226 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



ANKERITE (271A) 



Calcium-iron carbonate, CaC0 3 .FeC0 3 . Rhombohedral. 



Ankerite may be considered a dolomite in which the magnesia has 

 been more or less completely replaced by iron, and every gradation 

 is possible between a magnesia-free iron-lime carbonate and an iron- 

 free magnesia-lime carbonate. While ankerite doubtless occurs in 

 other parts of the State, its identity has been demonstrated by 

 analyses only on specimens from the Coeur d'Alene region in Sho- 

 shone County. 



SHOSHONE COUNTY 



The occurrence of ankerite in Shoshone County has been outlined 

 above under dolomite. Of a series of 10 gangue carbonates analyzed 

 from south and east of Mullan, mostly from copper prospects, 3 were 

 found to be siderite, 1 calcite, 4 ferriferous dolomite, and 2, while 

 containing magnesia, had a preponderance of iron over magnesia, and 

 hence can with propriety be called ankerites. 



The first of these is from the upper level of the Carney copper 

 prospect on Willow Creek, which has been described by Jones and 

 Calkins. 30 The specimen was collected as typical ore by Mr. Calkins 

 in 1912. It consists of a granular carbonate in grains 2 to 5 mm. in 

 diameter showing good rhombohedral cleavage. The carbonate is 

 white to faintly buff in color and contains abundant chalcopyrite 

 and a few thin seams of quartz. The quartz is certainly later than 

 the carbonate which it enters in cracks, and the chalcopyrite appears 

 to replace the carbonate. The results obtained upon analysis of the 

 carbonate, recalculated after deducting a little quartz, are as follows: 



Ankerite, Carney copper prospect 

 [Earl V. Shannon, analyst] 



The ratio of Fe + Mn to Mg is 1 to 0.81, and that of Fe + Mn + Mg 

 to Cais 1.00 to 1.04. 



The second specimen is from the Reindeer prospect and was col- 

 lected from the dump in 1912 by Mr. Calkins. It is medium grained 

 nearty white where fresh, but where slightly weathered is turning 

 brown. It contains later replacing masses of chalcopyrite and a 

 little of a gray sulphide, either tetrahedrite or chalcocite. Upon 

 analysis this gave the following results: 



36 F. C Calkins and E. L. Jones, jr. U. S. Geol. Survey, Bull. 540, pp. 201-202, 1914. 



