Chap. III. ] 
BRAUNITE. 
•75 
figures that are the most explicit in this respect are those for the Si02. 
If the replacement be gradual, the amoimts of silica found in a suffi- 
ciently large series of analyses of braunite should be distributed evenly 
over the whole of the range between the percentages corresponding to the 
two hmiting forms of braunite, namely between the values 7'91 and 9'98. 
The eight values of the siUca given above can be arranged in the following 
order — 
7- 68 near Tdl for 4Mn203-MnSiO;j- 
8 - 40 
8 '69, 872, 873, near 8 82 for 7Mn203-2MnSiOy. 
9- 80, 9-94, 10-02, near 9 98 for 3Mn203.MnSi03- 
It will be seen that all except one of these group themselves close to 
the values for one of the fixed ratios 3 : 1, 7: 2, and 4:1. Hence it would 
seem that braunites of these definite ratios, only, exist, and that the mixture 
is not truly isomorphous, owing perhaps to a definite combination taking 
place betweem the MnMn03 and the MnSiOs when the composition corre- 
sponds to these defijiite ratios. This is of course a question that can only 
be settled when a much larger nimiber of analyses have been made. I am 
inclined to think that the grouping shown by these analyses may be 
accidental and that the admixture of MnMnOs and MnSiOs may take 
place in any proportions between the limits 3 : 1 and 4:1. 
In deahng Avith the analyses of composite specimens of ore scattered 
through the descriptive portion of this Memoir, I have assumed for the 
braunite the most generally accepted formula, namely 3Mn203.MnSi03, 
containing a percentage of 9 '98, or practically 10, of silica. The advantage 
of this formula is that if the combined silica in the analysis be taken and 
multipUed by 10 the product gives very closely the percentage of braunite 
in the ore. And in the Central Provinces, where the ores are mixtures of 
braimite and psilomelane, it is foimd that by taking this formula the ore 
works out almost exactly in most cases to a mixture of braunite and 
psilomelane in a proportion that agrees with the appearance of the speci- 
men. Consequently if the complete analysis of a piece of Central Prov- 
inces ore be taken and the combined siHca multipUed by 10, this will give 
the percentage of braunite ; and the remainder, after taking out the P2O5 as 
apatite and any free siUca as quartz, will be psilomelane ; or as the amounts 
of apatite and quartz are usually insignificant, the remainder after subtract- 
ing the braunite can be reckoned to be psilomelane. This method can even 
be apphed to the calculation of the approximate composition of a sample of 
which only a partial analysis is available. Thus, the partial analysis (No. 3) 
of Ramdongri ore given on page 85H shows 5*19 % of combined silica. 
This corresponds to 51-9 per cent, or roughly 52 per cent, of braunite, 
