Mosandrite occurs in argiz}ine-neplieline syenite of Saima, Liaoning province. It is light yellow to yellow in colour, and tabulaa or long columar in crystal form. Specific gravity 3.23-3.30; hardness A-5. Biaxial positive with 2V=80º, Ng=1.661, Nm=1.653, and Np=1.651·Isotropismis observed at the center of this mineral. Chemical formula of the caystalline sample is (Na0.37 K0.24 H3O0.15)0.76 (Ca3.15 Na0.72 Sr0.07 Fe0.03 Nb0.03)4 (Ce0.4La0.32 Nd0.16 Pr0.05 Sm0.02 Y0.05 U0.01 Th0.04)0.95 Ti1.13[(Si3.18Al0.19)4O14a] [F2.01(OH)1.99]4. The cell dimensions measured by X-ray powder diffraction are a0=18.46A，b0=5.66A，c0=7.34A，a=89.5º，β=90.5º, γ=89.8 º, Z = 2, belonging to monoclinic or pseudo-orthorhambic system with the crystal structure being layered Ti-silicate. The structure of mosandrite is similar to that of 2M type muscovite. In mosandrite, Na+ and Ce3+ canons link `2:1’unit layers which are composed of.one Ca-O octahedral layer and two anion layers made up of Ti-O polyhedron,and Si-O tetrahedron pairs.The main infrared absorption spectra of mosandrite are 3550; 3380, 1630, 1040, 960, 845, 649 and 478cm-1. Its Raman spectrum. might be divided into three areas, with 310, 250, 200cm-1 peaks in the first area, 800, 660, 600cm-1 peaks in the second, and 1160, 1070cm-1 peaks in the third. There exists only one exothermic peak (600℃)on the differential thermal curve, resulting from partial crystallization of the non-crystalline mosandrite.