520 Van Horn and Cook — Neio Occurrence of Pearceite. 



Occurrence of the Pearceite. 



In 1906, while working along the northern part of the silver- 

 copper siliceous lime ore body, a fault was encountered having 

 a displacement of about 40 feet. Following along the fault 

 plane, silver-copper ores of great richness were discovered, 

 along with considerable barite as gangue mineral. Minerals 

 observed from this point were native silver, argentite, proust- 

 ite, pearceite, and erythrite. The latter occurrence is rather 

 peculiar, since it is the only oxidation product, if native silver 

 is excepted. Furthermore, there have been no other cobalt 

 minerals noted either from this mine or the district as a whole, 

 although it would seem as if some cobalt arsenides should be 

 present. There were said to be about 200 pounds of pearceite 

 crystals found, but on account of the heavier government tax 

 on high grade ores, practically the entire amount was ground 

 up and distributed through poorer grades. However, eight 

 specimens were presented by Mr. Cochran, while a ninth aggre- 

 gate was very kindly loaned to us by Senor Felipe Borrego, 

 formerly with the Yeta Rica mine, but at present foreman of 

 the Guadalupe mine at Cerro de San Pedro, San Luis Potosi, 

 Mexico. 



Crystallography . 



The crystallography of pearceite was studied first, in 1896, 

 by Penfield,* who worked on crystals from the Drumlnmmon 

 mine, Lewis and Clark County, Marysville, Montana. At this 

 time he proposed the name pearceite for polybasites in which 

 arsenic was in excess of antimony. He states that it crystal- 

 lizes in the monoclinic system and possesses a rhombohedral sym- 

 metry, due, in all probability, to twinning similar to that of 

 the micas. On account of this rhombohedral symmetry, which 

 was likewise exhibited by all the material under investigation, 

 as well as the similarity on angles for several forms, and the 

 imperfection of the crystals, definite orientation was found to 

 be impossible. Also, an attempt to obtain etching figures, 

 using nitric acid as the solvent, met with failure. Therefore, 

 although several undoubtedly new forms are present, it is 

 thought best to limit the present crystallographic report to a 

 discussion of a new twinning law. 



The pearceite occurs in aggregates of twin crystals, more or 

 less perfectly developed, which show striations and vicinal 

 planes. The individual crystals are tabular in habit with the 

 basal pinacoid as the predominating form ; the pyramids and 

 domes occurring as very narrow faces. Unfortunately most of 

 the edges of the crystals were broken off, as the mineral is very 

 brittle. The basal pinacoid is characterized by the presence of 

 * S. L. Penfield, this Journal, ii, 19, 1896. 



