ALUMINA. 'MtU 



Composition. Silica 59. 79, alumina 29. 4G, magnesia 4.00, peroxide of iron 1.80, water 

 5.62 {Hermann). 



This mineral was first found in the l^ral mountains, and received the name oi fibrous talc. 

 It was analyzed by nermann of Moscow. It difi'ers from talc in its behaviour before the 

 blowpipe. 



LOCALITY. 



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

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

 such a de<jrec as to become an inch thick.* 



MICA. 



[From the latin mico, to s/i'utc ; in allusion to its lustre.] 

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



MONO-AXIAL MICA. 



Rhombohcdral Talc-Mica. Jaiiicsv/i. — lilioinbocdrischcr Tale-Gliinmor. Mohs. 



f"'?- ^^5. Description. Colour generally dark green and brown ; sometimes 



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



l<^J ^ , which cleave with great facility in a direction perpendicular to the axis, 



<C?i "I i^, ' Fig. 385. It exliiliils one axis of doulilo refraction. Lustre pearly, 



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



rent to opaque. Scctile. 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 12.50, alumina 10.05, magnesia 25.97, jiotash 7.55, oxide of iron 

 4.93 {Rose). 



Black from Siberia. — ^Silica 42.50, alumina 11.50, magnesia 9.00, potasli 10.00, oxide 

 of iron 22.00, oxide of manganese 2.00 {Kkqrroth). 



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

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

 the follovving. 



* Mather. American Journal of Science. XXI. 'J7. 

 MiN. — P.\RT II. 47 



