Mineral Sulphides of Iron. 187 



The transformation of marcasite into pyrite. — More than 

 fifty years ago, Wohler* tried the experiment of heating both 

 minerals for four hours at the temperature of boiling sulphur 

 (about 445°) without observing any change in either of them, 

 Our own results indicate that marcasite undoubtedly changes 

 here, but very slowly. When marcasite was heated at 610° in 

 hydrogen sulphide gas for 3 hours, it lost about 2*5 per cent 

 sulphur and became strongly magnetic, owing, of course, to the 

 formation of some pyrrhotite. A finely ground sample, after 

 being thoroughly boiled out with hydrochloric acid, appeared 

 decidedly yellower and less lustrous than marcasite.f 



The comparison is best made by placing the sample to be 

 tested alongside of a fragment of marcasite which has had all 

 tarnish removed by recent boiling in hydrochloric acid (Stokes). 

 A finely ground and purified sample of the heated marcasite 

 gave the oxidation number 56 instead of 14 as previously. At 

 525°, a sample of marcasite heated 4^ hours in hydrogen 

 sulphide gave the oxidation number 55*8. Under similar 

 conditions at 450°, a sample of marcasite heated 4 hours gave 

 the oxidation number 15*7, which corresponds to 5*5 per cent 

 of pyrite. The sample was returned to the furnace and heated 

 again 5 hours. This time the top layer in the crucible gave, 

 after purification, the oxidation number 27, corresponding to 

 53 per cent pyrite, while a deeper layer in the same crucible 

 gave 31, which indicates about 61 per cent of pyrite. At 450°, 

 therefore, dry heating in H 2 S changes marcasite to pyrite 

 rather slowly — 50 per cent-60 per cent in 9 hours. Heated 

 to 410° for 4 hours, the oxidation number was 13*5, showing 

 that no measurable change had occurred. 



J. Konigsberger and O. ReichenheimJ found a marked 

 decrease in the electrical resistance of marcasite in the neigh- 

 borhood of 520°. They noted that the sulphide then possessed 

 a specific resistance of the same order as pyrite, and rightly in- 

 terpreted their results to mean that marcasite had changed into 

 pyrite and the change is irreversible. It is difficult to see in 

 their results, however, any support of their statement that the 

 change appears to begin between 250° and 300°, while our 

 results contradict it. 



An effort was made to effect the transformation at a lower 

 temperature in the wet way. At 350° marcasite heated in a 

 sealed tube with a small amount of dilute sulphuric acid partly 

 changed to ferrous sulphate and sulphur dioxide, but the solid 



* Ann. Chem. Pharm., 90, 256, 1854. 



f Pyrite is naturally a more lustrous mineral than marcasite. The duller 

 color of heated marcasite is to be ascribed to the very large number of 

 minute crystals in the product, and the lack of continuous surfaces. 



JNeues Jahrb., ii, 86, 1906. 



