606 



GEOLOGY AjS'D MINING INDUSTRY OF LEADVILLE. 



Table XII. — Alteration producis of galena andpyrite. 



1. From Maid of Erin mine, \inder White Porphyry. Contains 0.0048 Ag and trace of Au. 



2. From Morning Star (lorsaken) mine, under Gray Porphyry. Contains 0.0036 Ag. 



3. From Lower Waterloo mine, nn<ler Gray Porphyry. Contains 0.075 Ag. 



4. From Morning Star (Forsaken), under Gray Porphyry. 



5. From Morning Star (For.^aken), under Gray Porphyry. 



6. From Morning Star (Forsaken), under Gray Porphyry. 



7. From Silver Cord mine, under White Porphyry. 



REMARKS ON TABLE XII. 



Notwithstanding the great similarity in appearance of all the specimens of which 

 the above are analyses they are rather complex mixtures in very varying proportions 

 of several mineral substances. They all show a similar chemical behavior. Heated 

 in a closed tube, water is first evolved, the sub.stance then changes from an ocherous, 

 or sometimes brownish yellow, to dark brown, and later sulphuric and sulphurous acids 

 escape. The same changes occur in an open tube. On charcoal with soda there appears 

 sometimes a slight coating of arsenic trioside, accom]ianied by a smell of arsenic and the 

 reaction for lead. Entirely insoluble in boiling water. Nitric acid in the cold extracts 

 part of the lead oxide ; also, arsenic and phosphorus pentoxides and usually chlorine. 

 Continued boiling with nitric acid seems to decompose the iron minerals completely. 

 Warm hydrochloric acid efl'ects complete decomposition, and no trace of a ferrous salt 

 can be detected, even if solution has been effected in an atmosphere of carbon dioxide. 

 Caustic alkalies also decompose them completely, all the sulphur trioxide and arsenic 

 and phosphorus pentoxides going into solution, whereby the analysis is materially sim- 

 plified. Various tests, combined with a consideration of the three complete analyses, 

 show that the lead is present as anglesite and pyromorphite' : in No. 2 as the latter 

 mineral alone, the lead oxide, phosphorus pentoxide, and chlorine being in the 

 exact proportions required for the formula (3PbO, P20,) + PbCl2. The As.^Oj is noi 

 present in the corresponding chloro-arseniate of lead, as shown by the fact that 

 the proportion of P2O5 to that part of the CI not combined with silver is always the 

 same as in pyromorphite, and that there is insufficient lead for both phosphorus and 

 arsenic pentoxides together, as in No. 2, where it exactly sufiBces for the phos^jhorus 

 pentoxide. The arsenic pentoxide is therefore undoubtedly present as a hydrated 

 ferric arseniate. By combining in the first place chlorine and phosphorus pentoxide 

 with lead oxide and the remainder of the latter with sulphur trioxide, definite conclu- 

 sions may be reached as to the composition of the remainder of the mixture. Analyses 



'Tbo pyromorphite is sometimes visible in bunches of small crystals. 



