MINERALS WHICH FORM IGNEOUS ROCKS. 



27 



NAME. 



Garnet 



Idocrase 



Olivine 



Tourmaline 



Sphene 



Zircon 



COMPOSITION. 



Usually in rhombic dodeca- 

 hedrons, red or brownish. 

 A silicate of iron and alu- 

 mina, but in some rocks 

 containing magnesia 



Brown or green. Silicate of 

 lime and alumina, with a 

 little iron, manganese, and 

 magnesia 



Green or brown ; crystals in 

 prismatic system ; harder 

 than felspar, equals quartz ; 

 dissolves in sulphuric acid. 

 Consists of silica, magnesia, 

 and protoxide of iron. The 

 silica always less than the 

 magnesia. Only a trace of 

 alumina ; no lime. When 

 transparent called chryso- 

 lite. 



Usually black. A silicate of 

 alumina and magnesia, with 

 much boracic acid and oxide 

 of iron, and a little soda, 

 lime, and fluorine. This 

 mineral is also called schorl 



Green, brown, or black. Sili- 

 cate of lime and titanium 



Red or brownish. Silicate of 

 zirconia, with usually a 

 little iron, and occasionally 

 a little lime 



EOCKS IN WHICH FOUND. 



In eklogite, which is a compound 

 of green diallage and garnet ; in 

 garnet rock, a compound of gar- 

 net and hornblende ; occurs in 

 mica-schist ; also in granite, gra- 

 nulite, trachyte, perlite, and 

 chlorite-schist. 



In old lavas of Vesuvius ; serpen- 

 tine in Piedmont ; has been found 

 in slags of furnaces. Is not an 

 important rock constituent. 



In many basalts, as in the Eifel, 

 in lavas of Monte Somma, in 

 hypersthenite at Elfdalen in 

 Sweden, talc-schist at Katheri- 

 nenburg, in Lherzolite. Found 

 in slags of iron furnaces. The 

 rock in New Zealand called 

 dunite consists of olivine. 



In luxullianite, granite, mica- 

 schist ; not found in volcanic 

 rocks. 



In granite, syenite, zirconsyenite, 

 phonolite, trachytic rocks of 

 Laach, and in mica-schist. 



In several Scotch granites, in zir- 

 consyenite, in basaltic lavas at 

 Unkel on the Rhine. 



TJie Family of Zeolites. 



The zeolites are essentially felspars, which have been dissolved by 

 water slowly percolating through rocks in which those minerals occur, 

 and then have been redeposited in chemical combination with water, 

 in cavities, in volcanic and crystalline rocks. They are all silicates of 

 alumina, and several zeolites in addition contain lime, often with a 

 little soda or potash ; two others contain soda only, and one has a 

 large percentage of sulphate of baryta. These minerals are most abun- 

 dant in the vesicular basaltic lavas, but are also found in gneiss, syenite, 

 and granite, phonolite, and lavas of Vesuvius, &c. Occasionally they 

 form a large percentage of the rock, and furnish an instructive illus- 



