Chap. XVII.] 
GONDITE SERIES : COMPOSITION. 
351 
be, respectively, 2-65 and 3-42, it is easy to calculate tlie specific 
gravity of the remainder, which consists practically entirely of 
spessartite. The result thus obtained will therefore be very close to 
the true value for the specific gravity of spessartite — 'if anything jiist 
a trifle too low, because the other constituents mixed up with the garnet 
are all of lower specific gravity, except the rutile, which has practically 
the same value for this constant as spessartite. The value thus obtained 
is 4-24, which may be taken as a very good value for spessartite. 
Neglecting all the constituents of the rock except the quartz and spes- 
sartite, and calculating to 100, the composition by weight and by volume 
works out as follows : — 
Composition of gondite 
by weight. 
Spessartite 
Quartz 
57-94 
42-06 
Composition of gondite 
by volume. 
Spesstartite . . .46-30 
Quartz . . .53-70 
so that the piece of gondite chosen as being typical consists of about 
equal proportions of quartz and garnet by volume. 
From this analysis we can also calculate the composition of the 
spessartite. This works out to the figures shown in column I below, 
whUst column II gives the result of an analysis made by Mr. T. R. 
Blyth, of the Geological Survey of India, of a specimen of manganese- 
garnet from Chargaon in the Nagpur district : — 
Spessartite from 
Wagora. 
II 
Spessartite from 
Chargaon. 
MnO 
FeO 
CaO 
MgO 
AI2O3 
Fe203 
Mn203 
Si02 
Surplus 
MgO 
Oxygen assumed 
37-73 
100-00 
Nil 
100-00 
44-34 
MnO 
21-26 
AI2O3 
30-29-\ 
.. (=40-82 
4-97 C MnO 
2- 40 ) 
8- 38 [ =19-56 
9- 50) AI2O3 
34-71 
3- 00 
101-30 
0-96 
100-34 
It will be seen that the analysis of the Chargaon garnet does not 
agree very closely with the theoretical composition of garnets, namely 
that expressed by the general formula 3R"O.R"'203.3Si02, there being 
