The monoclinic space group of synchysite-(Y) was determined as C2/c by single-crystal crystallographic research using W eissenberg method. The unit cell parameters obtained and refined on a four-circle diffractometer are a 12.039(3)，b 6.950(1)，c 18.436 (6)A，and β 102.45 (2)°.From 1350 independent reflections collected by means of MoKa radiation on the same instrument, 1019 reflections with I>3σ (I) were corrected for Lorentz and polarization factors; however，no absorption correction was conducted. After the refinement with anisotropic temperature factors by the full matrix least square method, the residual index for the 1019 reflections was R=0.086. Synchysite-(Y) possesses a characteristic (001) layer structure, with layers of (Ca) and (YF) separated by layers of carbonate groups. The layers stack in a manner analogous to C2/c muscovite, where (Ca) layers in synchysite-(Y) are equivalent to (K) layers in muscovite, (CO3) layers in synchysite-(Y)，to layers of tetrahedra in muscovite, and (YF) layers in synchysite-(Y)，to layers of octahedra in muscovite. As in m uscovite,offsets of adjacent (CO3) layers are similar to those in 2M2,which results in the layer-stacking polytype structure.