A. K Rogers — Delafossite. 293 



tive determination of ferrous iron by titrating the sulphuric 

 acid solution of the mineral with potassium permanganate solu- 

 tion and obtained results which, calculated, give 34*10 and 34*62 

 as the percentages of ferrous iron. This might indicate that 

 all the iron was in the ferrous state, about 2-J- per cent having 

 been oxidized by fine grinding, were it not for the fact that a 

 solution of the coarsely crushed mineral gave distinct tests for 

 ferric iron. Another fact to be considered in this connec- 

 tion is that cuprous ions will also decolorize potassium per- 

 manganate solution. The results obtained by Mr. Bohart are 

 due to the combined effect of the ferrous and cuprous ions 

 and there is no way of determining the effect of the ferrous 

 ion alone. Copper, of course, should be removed when titrat- 

 ing an iron solution with permanganate, but it could not be 

 done in this case without changing the condition of the iron. 



My colleague, Professor G. H. Clevenger, suggested that the 

 original condition of the copper in the mineral under discus- 

 sion might be determined by using as a reagent silver sulphate 

 solution. In the Ziervogel process (a metallurgical process of 

 silver extraction) the presence of cuprous oxide, an undesirable 

 constituent, is indicated by testing the product with silver sul- 

 phate solution, in which case thin flakes of metallic silver 

 appear. This is known as the " spangle reaction."* 



The powdered delafossite when heated with a slightly acidu- 

 lated silver sulphate solution gave a beautiful spangle reaction 

 after standing about two days. Ferrous iron also gives this 

 reaction, as the writer proved with siderite. In fact, almost 

 any reducing agent may give it. But in delafossite the only 

 possibilities to consider are cuprous copper and ferrous iron. 

 JS T ow, in delafossite the iron is, in all probability, combined 

 with the oxygen as a radical and it probably does not act as 

 iron, but only as a radical. The composition of delafossite, 

 then, is probably cuprous metaferrite, Cu'Fe /// O a . 



Chalcopyrite, CuFeS 2 , and magnetite, FeFe 2 4 , also give the 

 spangle reaction with silver sulphate. With chalcopyrite it is 

 the cuprous copper, and with magnetite it is the ferrous iron 

 that acts as a reducing agent. This would indicate that these 

 minerals are, respectively, cuprous metasulfoferrite, Cu/Fe^'S,,, 

 and ferrous metaferrite, Fe'Fe/^C^. Some mineralogists have 

 considered these as the most probable formulae ; the spangle 

 reaction, perhaps, makes it more certain. 



Similar Compounds. — Magnetite, FeFe 2 4 , franklinite, (Zn, 

 Fe)Fe 2 () 4 , magnesioferrite, MgFe 2 4 , and jacobsite, MnFe 2 4 , 

 are well-known metaferrites, all isometric in crystallization. 



*Egleston, The Metallurgy of Silver, Gold and Mercury in the United 

 States, vol. i, p. 134, 1887. 



