he Dasliuigou tellurium deposit, Sichuan, southwest China, is the first deposit dominanted by tellurium in the world. Tetradymite is the most important ore-forming mineral in the deposit. It occurs as aggregates of big euhedral crystals with diameters of 0.3 to 6 mm, which form either almost pure tetradymite massive ore or breccia ore which is composed of dolomite breccias, pyrrhtite breccias, and interstitials of tetradymite together with telubisrnuth, wehrlite, native tellurium, native silver, native gold, chalcopyrite, calcite and quartz. All the tetradymite shows perfect cleavages along (0001) plane. In reflected light the tetradymite is creamy white, shows weak reflected pleochroism, and clear optical anistropy. The reflectance given fortetradymite in literature varies over a wide range. The values obtained here are O 59.9% and E 45.5 %. The measurements were made photometrically under oil with Leiz green filter (obsorption peak at 536Hm), using a carborundum standard. The mineral may take a good polish and its polished surface is often planar. The hardness of the tetradymite was measured by flaT——3 type micrometer of hardness as 20-80 kg/mm2, which is equal to 1 to 2 Moss hardness. Its specific gravities of 7.15 to 7.28 g/cm3 were obtained by the method of hydrostastic weighing in carbon tetrachloride. Chemieal composition Ten chemical analyses of tetradymite from the Dashuigou tellurium deposit gave: Bi 57.65 %-58.68 %, Te 35.20 %-36.40 %, and S4.59 %-5.36 %, and trace elements Ag 230-340μg/g, Se 160——280μg/g, Fe 310-2800μg/g, Sb 100-280μg/g, and An 1.4-10.7g/t. It may be noted that the noble elements An and Ag are quite enriched in the tetradymite. In fact, the tetradymites are so rich ill An, Ag and Se elements that they can be recovered as by-product in the mine.X-ray diffraction Five X-ray diffraction patterns for the tetradymite wereobtained with a copper target, 45 kV, and a current 45 mA. The diffraction patterns are similar to those of the tetradymite taken by Pauling (1975). Their unit cell dimentions are a: 4.241-4.252 and c: 29.501-29.612.Sulfur isotopes Ten samples of the tetradymite were chosen to be analysed for sulfur isotopes. The result shows that their gi4S ranges from -0.5‰ to 2. 1‰with an average of 0.5‰. Discussion and conclusion Tetradymite is one of rare minerals. Since its discovery in 1851 (Jackson, 1851), it has ben reported and/or studied in the United States, Japan, the former Soviet Union, Korea, China, India, and so on. Because tellurium is one of fears-scattered elements, tetradymite recognized in the past usually occur as rare mineral in some sulfide metal deposits, gold deposits, and tungsten deposits. It is in the Dashuigou tellurium deposit where tetradymite so greatly accumulates and occurs as massive aggregates of big crystals. The tetradymite in the Dashuigou tellurium deposit is an odinary mineral, but contains a lot of trace elements, such as Fe, Ph, Sb, Se, Ag and An. It should be emphasized that grades of Se, Ag and An in the ores composed predominantely of tetradymite reach economic value. Furthermore, the sulfur isotopes of the tetradymite experienced high homogenization and all the values are distributed near δ34S=0‰. This perhaps indicates that the sulfur was derived from some magmas.