Chap. VI.] 
MANGANESE-GARNETS. 
163 
several names, for varieties of garnets, that have no very explicit 
meaning and can usually only be applied to the garnet of the partis 
cular locality for which the term was invented. If any one need a 
name for a garnet that he has analysed and finds to depart consider- 
ably from any of the type formulae, he cannot as a rule use any of 
the existing varietal names, but is under the necessity of inventing 
anotuer and increasing still further the number of these indefinite 
varietal names. Ther is a very simple way out of this difficulty, 
which would, if adopted, do away with the necessity for all th^.se 
names. This consists in foirning compound names expressing the 
position of a given garnet between two of the type apecies. Thus 
there is a garnet to which the name folyadel-phite has been given. 
According to analysis 19 on page i43 of Dana's Mineralogy this 
garnet can be fairly regarded as a variety of andradite in whicn 
a certain proportion of the CaO is replaced by MnO, Analysis 
20 of another specimen of polyadelphite from the same locality, 
namely Franklin Furnace, N.J., U.S.A., contains, however, a much 
larger proportion of MnO replacing the CaO, whilst there is also 
a considerable proportion of Al^-Og replacing the Fe203. Although 
the garnet is still closer in composition to andradite than to 
spessartite, yet it differs considerably from the typical andradite 
and is in fact not so very far removed from being half-way in com- 
position between these two garnets. Instead of having a special 
name, polyadelphite, for this garnet it would be better to call it 
spessart-andradite, expressing the fact that it is really a compound of 
^ ^ ^ these two garnets. This could for brevity's 
' spandite '. Sake be contracted to spandite. If reference 
be made to the analyses i given in columns 3 
and 4, on page 168 , it will be seen that there is a considerable difference 
between the composition of the two garnets from the Vizagapatam 
District that these analyses represent. 
The Garbham garnet has the RO group composed about equally of CaO 
and MnO and in this respect is about half-way between spessartite and 
andradite. In the II2O3 group, however, the Fe203 is much larger in 
amount than the AI2O3, so that in this respect the garnet is fairly close 
to andradite in composition. It might then be thought that this garnet 
could be called a manganese-andradite. The Kotakarra garnet also 
1 One of these is an actual analysis of the mineral, and the other cal'j'ilated from 
the analysis of the rock in which it occurs. 
I " M 2 
