74 
MANGANESE DEPOSITS OF INDIA : MINERALOGY. 
Part 1 : 
3Mn203.MnSi03 ; so that some of the St. Marcel mineral must conform 
to this formula and some to the formula 4Mn203.MnSi03. 
The question arises as to whether all the braunites can be fairly said 
to conform with one of the definite formulae, in which the ratio of Mn203 
to MnSi03 is either 3 : 1, 7 : 2, or 4 : 1 ; or whether the admixture of these 
two portions of the braunite is really isomorphous within the limits 3 : 1 
and 4 : 1, so that the ratio of Mn203 to MnSi03 may have any value 
between these two limits. In Treadwell and Hall's Analytical Chemistry, 
I, page 115, reasons are given for supposing that the structural 
composition of Mn203 may be represented in the following manner 
This supposition is also supported by G. Bertrand* in order to allow 
of the replacement of Mn by Ba in the basic portion of the mineral, and 
of Mn by Si in the acid portion of the mineral. This is of course practically 
equivalent to a return to the idea of Rose that the Si02 can isomorpli- 
ously replace Mn02. It seems indeed as if the structure of bramiite 
can be very conveniently explained in this way, according to which it 
would be an isomorphous mixture of molecules of MnMn03, CaMnOs, 
BaMnOs, FeMnOs, MnSi03, FeSi03, ^^f - varying proportions, the 
limits to the replacement of molecules of the form RMnOs by molecules 
of the form RSiOs being that prescribed by the ratios 3 : 1 and 4:1. It is 
probable, in view of the fact that the varieties of braunite yet analysed 
do not show any very large amount of replacement of MnMn03 by 
FeMnOs or other similar molecules, that there is also a limit to this 
replacement consistent with the mineral still possessing the form of braunite. 
The extent to which this replacement of one molecule by another can take 
place so that the mineral still exhibits the characteristics of braunite is a 
point that will need a large amount of investigation and one that can pro- 
bably be solved only by an attempt to make the different varieties 
artificially. 
If, however, the views expressed above be correct one would expect 
the replacement of molecules of the form RMn03 by ones of the form 
RSiOs to take place gradually between the limits 3 : 1 and 4 : 1. The 
I Rcvui grnrrnlr d<: Chintie, Vlll, j). 208, (1905). 
