Chap. XIII.] 
KODURITE SERIES : ALTERATION. 
265 
that of the lime of the garnet, namely to be carried away as 
bicarbonate.^ 
The manganese-garnet or spandite seems to be the most stable mineral 
Replacement of the kodurite and many examples can be foimd of 
spandite. imaltered garnets in a matrix of partly or completely 
kaolinized felspar. When kodurite sufTers replacement the garnet is 
usually the last mineral to be replaced, whilst it often escapes altogether, 
as in the rocks from Kotakarra and Boirani (see fig. 2, Plate 8), 
where the felspar has been replaced by opal. When the replacement 
is effected by manganiferous solutions the felspar and apatite are often 
removed leaving perfectly fresh garnets in a matrix of psilomelane. 
Such garnet-studded psilomelane is especially common at Garbham and 
Ramabhadrapuram (fig. 4, Plate 8). From this photomicrograph 
it will be seen, however, that the garnet is also beginning to be replaced 
by the psilomelane, veinlets of which are seen traversing the garnet. 
But the garnet itself also often suffers alteration, and although the man- 
ganese-pyroxenes have in some places provided a certain proportion of 
the manganese required for the formation of the manganese-ores, yet the 
major portion of the manganese has been derived from the garnets. 
When kodurite is altered, the resultant rock is often crumbly lithomarge 
Alteration of span- containing abundant scattered garnets and apatites, 
dite. The alteration has, however, more often than not 
proceeded much further. The apatite has been completely removed 
whilst the garnet is still represented by little ochreous spots suggesting 
that the iron oxide is the last constituent of the garnet to be removed. 
The garnet from Garbham analysed by Mr. J. C. Brown contains the 
following constituents (for complete analysis, see page 167) : — 
Per cent. 
Si02 35-24 
Fe20.3 2.3 -no 
AhOi <3 '48 
MiiO 16-37 
BaO 0-18 
CaO 15-20 
MgO 2-04 
99 -41 
The composition deduced for the garnet in the kodurite of Kotakarra 
is given on page 258. 
iBoth apatite and Ca3(P04)2 are known to be distinctly sohible in water containing 
CO2 in sohition. See A.M. Comey's 'Dictionary of Chemical Solubilities Inorganic,' 
page 298, (1896). 
II 0 
