Cornwallite and cornubite were found in oxidized zone of the Debao skarn type Cu-Sn ore deposit in Guangxi. The former is jade green and bluish green in color and radial-fibrous, platy and botryoidal in form. The wet chemical analysis shows its composition to be Cu0 60.96, As25 30.53, A1203 0.13, Si02 0. 68, P2O5 0.50, Fe0 0.15, H2O+ 6.98, totally 99.93 (in percentage), with the ideal chemical formula being [Cu5(AsO4)2(OH)4.H2O]·The X-ray powder data agree with JCPDS 12-287, and the strong diffraction lines are 4.77(210)(3), 4.56(001)(7)，3.52(111)(10)，3.21( 4 O1)(8), 3.12( 3 11)(5)，3.02 (311)(5), 2.74 (411)(5), 2.49(511)(4), 2.43(601)(6)，2.30( 6 11)(6),1.888(3)，1.773(4)，1.651 (7)，1,532(3).Calculated cell dimensions a=17.547A，b=5,914A.c= 4.666A， β== 91. 6º，Z.=2;mononlinic; space group P21/a: H=4, D(mean)=4. 43. Refractive indices: Ng = 1 .846, Nm=1. 843, Np=1.833(±0.002).Biaxial negative, 2V=60º一 70º. The cornubite is green or greyish green in color and botryoidal in form. The electron, microprobe analyses of four grains are Cu0 62.29%，As2O31.54%, Feo 0.20% and sio2 0.13%(H2O+ undetermined), suggesting an ideal chemical formula of [Cu5(As04)2(OH)4], The X-ray powder data are basically consistent with JCPDS 12-228. The strong diffraction lines are 5.355(20), 4.711(100), 4.585(30), 3.587(30), 3.492(70), 3.101(30), 2.683(80), 2.557(40), 2.483(40), 2.300(50),1.746(30),1.571(30),1.519(30), 1.374(20).H=4. D(mean)=4.51; Biaxial negative, 2V=30º -40º (eye estimation).Refractive indices n=1.75- 1.85± (n=1.798 as calculated by Graston formula).