360 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. [ParT II 
with the concomitant decomposition of the garnet and leaching out 
of the constituents not necessary for the formation of the manganese- 
ores. Two of the commonest reagents concerned in the formation of 
mineral deposits are carbon dioxide and sulphuric acid. Considering 
first the carbon dioxide, we can suppose that in some parts of the mass 
of gondite or other rock it attacked the rock with the passage of the 
manganese into solution as bicarbonate, in a way similar to that 
supposed in the case of the decomposition of spandite (See page 266). 
This would mean the residual accumulation of the alumina as kaolin 
mixed with the quartz of the gondite. Kaolinic material is occasionally 
found in association with the deposits of the gondite series, but only 
rarely ; so that in this respect the theory is not supported by evidence. 
As in the case of the alteration of the kodurite masses, the solutions 
must have eventually become saturated with manganese bicarbonate. 
After this, on coming in contact with the other masses of gondite, the 
tendency would be for the solutions no longer to dissolve manganese 
but to deposit it instead, a replacement of the gondite being effected 
at the same time, with the passage into solution of its silica and 
alumina. The same difficulty arises as in the case of the kodurite 
series as to the removal of the alumina, which, according to the 
experience of the laboratory, would be precipitated in the presence of 
carbon dioxide or carbonates. As in the case of the kodurite series, 
we can suppose that under the conditions of the reaction in Nature 
alumina is slightly soluble in water containing carbon dioxide. 
Now let us consider the possibility of sulphuric acid as the reagent 
Sulph'iric acid as ^^^^ brought about the various changes that 
the reagent producing have affected the gondite series. In the first 
alteration. place it would be able to remove the alumina in 
solution in the form of sulphate. Moreover, according to the 
experiments of Karsten [Comey's ' Dictionary of Solubilities', page 360, 
(1896)], silica in the form of silicic acid is soluble in sulphuric acid. 
Further, it is well known that all the oxides of manganese are soluble 
in concentrated sulphuric acid with the formation of manganous sulphate, 
often with liberation of oxygen. But in Nature the acid, if present, would 
probably be dilute, even very dilute. Now the action of dilute sidphuric 
acid on the various oxides differs. MnOo is not attacked at all, whilst 
Mn203 and Mn304 give up a portion of their manganese, so that in each 
case Mn02 is left. Hence, during the decomposition of the garnet, if it 
took place in the presence of solutions of sulphuric acid, any manganous 
oxide in the RO group would go into solution as MnS04, whilst any 
