TALC CHLORITE HORNBLENDE. 103 



very flexible. Usually occurs in thin scales ; sometimes 

 in large plates. Harder than gypsum, not so hard as calc- 

 spar. S.G. 2 -5 to 3. Mostly fusible before B.F. Not 

 readily acted on by hydrochloric acid. In its composition 

 are silicates of alumina, with potash, magnesia, lime, iron, 

 manganese, &c. 



Talc. 



A greenish, yellowish white, or sometimes colourless 

 mineral of a pearly or resinous lustre. Is greasy to the 

 touch ; soft ; yields to the finger-nail ; can be cut into 

 laminse which bend but are not elastic. H. 1 ; S.G. 2 ( 6 

 to 2 - 8. Before blowpipe is infusible, but whitens. Becomes 

 red with nitrate of cobalt solution. Is not soluble in either 

 hydrochloric or sulphuric acid. Composition per cent. : 

 silica, 62 ; magnesia, 27 ; alumina, water, iron, &c. 



Chlorite. 



A dark green, generally foliated and scaly mineral. Streak, 

 greenish grey. H. 1 to 1'5 ; S.G-. 2'7 to 2-96. Soluble 

 in hot sulphuric acid. Contains silicates of alumina and 

 magnesia and water. 



Hornblende. 



There are many varieties of this mineral, mostly of a 

 greenish black, and also whitish colour (those containing 

 lime and magnesia, without iron, being light). Streak, 

 white or slightly coloured. Lustre, vitreous. H. 4 to 6. 

 S.G. 2-9 to 4. Scarcely acted on by hydrochloric or nitric 

 acid. Unaltered when heated in a closed tube. More or 

 less fusible before the B.F. Composed of silicates of lime, 

 magnesia, also iron, alumina, &c. 



Augite. 



A dark green or blackish mineral, in composition like 

 hornblende, of a pearly or vitreous lustre. Is met with in 

 volcanic rocks. 



Olivine. 



A green or brownish transparent or translucent mineral 



