370 DESCRIPTIVE MINERALOGY. 



DI-AXIAL MICA. 



Hemiprismatic Talc-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 Zmwald. — Silica 47.00, alumina 20.00, potash 14.50, 

 oxide of iron 15.50, oxide of manganese 1 .75 (Klaproth). 



Red or Lepidolite Mica. — Sihca 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. — Sihca 49.38, alumina 23.67, peroxide of iron 7.31, lime 

 6.13, potash 15.29, \\\h\^ Q .06 {Thomson). 



Geological Situation. This species forms an essential constituent of granite, gneiss 

 EUid 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. 



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 copjier-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 viflage 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. 



