The (\mathrm{CaO}-\mathrm{ZrO}{2}-\mathrm{SiO}{2}) phase diagrami has been the subject of several investigations; two publications have pointed oul the existence of (\mathrm{Ca}{2} \mathrm{ZrSi}{4} \mathrm{O}{12}) whereas two other publications deny its existence. This compound has been obtained by the conventional ceramic technique at (1400^{\circ} \mathrm{C}). The cell is monoclinic. space group (P 2{1} / m), with (a=7.399(1), b=13.651(2), c=5.312(1) \AA) and (\beta=108^{\circ} 9(20), Z=2). Its structure has been determined from 880 single-crystal data and refined to (R=0.016) with 94 parameters: it is based on rings of (\mathrm{Si}{4} \mathrm{O}{12}) with four conner-sharing tetrahedra belonging to the type (\mathrm{A}) classification of 'T. Zoltai and M. J. Buerger (Z. Kristallogr. 114, (1-8,1960) ); these rings are interconnected by (\mathrm{ZrO}{6}) octahedra, outlining tunnels paraliel to the (c) axis: calcium ions fill the 8- and 9-coordinated interstices. The low-point symmetry of the (\mathrm{Si}{4} \mathrm{O}_{12}) rings precludes any detailed interpretation of the infrared spectrum. which. however, may be compared to other cyclotetrasilicate spectra, and is tentatively subdivided into groups of lines belonging to a given type of vibration. 1993 Academic Press. Inc.