ALUMINA. 369 



Composition. Silica 59.79, alumina 29.46, magnesia 4.00, peroxide of iron 1.80, water 

 5.62 {Hermann). 



This mineral was first found in the Ural mountains, and received the name of fibrous talc. 

 It was analyzed by Hermann of Moscow. It differs from talc in its behaviour before the 

 blowpipe. 



LOCALITY. 



According to Mr. Mather, it occurs near West-Point, in a vein a foot in width, traversing 

 grey pyroxene. A fragment of it, an eighth of an inch in thickness, exfoliates by heat to 

 such a degree as to become an inch thick.* 



MICA. 

 [From the latin mico, to shine ; in allusion to its lustre.] 

 Under this name have heretofore been included two or three distinct species. 



MONO-AXIAL MICA. 



Rhombohcdral Talc-Mica. Jameson. — Rhomboedrischer Talc-Glimmer. Moks. 



Fig- 385. Description. Colour generally dark green and brown ; sometimes 



also black. Streak white or grey. It occurs in regular six-sided prisms, 



l<r i [_ ^\ which cleave with great facility in a direction perpendicular to the axis, 



VM~^~'T r^ J Fig. 385. It exhibits one axis of double refraction. Lustre pearly, 



^ ' inclining to metallic on the terminal faces of the prism. From transpa- 



rent to opaque. Sectile. Folia very flexible and elastic. Hardness from 2.0 to 2.5. Spe- 

 cific gravity from 2.80 to 3.00. Before the blowpipe, it sometimes fuses into a scoria, but 

 generally only becomes white and opaque. 



Composition. Silica 42.50, alumina 16.05, magnesia 25.97, potash 7.55, oxide of iron 

 4.93(7?ose). 



Black from Siberia. — Silica 42.50, alumina 1 1 . 50, magnesia 9.00, potash 10.00, oxide 

 of iron 22.00, oxide of manganese 2.00 {Klaproth), 



Geological Situation. It occurs in primitive districts, and in the ejected debris of Ve- 

 suvius. It is also found in the basalts of the Rhine ; but it is by no means so abundant as 

 the following. 



* Mather. American Journal of Science. XXI. 97. 



MiN. — Part II. 47 



