Nanpingite is a new cesium mineral discovered in No.31 muscovite-albite spodumene pegmatite of Nanping, Fujian province. The pegmatite is made up of eight zones, and nanpingite occurs along the border of the veinlets which intruded into the quartz-rriontebrasite assemblage in the interior part of pegma- tite and lie near the pollucite aggregates. These veinleth are generally 2mm- 4mrn in width and several meters in len;th. The main: n.ssociated minerals inc- lode late montebrasite, quartz and apatite. Nanpingite is micaceous in form, looking like muscovite. Diameter of its sheet is generally 1-5rnm, sometimes even up to lOmm. The aggregates take radiating, scaly, and at some places pseudo-hexa}onaf platy shapes. White in color, pearly-vitreous in luster, streak white, transparent. Hardness 3.More brit- tle than muscovilre. Cleavage {OOl} very perfect. Density 3.11 (2)g/cm3 (meas, in Clerici solution) or 3.198/cm3 (calc.).No pleochroism. Optically biaxial(一)， 2V=46º(2) (meas. on the universal stage) and 37.60º (calc.). Refractive indices (under white light):Ng=1.588(2)，Nm=1.584(2)，Np=1.551( 5 ) .Orientation; b=Ng. Very weak dispersion: r>v. Weissenberg single crystal analysis shows that nanpingite is monoclinic, space group; C2/c, polytype; 2M1.Ce11 parameters; a=5.362(3)A，b=8.86(1)A， c=21 .41(1)A，β=95.77(2), V=1012.13A3, Z=4. In X-ray powder diffraction pattern, the strongest lines are 2.664(100) (008)，2.129 (85) (0010)，2.122(16) (223), 2.654(14)(202),1.328(14)(067).a:b:c(calc.)=0:605:1:2.416. Energy spectra indicate that content of Cs is considerably higher than that of K. The BEI and elemental characteristic X--ray images demonstrate that Csi and K' are homogeneously distributed in nanpingite. Chemical composition is determined by electronic microprobe, atomic absorp- tion (Li, Rb), Penfield's method (H20*) and selective electrode (F).Electron probe standards; albite (Na, A1, Si), orthoclase(K)，wollastonite (Ca)，olivine (Mg)，TiO2 (Ti)，Mn0 (Mn)，FezO}(Fe), barite (Ba) and pollucite (Cs). The data give a structural formula of (Cs0.88Kb0.01 )0.95 (Al1.64Mg0.22Fe0.17Lj0.15)2.18[(Si3.16Al0.84)4O9.95] 〔(OH1.79F0.26]2.05 The DTA curve has a rather strong endothermic peak at 920℃.In TGA analysis, weight loss also takes place at 920℃. Absorption bands of infrared spec- tra are 3634, 3429, 1625, 1083, 1018; 911, 823, 788, 741, 663, 515, 467, 420, 350 cm(-1) By using the constants of Mandarino(1981)，it is obtained that the compa- tibility index 1-(Kp/Kc) is 0.0178, suggesting superior agreement between phy- sical and chemical data. Type specimen is preserved in the Institute of Mineral Deposits, Chinese Academy of Geological Sciences and the National Museum of Geology, Beijing, China.