426 E. V. Shannon — Notes on Boulangerite. 



paper by Foshag which is now in press. The formula 

 assigned to diaphorite by these authors in iPbS.SAgsS. 

 SSbsSs or with the ratio ES : SbsSs = 7:3. When this 

 result was first submitted to the writer's attention it was 

 thought possible that boulangerite might also have these 

 ratios, the difference being small. The compositions to 

 satisfy the two formulas are as follows : 



5PbS.2Sb,S3 TPbS.SSb.Sa. 



Lead (Pb) 55.41^o 54.01% 



Antimony (Sb) 25.72 26.87 



Sulphur (S) 18.87 19.12 



An examination of the analyses published by the writer 

 in the paper above cited shows that most of them agree 

 most nearly with the first formula although one or two 

 approach the second. The analysis of material from the 

 specimen bearing the crystals here described agrees very 

 closely with the 5 : 2 ratios which may not be significant 

 as the analysis was made upon the fibrous portion of the 

 specimen, the crystals not being sufficient in amount for 

 analysis. The problem of determining the correct compo- 

 sition of boulangerite now seems further complicated by 

 the probable existence of a lead sulphantimonite having 

 7 : 3 ratios. It is obvious that very exact analytical work 

 on material of undoubted purity will be necessary to 

 distinguish between these two compounds. Existing 

 knowledge of the minerals diaphorite and boulangerite 

 appears to indicate that they have different ratios of base 

 to acid and hence fall in different groups, the crystallo- 

 graphic similarity being accidental and not significant. 

 Any evidence as to the crystal form or physical proper- 

 ties of minerals of this general composition should be 

 carefully recorded as such data may ultimately lead to 

 the differentiation of two or more species. 



