258 ALBERT D. BROKAW 
b) Primary iron minerals other than sul phides.—Siderite is 
capable of precipitating gold from solution as follows: 
AuCl,+3FeCO,;+3H,.0=Au+3CO.+FeCl,+2Fe(OH), 
Of this reaction we have geological evidence in the form of gold 
pseudomorphs after siderite, in which gold gives a sort of skeleton 
of the siderite replaced, since its volume is less than that of the 
siderite. 
The specimen shown in Figs. 1, 2, and 3 is from the collection in 
Walker Museum, University of Chicago. The precipitation of gold 
by means of siderite was easily accomplished in the laboratory. A 
Fic. 1.—Gold replacing iron carbonate. XX 2 
cleavage block of light-colored siderite was placed in toc.c. of 0.5 
per cent gold solution, and after two weeks flakes of gold could be 
distinguished on the faces of the mineral. In another experiment 
sulphuric acid was added to the gold chloride solution. In this 
case the precipitation was more rapid, the siderite being nearly 
covered with shining flakes of gold after four days. After two 
weeks typical “‘wire gold” was noted, more or less coated with 
limonite. It is worthy of note that many occurrences of wire 
gold are found imbedded in a gangue of limonite. Ransome,’ 
in describing specimens of gold from the Breckenridge district in 
tF. L. Ransome, U.S. Geol. Survey, Professional Paper No. 75, pp. 81-82. 
