370 
DESCRIPTIVE MINERALOGY. 
DI-AXIAL MICA. 
or Common Mica. Jameson. — Hemiprismatischer Talc-Glimmer. Mohs. 
Description. Colour white, grey, green, yellow, red, brown 
and black. Streak white or grey. It occurs in oblique rhombic 
prisms, with transverse sections of about 120° and 60°, Fig. 
386. Cleavage parallel to the base. It exhibits two axes of 
double refraction. Lustre pearly. Transparent and translucent. Sectile. Flexible and 
elastic. Hardness from 2.0 to 2.5. Specific gravity 2.80 to 3.00. Before the blowpipe it 
loses its transparency, but does not fuse, except when it contains lithia, when it melts easily, 
and at the moment of fusion gives the flame a reddish tinge. 
Composition. Specimen from Zinwald. — Silica 47.00, alumina 20.00, potash 14.50, 
oxide of iron 15.50, oxide of manganese 1.75 {Klaproth). 
Red or Lepidolite Mica. — Silica 50.35, alumina 28.30, potash 9.04, oxide of manga¬ 
nese 1.23, fluoric acid and water 5.20, lithia 5.49 {Turner). 
From Orange county , N. Y. — Silica 49.38, alumina 23.67, peroxide of iron 7.31, lime 
6.13, potash 15.29, lithia 0.06 {Thomson). 
Geological Situation. This species forms an essential constituent of granite, gneiss 
and mica slate. It is also sometimes found in transition and secondary rocks. In some parts 
of Russia, it occurs in plates of a yard or more in diameter, and is known by the name of 
Muscovy glass. 
Hemiprismatic Talc-Mica, 
Fig. 386. 
LOCALITIES. 
The localities of mica in this State are of course very numerous, as the mineral occurs in 
all the primitive districts. I shall here notice only those which are the most interesting. 
Essex County. Mica, in dark coloured plates of moderate size, occurs at the Crag Har¬ 
bour mine, associated with hornblende and magnetic iron ore. It is also found, of the same 
dark colour, at many of the other iron mines in the vicinity of Port Henry. It is of common 
occurrence in several other towns of this county. 
Jefferson County. There is a locality near Henderson, where a yellow or somewhat 
copper-coloured variety of mica is met with, both in plates and in regular six-sided tables of 
large size. 
The dark copper-coloured variety is found on the farm of Mr. Wait, in the town of Antwerp, 
four and a half miles south of the village of Oxbow. 
On the banks of Vrooman lake, near the village of Oxbow, it occurs, in six-sided prisms or 
tables of a light copper colour. The crystals are from half an inch to two inches in diameter, 
and generally taper gradually towards one of the terminations. Sometimes they exhibit a 
cleavage parallel to the longer axis, and the hexagon separates into two pentagonal tables, 
