Koenenite[4NaC1·4 (Mg, Ca)CI2·5Mg(OH)24AI(OH)3]from syrinite ore deposit, Jiangcheng, Yunnan is light yellow in color, and becomes dark brown due to weathering. It is foliate with cleavage {0001}perfect. The specific gravity is 1.82-2.08, after soaking iii water it reduces to 1.97.The hardness is 2.06-2.1. No=2.529, reddish brown. Ne=1.557, colorless. Ne-No=0.028.It shows weakly, biaxial 2V<10°. The X-ray powder pattern of the mineral has strongest lines; 5.45(90)，3.25(50)，2.812(100)，2.380(70)，2.130(60).Koenenite consists of two trigonal substructure layers; [Na4(Ca, Mg02Cl12]4-and[Mg7AL4 (OH)22 ] 4+.The CI-lattice unit cell has parameters; a=4.070A，c=32.646A，v=541.23A，space group Ram. The OH-lattice unit cell has parameters; a=3.051A,c=10.882A， v=101.132A3，space group P3m1. The structure of koenenite, as viewed along c axis is one of regmarly alter- hating NaCI-like chlorine and brucite-like OH double layers. All possible octahedral sites are randomly occupied by the metal ions. The strongest lines of the X-ray powder pattern for the material formed by soaking koenenite in water are; 7.73(90),3.835(100),2.570(70),2.285(80)， 1，935(80)、1.526(80)，1.495(70)，1.316(50)，1.269(60)，1.200(50)，0.975 (50).The unit cell parameters a= 3 .048A，c=23.190A，v=186.578A3，space group R. Chemical analysis of a water absorbed product after soaking koenenite in water gave: SiO2 0.63,Fe203 0.96,A1203 23.98,Ca0 0.18,Mg0 32.49,Ms0 0.17,H20 25.30, Cl 15.52,sum 99.23%，leading to the formula, [Mg7Al4(OH)22[Cl4.H20].