592 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



Analysis and ratios of boulangerite from Nertschinsic, Siberia. 



These ratios show that, assuming the deficiency shown by the anal- 

 ysis to be in part due to antimony lost, the mineral like those preced- 

 ing had the composition expressed by the formula oPbS.SSb.Sg. 



The foregoing three original descriptions offer strong evidence in 

 support of the conclusion advanced by Sjogren — namely, that the 

 true formula for boulangerite is SPbS.SSbjSs- Later analyses of 

 this mineral from a large number of localities agree almost exactly 

 with this formula. It is not improbable that the compounds 

 SPbS.SbjSg and lOPbS.SSbaSg also occur as natural mineral species, 

 but in the absence of distinctive physical and crystallographic data 

 they have been confused with boulangerite, owing to the close 

 similarity in chemical composition existing between the three com- 

 pounds. The formula 10PbS.3Sb._,S3 has been assigned by Frenzel ^ 

 to the substances plumbostib and embrithite, but the analyses are far 

 from satisfactory. An arsenic analogue of this compound has been 

 found in the mineral guitermanite. Specimens of boulangerite from 

 Przibram, Bohemia, analyzed by Boricky and Helmhacker,^ in part 

 agree fairly well with the formula SPbS.SbjSg, as shown in the 

 following table. An equal number of the same series of analyses, 

 however, yields more nearly the 5 : 2 formula. 



Analyses of boulangerite from Przibram, Bohemia. 



'Journ. Pract. Chem., vol. 2, p. 

 > Ber. Akad. Wien.. vol. 56, 1867. 



