AKT. 24 



BENJAMINITE^ A NEW MINERAL SHANNON 



Comparison of benjaminile with other minerals. 



The low summation of the benjaminite analyses deserves comment 

 as do the low sulphur determinations. In each analysis a single 

 small portion only of the powdered and purified mineral was avail- 

 able. Sulphur had to be determined in an aliquot small portion of 

 the solution used for general analysis, a procedure wldch tends to 

 give low results both from loss of some sulphur on solution in acid, 

 probably as hydrogen sulphide, and by incomplete precipitation by 

 barium chloride. The results for this constituent are consequently 

 only approximate and may be 1 per cent low in each case. 



The low summation is in part due to this cause and in part to 

 hygroscopic or absorbed substances in the fine powder, particularly 

 methylene iodide used for the gravity separation which seems to 

 adhere to the surfaces of the grains. The absence of zinc, manga- 

 nese, tin, antimony, arsenic, and other probable metals was definitely 

 determined. 



The mineral seems unquestionably distinct from any established 

 species. The nearest approach to the composition found in the 

 literature is in certain cupriferous and argentiferous cosalites quoted 

 in Dana. Cosahte is a lead mineral, the composition of wliich is 

 definitely established and, while the lead is probably susceptible of 

 replacement by metals of like valence, including cupric copper, 

 cupric sulphide is relatively rare as a constituent of the sulpho-salt 

 minerals and the copper commonly occurs in the cuprous form 

 isomorphous with silver. The non-isomorpliism of metals of unhke 

 valence in minerals of this class is now widely recognized ^ and these 

 analyses of ''cosalite" wliich show amounts of silver and copper in 

 excess of 1 or 2 per cent are doubtless other minerals or mechanical 

 mixtures and required to be reexamined by modern methods. 



The benjaminite falls in the 3:2 division, klaprothoUte group of 

 Wherry and Foshag.^ The members of this group now are as follows: 



Klaprotholite 3Cu2S 2Bi2S3 



Schirmerite 2Ag2S. PbS 2Bi2S3 



Rathite 3PbS 2AS2S3 



Benjaminite 2PbS. (Cu, Ag)2S 2Bi2S3 



> William F. Foshag. The isomorphic relations of the sulphosalts of lead, silver and copper. Amer. 

 Journ. Sci.,vol. 1, pp. 444-443, 1921. 



' Edgar T. Wherry and William F. Foshag. A new classification of the sulfo-salt minerals. Journ. 

 Wash. Acad. Sci., vol. 11, pp. 1-8. 1921. 



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