Fluorannite is a new mineral of trioctahedral mica group, belonging to the F_rich analogue of annite. It is present in the fluorannite differentiates at the top of the Suzhou A-type granite located in the western suburb of Suzhou City. It was previously named Li-bearing biotite, an361 nite or F-rich annite-like biotite. Flourannite occurs as enhedral-subhedral tabular-sheet crystals, iron_black in color with a submetallic luster.D(meas.)=3.18 andD(calc.)=3.23 g/cm3. Optical properties under the condition of Na light are: biaxial negative,Ng=Nm= 1.648,Np=1.596, and 2V(meas.)≈0°, 2V(calc.)=0°, Y=b=Nm. The mineral shows strong pleochroism:Ng,Nm andNp are reddish, dark green and pale brown respectively. Chemical constituents (wB%) calculated by means of EPMA, wet analysis and atomic ab- sorption spectrometry are SiO234.12 (32.38~35.98), Al2O313.89 (12.68~14.50), FeOTotal33.26 (32.97~34.88), FeO 26.19, Fe2O37.86, MgO 1.49 (1.24~1.82), MnO 0.68 (0.58~0.92), Li2O 0.47, TiO21.29 (1.08~1.52), ZnO 0.27 (0.14~0.28), NiO 0.01 (0.00~0.04), K2O 8.73 (7.98~9.10), Na2O 0.19 (0.10~0.34), Rb2O 0.42, CaO 0.02 (0.00~0.10), BaO 0.44 (0.00~0.51), SrO 0.01 (0.00~0.07), H2O+0.91, F 3.91 (2.98~4.34), Cl trace, and S 0.00, O=F 1.65, totally 99.25. An empirical chemical formula formulated on the basis of (O+F+OH+Cl+S)=12 and the assumption that the sum of tetrahedral cations equals 4 is (K0.92Na0.03Rb0.02Ba0.01)0.98(Fe2+1.82Fe3+0.49Al0.19Mg0.18 Li0.16Ti0.08Mn0.05Zn0.02)2.99(Al1.17Si2.83)4.00O10(F1.03OH0.50O0.47)2.00, with the simplified formula being KFe2+3AlSi3O10F2. X_ray single-crystal structure analyses with an antomatic four-circle diffractometer shows fluorannite to be monoclinic, space groupC2/m,Z=2; the mineral is a 1M polytype,a=0.5369(8)nm,b=0.9289(3)nm,c=1.0153(8)nm;β=100.49(1)°,andV=0.4980(2) nm3. Crystal structure determination, infrared spectral analysis and M ssbauer spectral analysis of fluorannite were conducted. In fluorannite monomineralic differantiates, the overwhelming majority of fluorannite crystallized directly from a silicate melt that evolved from an A_type granitic magma at the late stage. It had abundant F, Fe and lithophile ore-forming elements, and was relatively enriched in alkalis (K≥Na), but was depleted in water. The new mineral fluorannite and its name were approved for publication by IMA CNMMN 999-048.