Hemimorphite has been found in the oxidized zone of the Lehong Pb-Zn deposit, Yunnan Province. It occurs as veinlet-like or fine_grained botryoidal aggregates in fissures of cave walls within the oxidized ores. The mineral was studied by such means as physical and optical examination, chemical analysis, X_ray powder diffraction, IR spectral analysis, TG and DTA. Microscopic studies give the following optical data:Ng=1.637,Nm=1.618,Np=1.615,(+)2V with optic angle of 45°. Chemical analyses show that the formula of this mineral is Zn3.98(Si2.01Al0.01)2.02O7(OH)2.03·0.88H2O, and X-ray powder diffraction data indicate that its unit cell parameters area=0.8380(4)nm,b=1.0718(7)nm,c=0.5125(6)nm. The mineral assemblage of the oxidized ore mainly includes limonite, goethite, hemimorphite, zinkspath and secondary carbonate. Studies show that most former zinkspaths were altered into zinc bicarbonate [Zn(HCO3)2] and zinc sulfate (ZnSO4) under the strong acid oxidizing-leaching condition, whereas ZnSO4and Zn(HCO3)2were combined with the silica gel in the oxidized zone to form hemimorphite. Therefore, it is considered that the development of hemimorphite depends on strong oxidizing-leaching process in the Pb-Zn deposit, that is to say,the stronger the process, the more favorable the formation of hemimorphite. Hemimorphite is not only the main economic secondary zinc mineral but also an important indicator for the prospecting of primary Pb-Zn deposits.