204 ALBERT D. BROKAW 
Feldspars Olivine Calcite Vesuvianite 
Micas Serpentine Aragonite Noselite 
Pyroxenes Talc Dolomite Sodalite 
Nephelite Tourmaline Magnesite Hauynite 
-Spodumene Apatite Siderite 
It seems certain that if gold in solution escapes precipitation 
by sulphides, ferrous compounds, etc., and if gold and manganese- 
bearing solutions remain in contact with ordinary wall rock or 
gangue minerals for sufficient length of time, the acidity will be 
sufficiently reduced to allow the simultaneous precipitation of gold 
and manganese dioxide in the ore body. 
It must be noted in this connection that the amount of man- 
ganese dioxide mixed with the gold is usually far in excess of that 
required by the equation given on p. 262. 
Ferric salts, the most abundant oxidizing agents in mine waters, 
do not oxidize manganous salts to manganese dioxide, even in 
alkaline solutions. This was shown experimentally by making 
a solution of manganous sulphate and ferric sulphate, and slowly 
neutralizing the solution. Iron hydroxide was precipitated at 
once, but the manganese remained in solution until the solution 
became alkaline, when it was precipitated as manganous hydroxide. 
Wellst has suggested an explanation for the precipitation of 
manganese oxides below the zone of acid waters, by the presence 
of small amounts of permanganates in the mine waters, which 
react with manganous salts to form manganese dioxide as follows: 
2KMnO,+3MnSO,+2H.0=5MnO.+K.SO,+2H.SO, 
The acid liberated is taken up by alkalies in case of an alkaline 
solution. This reaction has been used in the volumetric deter- 
mination of manganese and is doubtless important if permanganates 
are present. No data available show permanganates in mine 
waters, and this theory needs verification before it is accepted 
as correct. However, it seems to be the nearest approach to an 
explanation of the precipitation of the excess of manganese dioxide 
over the stoichiometric proportion to gold, or the precipitation 
of the substance in any conspicuous amounts. The order of pre- 
tR. C. Wells, cited by Emmons, Trans. Am. Inst. Min. Eng., XLII, 29 (1911). 
