178 
MA-NOANESE DEPOSTTS O? INDIA: MiNERALOOy. [ PaRT I 
One of the commonest of these is a colom-less highly refracting mineral, 
which is probably apatite. Black inclusions are also sometimes present. 
These may represent a residue of manganese oxide over that required 
for the formation of the garnet, similar to the case of the hematite 
inclusions mentioned above. 
Spessartite, belonging to the isometric or cubic system like all 
Anomalous double the garnets, should be isotropic, i.e., remain 
refra tion. didi,v^ as viewed between crossed nicols during a 
complete revolution under the microscope. Many garnets, however, 
exhibit anomalous double refraction, and the Indian spessartites are no 
exception in this respect. This phenomenon, which is considered to be 
of secondary origin, is most often seen in the spessartite of KajUdongri, 
and causes the part of the crystal exhibiting it to polarize in a very low 
grey, in which there may be a yellowish tinge owing to the superposi- 
tion on it of the yellow colour of the garnet. The whole of the 
spessartite is sometimes affected by this phenomenon, the garnet then 
being split up into small areas of different optic orientation as if it 
were an aggregate of different individuals. In places it shows signs of 
interference crosses, which in some cases are very distinct. In one 
case where the garnet was zoned, colourless inside and yellow outside, 
the colourless portions showed anomalous double refraction to a 
much less marked degree than the yellow portions. 
Of the characters of this mineral observable under the microscope 
in thin sections, the colour and anomalous double 
Microscopic cisoGct'* • 
refraction have already been discussed. The other 
noticeable features are : — the high index of refraction, causing the min- 
eral to stand up in the sections and to exhibit very pitted surfaces ; 
and the great tendency to idiomorphism that it displays. When 
the whole rock is composed of spessartite, the separate garnets 
mutually interfere with each other ; but when there is another 
mineral, such as quartz, to act as a cement to the garnet, the latter shows 
its crystal outlines, which are usually polygonal. The garnets are then 
about equally developed in all directions and are doubtless almost 
always the cross sections of trapezohedra. When rhodonite is present 
the garnet is almost invariably idiom orphic with regard to the rhodonite 
in which it is often enclosed. Occasionally, however, the relation 
of these two minerals is reversed. In one case, namely in a rock from 
Kajlidongri, the garnet seems to have been formed later than the quartz ; 
for it includes the latter, so that the garnet forms the meshes of a net 
