycrystalline sample of Ca 3.1 Cu 0.9 RuO 6 has been synthesized by standard ceramic techniques. The crystal structure, determined at room temperature by neutron powder di4raction, is a monoclinic distortion of the rhombohedral, pseudo one-dimensional K 4 CdCl 6 structure: space group C2/c, a โซุโฌ 8.99775(4), b โซุโฌ 9.25809(4), c โซุโฌ 6.47424(2) A s , and โซ.3)1(926.19ุโฌ The octahedrally coordinated cation site is occupied by Ru and the trigonal prismatic site by 10% Ca, 90% Cu. The latter cations are displaced in an ordered manner away from the center of the prism towards one of the rectangular faces; their coordination number is thus reduced to 4. Neutron di4raction data show the onset of long-range magnetic order at 42(2) K. The data suggest collinear ferrimagnetic ordering of the Cu and Ru spins within each chain and antiferromagnetic interchain coupling. However, a small remanent magnetization of 9ุ10 ุ3 B per formula unit was measured by SQUID magnetometry at 5 K. The magnetic structure of Ca 3.1 Cu 0.9 RuO 6 contrasts with the 1203 spin structure adopted by other K 4 CdCl 6 phases that do not contain a Jahn+Teller cation; in those cases, the frustration inherent in the lattice is not lifted by a structural distortion.