1894.] -"'^ ' [Brown. 



are the most regular. No very marked difference in the rate of oxidation 

 is brought out by this series of experiments, the amount of sulphur oxi- 

 dized never having reached the critical point in pyrite, as shown by Prof. 

 Smith's oxidations with the current already described. This point at 

 which the rate of oxidation of sulphur in pyrite suffers a change was 

 found by Prof Smith to be between 21 and 22 per cent, from the results 

 of a very large series of current oxidations.* The explanation for this 

 being the critical point in the oxidation of pyrite will be given in the dis- 

 cussion of its constitution. The experiments with permanganate oxida- 

 tion simply show then that up to near this point (the highest point reached 

 in the pyrite oxidation was nearly 15 per cent.) the relative rates of oxi- 

 dation of the two minerals do not differ widely from each other, but that 

 marcasite oxidizes somewhat faster than pyrite. This is simply what has 

 been long known and recognized in regard to atmospheric weathering. 



As will be seen when the constitution of these minerals is considered, 

 marcasite cannot have a critical point in regard to oxidation of its sulphur. 



The experiments thus far described had for their object the removal of 

 sulphur. On the other hand, a number of ways of attacking the iron 

 were tried and with more interesting results. In these trials reagents 

 were selected which would attack the iron more energetically than the 

 sulphur. Among these may be classed the experiments of solubility in 

 acids. 



Hydrochloric acid (hot or cold, concentrated or dilute) has little action 

 on these minerals. Pyrite was treated for one hour with boiling concen- 

 trated HCl, of specific graviiy 1.20 in covered beakers and showed in the 

 solution only 2.56 per cent.' of iron out of 4(5.67 per cent. Marcasite, 

 treated in the same way, gave an identical result. Similar experiments 

 at the ordinary temperature were tried with both minerals, by digesting 

 for three days with excess of concentrated hydrochloric acid and with 

 excess of 2 UCl -f SH^O, but even after three days the action was very 

 slight in both cases. Pyrite gave with both concentrated and dilute acid 

 the same result — a solution of 1.51 per cent, of iron. Marcasite gave 

 almost identical results. The concentrated hydrochloric acid solution 

 showed 1.51 per cent, of iron, the dilute solution 1.&9 per cent. No evo- 

 lution of hydrogen sulphide was detected by lend paper in either case. 

 Concentrated sulphuric acid at boiling temperature decomposes both 

 minerals, with evolution of suli)hur dioxide and the separation of sulphur, 

 but the action is very slow and seems to take place more readily with 

 pyrite than with marcasite. Pyrite digested with concentrated sulphuric 

 acid at boiling temperature for one hour showed 14.81 per cent, of the 

 iron dissolved, but marcasite under like conditions was only attacked in 

 one hour to the extent of 12.77 per cent, of iron. Trials were also made in 

 the cold, but did not differ materially from the results obtained with IICl. 

 More important results were obtained by conducting dried hydrochloric 



* Private communication from Prof. E. F. Smith, 1893. 



