CaAl 12 Si 4 O 27 , a new high-pressure phase in the CaO}Al 2 O 3 }SiO 2 system, was prepared at 15503C and 14 GPa. It has trigonal symmetry, P-3, with a β«Ψβ¬ 7.223(1) and c β«Ψβ¬ 8.614(3) A s . Its structure was solved using crystal chemistry principles and re5ned using the Rietveld method applied to powder X-ray di4raction data, R wp β«Ψβ¬ 6.1%, R B β«Ψβ¬ 2.7%. The structure is a new type based on a close-packed anion lattice with a mixed layer stacking sequence ABACA,(hc) 2 and with Ca atoms ordered in one-seventh of the anion sites in alternate c-stacked layers. The Si atoms are ordered in both octahedral and tetrahedral sites while the Al atoms are ordered in two independent octahedral sites. Octahedral edge-shared clusters, Al 6 O 19 , occur in the structure, interconnected within the closepacked layers by corner-linking to SiO 4 tetrahedra. The clusters are also connected by corner-sharing to nine-member rings of edge-shared Al-and Si-centered octahedra in adjacent layers. The new phase has structural features in common with the barium titanium ferrite of similar composition, BaFe 11 Ti 3 O 23 , as well as with the mineral simpsonite, Al 4 Ta 3 O 13 (OH), and alkali metal niobates which contain Nb 6 O 19 clusters.