The arrangement of atoms in synchysite-(Ce) has been solved and refined to R = 0.036 using three-dimensional X-ray-diffraction data. In contrast to the putative hexagonal cell, weak X-ray-diffraction maxima on precession films demonstrate that synchysite-(Ce) is monoclinic, C2/c, a 12.329(2), b 7.110(1), c 18.741(2) Aa, beta 102.68(1) degrees . It possesses a (001) layer structure, with layers of (Ca) and (CeF) separated by layers of carbonate groups. The layers stack in a manner analogous to C2/c muscovite, where (Ca) layers in synchysite are equivalent to (K) layers in muscovite, (CO 3 ) layers in synchysite, to layers of tetrahedra in muscovite, and (CeF) layers in synchysite, to layers of octahedra in muscovite. As in muscovite, the monoclinic symmetry results from the oblique stacking of the individual layers. Also as in micas, offsets of adjacent (CO 3 ) layers are similar to those in 2M 2 polytypes; polytypism similar to the micas may also exist in synchysite.