1S4 Ford ami Bradley — Pseudomorphs after Stibnite 



Abt. XVII. — Pseudomorphs after Stibnite from San Luis 

 Potosi, Mexico ; by W. E. Ford and W. M. Beadlky. 



Recently the Brush Mineral Collection acquired from the 

 Scott Mineral Company of New York City an interesting 

 crystal which, on study, proved to be a pseudomorph after 

 stibnite. Through the courtesy of Mr. Scott two other crystals 

 were placed at the disposal of the writers for study, and suffi- 

 cient material was donated for an analysis. As no stibnite 

 pseudomorphs as large or as perfect as these have been previ- 

 ously known, it was felt that a brief description would rje of 

 interest. 



The specimens are found about ten miles to the west of 

 Charcas, San Luis Potosi, Mexico. They occur in small veins 

 which traverse a limestone. In the majority of cases the long 

 prismatic crystals cross the veins from wall to wall and are 

 consequently unterminated, but occasionally, when the vein 

 cavity enlarges, well-developed crystals are found. 



The three crystals studied averaged 10 cm in length and varied 

 from 1 to 2 cm in thickness. The prism zone on the crystals 

 was deeply grooved and striated in a vertical direction, but in 

 places the different faces occurred with sufficiently wide and 

 smooth planes to admit of their identification. In each case 

 only one-half of the faces of the terminating pyramids were 

 developed. Because of the dull, earthy luster shown by the 

 crystals, measurements could only be made with a contact goni- 

 ometer. The faces, however, were smooth enough in character 

 and sufficiently large to permit of reasonably accurate measure- 

 ments. In this way the presence upon the crystals of the fol- 

 lowing forms was determined : b (010), n (210), m (110), o (120), 

 and t(343). On the crystal represented in figure 3 two faces 

 of a second pyramid were observed, which, however, could not 

 be identified with any known form of stibnite. The faces of 

 this pyramid were small but of good enough quality to insure 

 contact measurements that would be accurate to within one or 

 two degrees. The angles measured between the two faces of 

 this pyramid, and between them and the prism faces, were 

 always within a degree of the calculated values for a form hav- 

 ing the symbol (6.10.15). But the character of the faces and 

 the fact that they were observed on only one crystal must leave 

 the identity of this form in doubt. 



The material of which these pseudomorphs are composed is 

 fine-grained in texture and of a pale, dirty yellow color. When 

 finely powdered and immersed in Canada balsam, it appears 

 of uniform character under the microscope. The larger grains 

 were quite opaque, and only the smallest particles were trans. 



