44 F. R. Tan Horn — Pisanite and Arsenopyrlte. 



Schaller* has shown to be isomorphous with melanterite form- 

 ing pisanite. Of incidental interest is the fact that the forma- 

 tion of both green and bine pisanite have caused a secondary 

 enrichment of the copper by oxidation, since the sulphide ores 

 of the district contain an amount which averages slightly under 

 two per cent. The small percentage of magnesia is seen to be 

 fairly constant in both varieties. When we calculate this MgO 

 as FeO, the analysis becomes as follows : 



Kecalculated 

 analysis 



S0 3 ._ 28-72 



(FeO,MgO) 22-14 



CuO 3-83 



HO 45-58 



Combining 



Molecular 



weights 



ratio 



•359 



7-48 



•308 



6-42 



•048 



1-00 



2-530 



52-71 



100-27 



About the nearest logical formula for the green pisanite 

 would seem to be seven times that of the melanterite formula, 

 or 6FeO.CuO.TS0 3 + 49H 2 0. In this analysis it would seem 

 that the water is even larger than in the blue mineral although 

 the same is nearly a per cent lower. In all published analyses 

 of the pisanite the size of the molecule seems to vary, although 

 all occur in multiples which adhere closely to the general 

 formula (RS0 4 + 7H 2 0) S . It is very clear from the analyses 

 of the Isabella varieties as well as those from other localities 

 that iron and copper have no fixed relation to each other, but 

 that they may replace each other in any proportion. 



Crystallography . — Although the analyses of the mineral 

 showed quite conclusively that the substance was pisanite, and 

 not a mechanical mixture of melanterite and chalcanthite, as 

 might be claimed, nevertheless, it was thought advisable to-see 

 if any measurable crystals could be obtained. The writer, 

 after breaking up several specimens of the blue pisanite, was 

 able to find some cavities lined with well-defined crystals. 

 Because of his previous experience with pisanite from Cali- 

 fornia, Dr. W. T. Schallerf of the IT. S. Geol. Survey, 

 Washington, D. C, was asked to see if he could obtain any 

 crystallographic results on the material selected. I wish to 

 express my thanks to Dr. Schaller for his data which follow, 

 and also especially for his drawings, figs. 3, 4, and 5. Dr. 

 Schaller has observed seven -forms which are fairly common 

 for pisanite with the exception of <t (121), which was first dis- 

 covered by him on the mineral from the Alma mine, Leona 

 Heights, California. Dr. Schaller's results follow : 



*Op. cit., p. 207. fOp. cit., pp. 199-207. 



