S. L. Fenfield — Pearceite^ etc. IT 



Art. IY. — On Pearceite^ a, Sidpharsenite of Silver and on 

 the Crystcdlization of Polyhasite ; bj S. L. Penfield. 



1. Pearceite. 



The mineral to be described as pearceite in the present arti- 

 cle is a sulpharsenite of silver, AggAsSg or OAg^S, As^S^, anal- 

 ogous to polybasite AggSbSg, and like the latter characterized 

 by having a part of the silver replaced by copper and often by 

 small quantities of zinc and iron. It can not be claimed to be 

 a new mineral^ for as an arsenical variety of polybasite it has 

 previously been recognized although no special name has been 

 assigned to it. H. Rose* first described polybasite and gave 

 the name to the species in 1828, and in 1833 he publishedf an 

 analysis of a specimen from Schemnitz containing arsenic, with 

 only a trace of antimony, while in the original polybasite from 

 Durango, Mexico, described by him, both antimony and 

 arsenic were present, and he recognized the fact that these ele- 

 ments were isomorphous and could mutually replace one 

 another. The polybasites from Durango in Mexico, Freiberg 

 in Saxony, Pribram in Bohemia, the Two Sisters' mine near 

 Georgetown, the Yankee Boy mine near Ouray, and the Sheri- 

 dan mine near Telluride in Colorado, the Comstock Lode in 

 ^N'evada, and apparently from most localities, are essentially the 

 antimony variety, and in mineralogical literature the composi- 

 tion of polybasite is usually given as a sulphantimonite of sil- 

 ver. Rammelsberg:|: gives an analysis by Joy of polybasite 

 from Cornwall, England, where antimony and arsenic are pres- 

 ent in about equal molecular proportions, and the author in 

 connection with Mr. Stanley H. Pearce, has published§ analy- 

 ses of arsenical polybasite {pearceite) from the Mollie Gibson 

 mine, Aspen, Colorado. This latter material was not distinctly 

 crystallized, but was found in great quantity and was the min- 

 eral which carried the bulk of the silver in the most produc- 

 tive silver mine in Colorado at that time. 



The author's attention has recently been called to the occur- 

 rence of beautifully crystallized pearceite, or arsenical polyba- 

 site from the Drumlummon mine, Marys ville, Lewis and 

 Clarke Co., Montana. The mineral was first sent by Mr. K. 

 F. Bayliss, of the Montana Mining Co., to Dr. Richard Pearce, 

 of Denver, with the request that it should be investigated, and 



* Pogg. Ann., XV, p. 573, 1829. f Loc. cit.. xxviii, p. 56, 1833. 



X Mineralchemie, p. 102, 1860. § This Journal, xliv, p. 15, 1892. 



Am. Jour. Soi. — ForRTH Series, Yol. II, No. 1. — July, 1896. 

 2 



