311 Foote and Bradley — Solid Solution in Minerals. 



The analyses show that all specimens contained more or less 

 ferrous carbonate, and one, manganese carbonate also. We 

 consider that these components replace magnesium carbonate. 

 The ratios are as follows : 



I II III 



MgC0 3 -397 '373 -364 



FeC0 3 -052 -073 "030 



MnCO s .... -028 



CaC0 3 ° -606 -607 '630 



The ratios /TT — ^ — _ * „ calculated from these results are : 

 (Mg,Mn,Fe)CO, 



I II ill 



1-349 1-361 1-492 



These ratios show a somewhat surprising excess of calcium 

 above the dolomite ratio, and the excess is variable. Since the 

 material was homogeneous, but deposited directly on calcite, 

 we see no reason why the ratios do not represent approximately 

 limiting values for calcium carbonate in dolomite. From the 

 fact that dolomite occurs so generally in the 1 : 1 ratio, Ret- 

 gers* assumed that this ratio could not be much exceeded, but 

 this appears not to be the case. The variable ratio is not 

 unexpected and should be due in large part to the influence of 

 temperature at the time of formation. These were all the 

 specimens analyzed, in which dolomite was clearly the secondary 

 mineral. One specimen was obtained in which there was pri- 

 mary formation of dolomite followed by the formation of a 

 single large calcite crystal. (See calcite anal. ]STo. 6.) About 

 the base of the calcite, a small quantity of dolomite had subse- 

 quently formed. Both deposits were similar in appearance 

 and could not be separated. A. sample, chipped off near the 

 base of the calcite crystal and containing some of each deposit, 

 gave the following results on analysis : 



Sp. gr. 2-834-2-868 

 Joplin, Mo. 



a b Average 



MgC0 3 40-31 40-13 40-22 



FeC0 3 1-91 1-93 1*92 



CaCO. 58-19 58-13 58-16 



100-41 100-19 100-30 



CaCO 

 The ratio .__. ^ N i,^ calculated from these results is 1-176. 

 (MgFe)C0 3 



*Loc. cit., vi, 227, 1890. 



