444 W. F. Fosliag — Isomorphic Relations of the 



Akt. XXXII. — The Isomorphic Relations of the Sulpho- 

 salts of Lead and Copper; by William F. Foshag.^ 



The sulpliosalt minerals are chiefly compounds of lead, 

 silver and copper with sulphur and antimony, arsenic and 

 bismuth. Rarely iron, zinc, mercury, thallium and man- 

 ganese enter into the composition of these minerals to 

 any considerable extent, while selenium and tellurium 

 may replace some of the sulphur. In these compounds 

 sulphur, selenium and tellurium are strictly isomorphous, 

 as are also antimony, arsenic and bismuth. A critical 

 survey of all reliable data, however, indicates that the 

 lead, and silver and copper are not isomorphous and that 

 the formation of mixed crystals does not take place. This 

 fact does not seem to have been generally recognized and 

 our standard text-books contain formulas of the 

 type (Pb,Ag,),Sb,S,i (diaphorite) and (Cu,Pb)3Sb,S, 

 (bournonite). We find, where the analyses are reliable, 

 that the silver minerals, pyrargyrite, stephanite, etc., are 

 remarkably free of lead and that the lead minerals rathite, 

 sartorite, etc. are free of copper and silver. H. Rose was 

 the first to state this fact. He says,- ' ' I have never seen 

 lead sulphide occurring together with the other metallic 

 sulphides except copper sulphide and sometimes with 

 iron sulphide, which, however, are in such small amounts 

 that they do not appear to belong to the compound. The 

 compounds that do not contain lead sulphide are com- 

 pletely free of lead even when they are surrounded by 

 galena or the crystals rest upon galena." 



These deductions of Rose seem to have received little 

 attention other than occasional denial in the early litera- 

 ture. We shall see that his generalization is correct 

 except for a certain type of compound. 



In order to determine whether there is considerable 

 miscibility between lead on the one hand and silver and 

 copper on the other the writer undertook to collect a 

 number of analyses of the sulphosalts, noting the kind 

 of material upon which the analyses were made. Unfor- 

 tunately the descriptions of the material analyzed were in 



^ Published by permission of the Director of the Smithsonian Institution. 

 ^Ueber die in der ISTatur vorkommenden nicht oxvdirten Verbindungen 

 des Antimons und des Arseniks, Pogg. Ann., 15, p. 469. 



