Abstract <p>A study of the crystal structure of the high-calcium eudialyte from the Tamazeght massif (Morocco) revealed some structural features reducing the symmetry of this mineral. The trigonal unit-cell parameters are <i>a</i> = 14.1863(1) Å, <i>c</i> = 30.0703(2) Å, <i>V</i> = 5240.95(4) ų, sp. gr. <i>P</i>3. The structure, whose unit cell contains 161 independent atomic sites, was refined to the final residual factor <i>R</i> = 4.4% in the isotropic approximation of atomic displacements using 8202 reflections with <i>F</i> &gt; 3σ(<i>F</i>). Ordering of Ca and Mn atoms in octahedra of six-membered rings; Fe and Mn atoms in <i>М</i>2 sites between translation-linked rings; as well as Na, Н₃O, La, and K in extraframework <i>N</i> sites was established. The chemical composition of the mineral studied is close to those of golyshevite and feklichevite, but differs from them by a lower calcium content and a higher manganese content. The main features of the mineral in comparison with the high-calcium eudialyte-group minerals investigated previously within the sp. gr. <i>R</i>3 are discussed. It is shown that the symmetry lowering in the structural model of high-calcium eudialyte-group representatives may be caused by the ordering of the cations occupying <i>N</i> sites of the extraframework part of the structure.</p>