Mineral Sulphides of Iron. 



183 



for a long time with 20 per cent hydrochloric acid. The res- 

 idue was dense and brassy-yellow. It was finely ground, puri- 

 fied as usual, and tested by Stokes's method. It gave an oxi- 

 dation number of 55*1, which corresponds to purepyrite within 

 the limits of error of the method. To make sure that no mis- 

 take had been made in this test, some pure natural pyrite was 

 compared the next day with the same ferric sulphate solution. 

 100 cc of the sulphate solution, after it had been reduced with 

 the synthetic pyrite, required 42'51 g. of permanganate solution. 

 100 cc which had been reduced by natural pyrite took 42*56 g. 

 of the permanganate. 



The action of sulphur on pyrrhotite was tried again at 300°, 

 where the reaction was of course much more rapid than at 70°. 



Into the tube were put 5 g. powdered pyrrhotite, 1*75 g. sul- 

 phur, 0*2 g. JSTaHCOg, and 100 cc water partially saturated with 

 H 2 S. The tube was heated four days at 300°. The oxidation 

 number of the purified product was 52*0, corresponding to 

 about 95 per cent pyrite. It is possible that these products 

 contained a very little undecom posed pyrrhotite or perhaps 

 amorphous FeS 2 , both of which would have undoubtedly had 

 a similar effect as marcasite in lowering the oxidation number. 



Table III. 



The oxidation numbers of FeS 2 formed from alkali poly sulphide solution. 



Time 



Water 



FeS0 4 . 

 7H 2 



Na 2 S 2 



Sulphur 



Temper- 

 ature 



Oxidation 

 number 



Py- 



rite 



3 days 



100 cc 



5g. 



plain 

 excess 



0-75 g. 



300° 



54 



97* 



5 days 



100 cc 



3g- 



plain 



excess 



•75 g. 



200° 



405 



71* 



1 days 



100 cc 



3g- 



plain 

 excess 



•75 g. 



]00° 



26 



51* 



The action of alkali poly sulphides on ferrous salts. — 

 Senarmont* in 1851 showed by analysis that the product of 

 the action of alkali polysulphides on ferrous salts at 180° is 

 FeS 2 . The question of the crystal form was not investigated. 

 The black amorphous precipitatef which is obtained at room 

 temperature by the above reaction appears to be a mixture of 

 sulphur and ferrous sulphide, at least it decomposes with dilute 

 acids, giving a residue of amorphous sulphur, while hydrogen 

 sulphide escapes. On heating, disulphide of iron gradually 



* Loc. cit. 



f Gedel (loc. cit.) claims that this precipitate is Fe 2 S 3 . 



