254 ALBERT D. BROKAW 
Hydrogen sulphide may be formed by the interaction of acid 
mine waters on mineral sulphides, for example, on zinc blende, 
as follows: 
H.SO,+ZnS = H.S+ZnSO, 
This reagent is doubtless an important precipitant for gold in 
such waters. Why then, does it throw down metallic gold in 
nature, instead of gold sulphide, as in the laboratory ? 
A number of factors may be mentioned as probable causes. 
Hydrogen sulphide is given off very slowly by most of the mineral 
sulphides except pyrrhotite and zinc blende, even in fairly strongly 
acid solutions. It has been shown that in the presence of auric 
chloride, gold sulphide is oxidized to metallic gold and sulphuric 
acid, with the further precipitation of gold from the solution, 
probably according to the equation: 
Au.S.+4AuCl,+8H,0 =6Au+ 2H.SO,+12HCl 
Since the hydrogen sulphide is very slowly evolved, it may be 
that the gold sulphide is momentarily formed and immediately 
oxidized by the adjacent gold chloride molecules, which have not 
yet been attacked by the hydrogen sulphide. The reaction was 
tested experimentally by placing in a dilute solution of gold chloride 
fragments of pyrrhotite and zinc blende, the mineral sulphides 
which give off hydrogen sulphide the most readily and are there- 
fore the most favorable for the precipitation of gold sulphide. In 
the case of zinc blende the surface of the crystal was covered with 
shining flakes of gold after standing 24 days. No gold sulphide 
could be distinguished. In the case of pyrrhotite the action was 
more rapid and the gold was completely precipitated in 3 days, 
but here again no gold sulphide was detected. The pyrrhotite 
was covered with a dull yellowish-brown coat, which, under the 
microscope was shown to be made up of minute crystals of gold, 
partly imbedded in material resembling limonite. . 
A second possibility is that ferric salts may oxidize the gold 
sulphide in a similar fashion. With this in mind, a solution of 
gold chloride and ferric sulphate was treated with hydrogen sulphide 
solution until a dark precipitate began to form. A small amount 
t Gmelin-Kraut, Joc. cit. 
