Mineral Sulphides of Iron. 185 



denser, and in direct sunlight showed a decided metallic luster, 

 while their oxidation coefficients approach those of pure pyrite. 

 It may therefore be safely stated that the product of the union 

 of ferrous sulphide and sulphur from an alkali polysulphide 

 solution is at first amorphous disulphide of iron which gradu- 

 ally crystallizes to pyrite.* 



The formation of iron disulphide by the action of sodium 

 thiosulphate on ferrous salts. — In the endeavor to explain the 

 formation of pyrite and marcasite in nature, the following 

 hypothesis presented itself. Iron disulphide of either form 

 may oxidize under surface conditions to ferrous thiosulphate 

 by direct addition of oxygen ; this is transported by circulating 

 waters to some point where it is reduced again to its former 

 condition. In a partial study of the oxidation of marcasite, 

 no trace of thiosulphate was discovered. At the same time 

 the effort to obtain the disulphide of iron by reduction of 

 the thiosulphate was successful. When water solutions of 

 ferrous sulphate and sodium thiosulphate are heated in sealed 

 tubes, even to temperatures under 100°, the iron disulphide is 

 precipitated with sulphur. By quantitative experiments which 

 follow, the reaction is proved to be : 



4Na 9 S a O, + FeS0 4 = FeS 2 + 3S + 4Na 2 S0 4 . 



Exp. 1. 5 g. FeS0 4 .7H 2 0, 18 g. Na 2 S 2 3 .5H 2 0, and 25 cc 

 water were heated in a sealed tube for 9 days at 90°. All but a 

 trace of the iron was precipitated. The precipitate of FeS 2 + S 

 was washed in air-free water and dried in vacuo. The sulphur 

 was extracted by carbon disulphide and the residue of FeS 2 was 

 weighed. 





Found 



Cal. from the 

 above equation 



FeS 2 +S 



FeS 



3-82 

 2-17 



3'85 

 2*16 



Exp. 2. 2 g. FeS0 4 .7H 2 0, 18 g. Na 2 S 2 3 .5H 2 and 35 cc water 

 were sealed in C0 2 and heated for one day at about 150°. The 

 precipitate was filtered and washed with air-free water. The 

 solution was boiled iu a current of carbon dioxide to remove a 

 trace of hydrogen sulphide, and an aliquot part was titrated 

 with standard iodine solution. 



No. 1. 1/5 of the solution required 16*135 g. iodine solution. 

 Cal. for th > whole, 80*675 g. iodine solution. 



No. 2. 2/5 of the solution required 32*332 g. iodine solution. 



* In a recent paper (Zs. angewandte Chem., xxiv, 97, 1911), " Die Bildung 

 von Eisen Bisulphide in Losungen und die Enstehungen der naturlichen Pyrit- 

 lagern," W. Feld states that whenever sulphur and ferrous sulphide are boiled 

 in neutral or weakly acid solutions, pyrite forms. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 195.— March, 1912. 

 13 



