^30 ANALYSES OF IfALC AND MICA, 



TThe results of its analysis were 



Component Silex ....48 



P»^'«- Alumine 34-25 



Oxide of iron »•••• 4'5 



Magnesia, mixed with a little oxide 



of manganese* ••»••• 0*5 



i>otc.»i.. 8*75 



Loss by calcipation ............ li*25 



97-25. 



4. Black Siherian mica. 



Black MuscoTy Another variety, found in similar situations with the pre- 



* "^ ceding, is the black mica, or black Muscovy glass. This 



differs both from the preceding' and the common mica, not 



only in appearance, but in the proportions of its component 



parts. The following is Karsten's description of it. 



When in large masses it appears black, but in thin plates 

 it is a deep olive green. Before the laminae are separated, 

 they exhibit metallic reflections of green, blue, an(^ red, on 

 being held under different angles to the light. It may be 

 obtained in large plates, and these split into thinner, which, 

 by their tendency to form rhombs, indicate a secondary 

 juncture. The principal fracture is lamellar, with very 

 shining laminae of a greasy and semimetallic lustre. This 

 mineral is very tender, extremely smooth, and perfectly 

 transparent when the laminse are very thin, though entire 

 pieces are opake. The laminae have a perfectly elastic flex- 

 ibility. 



This substance is employed scarcely for any thing but 

 lining little boxes either of wood or pasteboard. 

 Action of heat. Before the blowpipe on charcoal it does not appear to 

 undergo fusion except at the edges of the laminae. If larger 

 plates be heated redhot in a crucible, they acquire a tombac 

 brown colour with a metallic brilliancy. The leav'es split, 

 and appear friable. They lose one per cent. 



the 



