CRYSTALLOGRAPHIC NOTES ON STEPHANITE IN A SILVER 

 ORE FROM MEXICO. 



By Earl V. Shannon. 



Assistant Curator, Department oj Geology, United States National Museum. 



The following short note contains the results of a crystallographic 

 examination of several specimens of silver ore labeled as having come 

 from the Campania mine, Sultepec, Mexico. The specimens were 

 received by this department from the Division of Mineral Technology 

 and probably originally formed part of the exhibits at the Panama 

 Pacific Exposition (San Francisco, 1915). The measurements 

 identify the silver mineral as stephanite, although its appearance 

 and habit are such that its identity was not suspected prior to the 

 crystallographic examination. The number assigned the material 

 in the museum catalogue is 90,937. The best specimens have been 

 transferred to the mineralogical collections, while a larger one of 

 poorer quality is preserved in the study collection of silver ores. 



The stephanite forms brilliant striated and highly modified prismatic 

 crystals, elongated in the vertical direction (c axis), very unlike 

 the tabular habit usually assumed by this mineral. Although other 

 localities have frequently furnished twins according to several laws, 

 especially pseudohexagonal forms produced by twinning on m (110), 

 no evidence of twinning was seen in the present specimens unless the 

 arrangement of striations on a single face of one crystal contains 

 such evidence. 



The crystals, which reach 1.5 cm in length, with a diameter of 0.4 cm, 

 lie scattered through coarsely crystalline white to colorless or slightly 

 greenish calcite and project into small vuggy cavities -in a manner 

 suggesting that the silver mineral and its gangue were deposited 

 simultaneously. The only other silver mineral contained in the 

 specimens is polybasite, which occurs in a single small vug as incom- 

 pletely developed crystals, which rest upon and are evidently younger 

 than the stephanite. Included in the calcite-stephanite vein filling 

 are numerous crushed fragments of a wall rock which has the appear- 

 ance of a highly sericitic fine-grained schist or shale. These inclusions 

 contain more or less pyrite, either as scattered crystals or a fine- 

 grained replacement. The stephanite is dark lead-gray in color and 

 shows a brilliant conchoidal fracture with only traces of cleavage. 



The mineral gives the usual blowpipe reactions for stephanite, and 

 the essential constituents were found by appropriate chemical tests 

 to be silver, antimony, and sulphur, while arsenic and lead were 

 proven absent. 



No. 2479. Proceedings U. S. National Museum, Vol. 63. Art. II. 



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