CllAP VII.] 
OTTRELITE. 
199 
On comparing the pleochroism of these Indian rose and crimson 
micas with that of the alurgite from St. Marcel in Piedmont, it wiU be 
seen that although the pleochroism schemes are somewhat different, 
the colours corresponding to each elasticity axis have a general resem- 
blance to one another. Thus the brownish tint in the red of the b axis 
of the typical alurgite corresponds to the yellowish, brownish, and 
orange, tints in the otherwise pink colour corresponding to the b axis 
of the Indian micas. It ^viIl also be noticed that the optic axial angles 
of the Indian micas bear a general resemblance to those of the St. 
Marcel mineral, especially in the case of the mica from Sitapathur. 
Hence these Indian micas can be provisionally regarded as alurgite, 
until they have been subjected to the test of analysis. 
The mineral from Jothvad of wliich the pleochi'oism is given is not one 
of the crimson micas, but is a rich brown. Its pleochroism, however, 
coiTesponds closely, except in the ,i axis, with, that of the orange laminae 
in the crimson mica of Kajlidongri. 
Ottrelite. 
This mineral belongs to the clintonite group of the mica division 
of the hydrous silicates. Its formula is doubtfully given as H2(Fe, 
Mn)Al2Sio09, and the analyses given by Dana show 0'93 to 8-96 % of 
MnO. For the purposes of microscopic work, however, chloritoid and 
its varieties are also grouped imder ottrelite in Rosenbusch-Idding's 
' Microscopical Physiography ', 4th edition, p. 289, (1905), and the 
formula H2(Fe,Mg,Mn)Al2Si07 given for the whole group of minerals. 
All ottrelites contain manganese and only some clintonites, but as it is 
often possible to distinguish the minerals only by chemical analysis, 
Rosenbusch-Idding's practice will be adopted here, with the understand- 
ing that ottrelite proper contains manganese. It is characteristically 
found in phyllites and schists that have been formed by the metamor- 
phism of argillaceous sediments, though it is not a common mineral. I 
am not aware if any of my colleagues have met with this mineral in 
their studies of Indian rocks. I have myself found that it is not uncom- 
mon in the phyllites and mica-schists forming parts of the Chilpi Ghat 
series in the Central Provinces, and in their more metamorphosed 
equivalents, the mica-schists, usually mapped as a part of the meta- 
morphic and crystalline complex. These Chilpis, as is explained 
elsewhere in this Memoir (page 282), are the equivalents of the Dharwai' 
of Southern India and the Aravallis of Rajputana : hence future 
