MICA. 443 



light It is found of nearly all colours. The garnet and staurolite crystals 

 are shown (figs. 4/0, 471). 



The former is silicate of alumina with the silicate of some other oxide, 

 which is not always the same. This change, of course, gives us a series, as in 

 the case of alum above mentioned. 



The red varieties, called almandine, or precious 

 garnets, are distinguished from the duller, " common " 

 species by their clear colour. Bohemia is the most 

 productive soil for the garnets. 



MICA includes, as we have already noticed, a group 

 of minerals which have a peculiarly laminated structure. 

 Fig. 47.-Gamet crystal, jhcgg layers are by no means all alike, but they present 

 a smoothness to the fingers which is highly characteristic. The chief con- 

 stituents are alumina and silica, occasionally with magnesia. Mica slate is 

 very common, and is often used instead of glass in window-frames. Musco- 

 vite, Icpidolite, and phlogopite are all micas of the " potash," " lithia," and 

 " magnesia " varieties. 



In the list of minerals associated with the lighter metals, we need only 

 now mention the Gems, so well known. These stones are very hard in 

 many instances, infusible, and exhibiting beautiful 

 colours. Amongst them are diamonds, sapphires, and 

 rubies, of which we have spoken; the topaz, noticed 

 under corundum. The chrysoberyl (of a pale green, or 

 occasionally reddish hue), of which the alexandrite of 

 Siberia is a variety, is a compound of glucina with 

 alumina; the beryl, a silicate of the same, and the 

 emerald of beautiful green. Zircon is another gem, and 

 "hyacinth" is its most valued form. The latter is F '*- 47i.-Stauroihe crystal, 

 found in basaltic rocks. The emerald crystallizes in the hexagonal 

 system. 



\Vc may now consider the minerals formed by the heavier metals, such 

 as Iron, Copper, Nickel, etc. 



IRON. This well-known metal fills a very important place in our 

 mineral arrangements, for the substances formed with iron ores occur in 

 great variety of structure, and occasionally in very large masses. They are 

 highly magnetic, and very hard. Were we here treating of iron as a metal, 

 we could give some information respecting its extraction and manifold uses. 

 All we need mention here is the fact that iron occurs in nature in various 

 ores which are essentially composed of iron and oxygen. The iron is 

 extracted in the blast furnace, in which the process is continued for years. 

 The " slag," or glassy scum, protects the molten iron from the air ; its 



