Chap. XVII.] 
GONDITE SERIES : ALTERATION. 
357 
those of Jothvad, or whether the alteration has continued in part 
almost down to recent times. 
But on page 363 the absence of any except rare open spaces in the 
Probably formed in ore-bodies is taken as indicating that the larger part 
Archaean times. of this alteration took place in Archaean times 
towards the end of the Dharwar tectonic disturbances. 
The Way in which the Manganese-silicate-rocks alter to Manganese- 
ores. 
We can now consider the way in which the alteration of manganese 
silicate- rock to manganese-ore took place. From the consideration of 
the rock undergoing alteration and the products of the alteration, it 
is evident that, in the case of spessartite, the result of the alteration is 
a removal of : — • 
(1; most of the alumina ; 
(2) a very large proportion of the silica, sufficient of this being left 
behind for the formation of braunite ; 
(3) a certain proportion of the small quantity of lime and magnesia 
present. 
As the manganese was to a large extent present in the garnet in 
The formation of ^i^q iovm of protoxide, there must have also been 
oraimite and psuo- . i • <• • i • i • • • 
melane. an mtroduction of oxygen m solution, this raising 
the state of oxidation of the manganese. We see from this that the 
reactions taking place must have been somewhat analogous to those 
hypothecated to explain the alteration of the kodurite rocks. The 
formation of braunite might be expressed thus : — 
II (3MnO.Al203.3Si02) i- 170 = 3(3Mn203.MnSi03) +4Mn2Mn05 + IIAI2O3 + 30SiO2. 
Spessartite. Oxj'gen. Braunite. Psilomelaue. 
So that the equation should not be too complicated I have put the 
formulae of spessartite, braunite, and psilomelane, in their simplest 
forms. The small amoimts of lime and magnesia that enter into the 
RO group of the spessartite, partly found their way into the psilomelane, 
replacing a portion of the manganous manganese, and perhaps to a very 
small extent into the braunite. Any portion of these constituents 
left over must have been removed in solution. Any ferrous iron in the 
RO group of th(3 spessartite was no doubt oxidized to the ferric condition. 
Any manganic manganese in the R2O3 group of the spessartite went to 
form a portion of the braunite or psilomelane, whilst any ferric iron in 
this group probably went mostly into the braunite and to a smaller 
