Penfield and Ford — Calaverite. 215 



demanded by the formula is 134, whereas the published analy- 

 ses show from 11'50 to 13*86 per cent. An analysis of 

 sylvanite from Cripple Creek by Palache* yielded 1249 per 

 cent of silver and a ratio of Au : Ag=l : 0*87. As most analy- 

 ses of sylvanite indicate less than 12 per cent of silver, their 

 agreement with the formula AuAgTe 4 is only approximate. 

 Calaverite, on the other hand, is a nearly pure gold telluride, 

 conforming closely to the formula AuTe 2 , though always con- 

 taining some silver, and at times, as indicated by the analyses, 

 as high as 3*5 per cent. Although chemically there is not a 

 very great difference between calaverite containing 3*5 per 

 cent of silver and sylvanite containing 11*5 per cent, still there 

 is evidently a tendency for sylvanite to conform closely to the 

 formula AuAgTe 4 and calaverite to the formula AuTe 2 . 

 Thus from a chemical standpoint alone calaverite may well be 

 regarded as a distinct mineral species. The relation between 

 the two minerals is analogous to that existing between calcite 

 and dolomite, and as the latter are closely related crystallo- 

 graphically, so calaverite and sylvanite are closely related as 

 indicated by their similarity in axial ratios and twinning. 



In order to distinguish between sylvanite and calaverite, the 

 following method of testing, which has been applied to all of 

 the occurrences examined by us, may be recommended : The 

 powdered mineral when boiled in a test-tube with concentrated 

 nitric acid is quickly oxidized, the silver and tellurium going 

 into solution, gold being left. The clear solution, decanted 

 into another test tube, diluted, and tested with hydrochloric 

 acid, gives a precipitate of silver chloride, which is consider- 

 able if the mineral is sylvanite, but is slight, or amounts per- 

 haps only to a turbidity, if the mineral is calaverite. After 

 washing the gold by decantation it may be dissolved in a few 

 drops of aqua-regia, indicating a complete separation of gold 

 and silver. 



Specific gravity determinations and quantitative estimations 

 of gold and silver have been made on specimens from the 

 .Monument Mine and from the unknown Cripple Creek Mine 

 designated as Occurrence N"o. 2. Of material from the lirst 

 source a single fragment weighing 0*7729 grams was employed, 

 while from the second source a number of fragments weighing 

 1*2532 grams were available. The specific gravity determina- 

 tions were taken with much care on a chemical balance. The 

 results by Penfield are as follows : 



*This Journal (4), x, p. 422, 1900. 



