18 Organic Acids in the Examination of Minerals. 



a purer specimen was not attacked. With bournonite the 

 reaction is feeble. 



Pyrite, marcasite, molybdenite, chalcocite, cinnabar, argen- 

 tite, niccolite, smaltite, chalcopyrite, ullmannite, arsenopyrite, 

 and tetrahedrite resist the action of citric acid. Tartaric and 

 oxalic acids act in a similar manner, both in the cold and on 

 boiling. A comparison of the behavior of nine organic acids 

 with stibnite gave the following results : 



(a) Stibnite heated with citric, tartaric, and. oxalic acids yields sul- 

 phureted hydrogen freely and goes into solution. 



(&) With malic, benzoic, and pyrogallic acids, sulphureted hydrogen 

 comes off feebly and the mineral dissolves imperfectly. 



(c) With formic and acetic acids no gas is evolved and the mineral is 

 not dissolved. 



(d) With picric acid no gas is evolved, but the mineral is partially 

 dissolved. 



It is noticeable that the liquid acids are powerless to effect 

 decomposition. (§ 8.) 



13. The action of the organic acids on mineral sulphides 

 is not so decided as that of the mineral acids ; but this is no 

 disadvantage, since it affords additional means of determining 

 them. On examining the reactions recorded in the preceding 

 section, it will be found that bornite and pyrrhotite are decom- 

 posed by citric acid, while their kindred compounds, pyrite 

 and chalcopyrite (as well as chalcocite) are not. In order to 

 establish satisfactorily this difference of behavior, several 

 specimens of each of these minerals, from various localities, 

 were carefully tested. 



(a) Four samples of bornite were heated with a concentrated solution 

 of citric acid, and each gave a strong reaction for sulphureted hydrogen ; 

 of four specimens of chalcopyrite, treated in the same manner, two gave 

 no traces of this gas, and two gave mere traces on long boiling ; three 

 samples of chalcocite yielded no traces of sulphureted hydrogen. 



(b) Three specimens of pyrite heated with citric acid gave no traces of 

 the gas, and three of pyrrhotite liberated it both in the cold and freely on 

 boiling. On the other hand all the specimens named are decomposed by 

 hydrochloric acid, except one specimen of pyrite (from Germany). 



It is evident, then, that citric acid may be used to distin- 



