EARTHY MINERALS. 61 



120. Silicates of Magnesia. These are of two classes, 

 the hydrous and the anhydrous. In them all, which are 

 numerous, there is not merely a silicate of magnesia, but 

 there are also other oxyds, some in one and some in an- 

 other, as oxyd of iron, alumina (oxyd of aluminum), lime 

 (oxyd of calcium), the oxyd of manganese, etc. These 

 additional oxyds are generally small in amount, but in 

 some cases they make quite a considerable portion of the 

 mineral. I will notice only the most important of these 

 silicates. 



121. Talc. This is one of the softest of minerals, being 

 easily cut with a knife. It is commonly of a light green 

 color, and has an unctuous feel. One of its varieties, 

 scaly talc, is the French chalk so familiar to us. Anoth- 

 er, the soapstone (steatite), is found in extensive beds, 

 and is used for many purposes fireplaces, linings for 

 stoves, etc. It is, you know, rather soft, but heat hard- 

 ens it. Powdered soapstone is used for lessening the 

 friction of machinery. 



122. Serpentine. This, when so pure as to be called 

 precious serpentine, is almost a pure hydrous silicate of 

 magnesia, the only additional oxyd in it being that of 

 iron in the small quantity of 0.2 per cent. that is, -^Ju-th 

 of the whole. When polished it is a very rich-looking 

 stone, the color being green. Other varieties contain 

 more of the oxyd of iron. There are some rocks com- 

 posed wholly of serpentine, and others that have ser- 

 pentine mingled with other minerals. The verd-antique 

 marble is granular limestone with serpentine scattered 

 through it. Serpentine was so named from its resem- 

 blance to the skin of a serpent, being streaked or spotted. 



123. Chlorite. This is a dark green hydrous silicate 

 of magnesia, alumina, and iron, there being in it 17 per 

 cent, of alumina and 34 of magnesia. In some parts of 

 the earth there are extensive deposits of this mineral, 

 and also of a slaty rock, which is called chlorite slate, 

 because chlorite is its chief constituent. 



