26 



MICACEOUS MINERALS. 



NAME. 



COMPOSITION. 



HOCKS IN WHICH FOUND. 



Chlorite 



Muscovite or 

 Potash-mica 



Lepidolite or 

 Lithia-iriica 



Phlogopite 



Biotite or 

 Magnesian- 



Dark olive green. Hydrous 

 silicate of magnesia and 

 alumina, with 12 per cent, 

 of water. Dissolves in hot 

 sulphuric acid 



White or brown or black. 

 Silicate of alumina with 10 

 per cent, of potash, and 

 rarely more than 5 per 

 cent, of iron oxides, with a 

 little water 



Red or violet. Composition 

 similar to muscovite, but 

 with lithia, hydrofluoric 

 acid, and protoxide of man- 



Brown, or reddish brown. 

 Silicate of alumina, mag- 

 nesia, potash, sometimes 

 with fluorine and a little 



Usually dark green, brown, 

 or black. Silicate of mag- 

 nesia, alumina, potash, and 

 oxide of iron, with some 

 water, dissolves in hot sul- 

 phuric acid 



In chlorite slate, protogine, proto- 

 gine gneiss, diabase, correspond- 

 ing to mica as a rock constituent, 

 closely related to soapstone, found 

 in serpentine at the Lizard. 



Granite, gneiss, mica-schist, some 

 lavas of Vesuvius ; has been found 

 in slags, and has been formed in 

 clayey sandstone walls of iron 

 furnaces. 



In granite in Cornwall, in gneiss, 

 and greisen. 



Chiefly in metamorphic limestone, 

 as in the Vosges j also in ser 

 pentine. 



In granite gneiss, trachyte, basalt, 

 miascite, in lavas of Vesuvius. 



Many other micas are met with. 



The Family of Garnets. 



The garnets are a group of silicates of variable composition. The 

 typical garnets have on this account been grouped under six varieties, 

 as lime garnet, which is a double silicate of lime and alumina ; mag- 

 nesia garnet, which is a silicate of magnesia, iron, and alumina ; iron 

 garnet (or common garnet), which is a silicate of protoxide of iron and 

 alumina ; manganese garnet, which is a silicate of manganese, alumina, 

 and iron ; iron-lime garnet, which is a silicate of iron and lime with a 

 little alumina and manganese ; and lastly lime-chrome garnet, which is 

 emerald green, and is a silicate of lime and chrome with a little alumina 

 and iron. These, however, are mostly of rare occurrence in igneous 

 rocks, and it will be convenient to assume that the common almandine 

 garnet is the kind usually met with. The Vesuvian mineral idocrase 

 is closely allied to the garnets, in being a silicate of lime, alumina, iron, 

 usually with a little magnesia. Here also may be placed, though ii 

 no near association, tourmaline, sphene, and zircon. Garnets oftt 

 occur embedded in talc and mica, and they may be regarded as m< 

 closely allied to that group of minerals. 



