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MANGAITESE DEPOSITS OF INDIA : GKOLOGY. [ PaRT II : 
composition of which is expresed by the formula : — 3MnO.A]203. 
3Si02. As will be seen, however, from the analyses given on page 351, the 
manganese is usually partly replaced by calcium and magnesium, and 
the alumina by iron. The formation of the spessartite of the theore- 
tical composition may be expressed by the following equation : — 
2H20.Al203.2Si02 + 3MnO+Si02 = 3MnO.Al203 3Si02 + 2H2O. 
Kaolin or clay Quartz Spessartite 
By the introduction of oxides of calcium, magnesium, and iron, repre- 
senting calcareous, magnesian, and ferruginous impurities, respective- 
ly, in the original sediments, into both sides of the equation, the form- 
ation of spessartite containing these constituents could be shown. This 
shows that when the sediments admixed, interstratified, or in contact 
with, the manganese oxides were largely argillaceous, the chief mineral 
formed was spessartite. Any excess of quartz in these sediments over 
the kaolin crystallized out as quartz with formation of spessartite-quartz- 
rock, if all the manganese had already been used up in combining with 
the kaolin. Whilst, if any manganese oxide were still left over, it com. 
bined with this excess of quartz with the formation of the triclinic man- 
ganese-pyroxene, rhodonite, of the theoretical composition expressed 
by the formula MnSi03. No analysis has yet been made of Indian 
rhodonite, so that it is not possible to say if the manganese of this mineral 
is replaced by any other element to any considerable extent. Assuming 
the theoretical formula, the following equation will express the form- 
ation of this mineral : — 
Mn02 + Si02 = MnSiOs- 
Quartz Rhodonite 
If, after this equation had been carried out, there were any silica left 
unaccounted for, it would crystallize out as quartz, and the rock then 
formed would be spessartite-rhodonite-quartz-rock. If the quantity of 
manganese were such that there would be an excess of manganese oxide 
left over after supplying that required to combine with all the sand and 
clay in the formation of spessartite and rhodonite, then the last 
portions of the silica, instead of forming rhodonite by combining with 
one molecule of manganese oxide for every molecule of quartz, would 
probably combine with a much larger quantity of manganese oxide 
with the formation of braunite according to the following equation : — 
4MnO + 3Mn02 + Si02 = 3Mn203.MnSi03 
Braunite. 
