10 Yd ii Horn — Discussion of the Formulas of 



Art. V. — A Discussion of the Formulas of Pearceite and 

 Polybasite ; by Frank R. Van Horn. 



The calculation of the formula of the pearceite from the 

 Veta Rica mine, Sierra Mojada, Coahuila, Mexico, which was 

 given in a preceding paper,* showed conclusively that the 

 analysis conformed more closely to the formula (Ag,Cu), 6 As 3 S u 

 than to that of the molecule (AgjCuJ^As^,., . The latter was 

 assumed to he isomorphous with polybasite, (Ag,Cu) 18 Sb 2 S„ , 

 the formula of which was proposed by Heinrich Rose in 1829, f 

 and has been accepted by most mineralogists ever since. ,The 

 writer, while looking up the literature on the subject, found a 

 few words relative to the matter in Hintze's Mineralogy.:}: 

 The statement was made that, in 1875, Rammelsberg in his 

 Mineralchemie gave two formulas for polybasite, namely : 

 (Ag,Cu) ls Sb 2 S 12 and (Ag,Cu) 10 Sb„S n ; further, in the last edition 

 of the same book in 1895, the latter formula was given alone, 

 evidently discarding the Rose formula. Hintze also pointed 

 out that Bodlander in 1895 showed that in polybasite there 

 was a variation in the proportion of R„S : R,S 3 which ranged 

 between 7'22 and 101. Bodlander therefore concluded that 

 polybasite was a mixture of 7R 2 S.Sb 2 S 3 and 9R 3 S. As S 3 . 

 With the formula of the new occurrence of pearceite from 

 Sierra Mojada, as well as the statements of Hintze in mind, 

 the writer became interested in recalculating all previous anal- 

 yses of both pearceite and polybasite in order to ascertain what 

 the actual combining ratios were. 



Pearceite. 



Upon investigation, it was found that there have been but 

 five analyses published prior to that of the preceding paper. 

 These five, with that of the Sierra Mojada pearceite, will be 

 given in chronological order beginning with the oldest, as fol- 

 lows : 



1. Scheuanitz, Hungary, H. Rose, Pogg. Ann.,xxviii, 158, 1833. 



2. Arqueros, Chile, crystals, Domeyko, Min., 1879, 393. 



3. Molly Gibson mine, Aspen, Colorado, crystallized, Penfield 

 and Pearce (analysis by Penfield), this Journal, xliv, 17, 1892. In 

 this analysis 12-81 per cent of impurities were deducted, and the 

 analysis was recalculated by Penfield in the present form. 



4. Same locality as 3, but massive with 28" 18 per cent deducted 

 for impurities. Penfield and Pearce as above (analysis by Pearce). 



* A New Occurrence of Pearceite ; by Frank R. Yan Horn and C. W. 

 Cook, this Journal, xxxi, 518, 1911. 

 t H. Rose, Pogg. Ann., xv, 575, 1829. 

 X C. Hintze, Handbuch der Mineralogie 1904, i, 1171. 



