SILICATES, TITANATES, ETC. 



435 



tinolite are lound in schists and granular limestones and are asso- 



I dated with metamorphism only, while I lornblencje occurs both as a 

 primary mineral in granites, syenites, diorites, and gabbros, as well 

 as in the <;neis.-.es and schists. The dark basaltic hornblendes 

 an igneous only, and form phenocrysts in ferruginous lavas, 

 l>a>alts, and gabbros, us at Aussig in Bohemia, or Monte Somma, 

 Yesuviu-. The soda-iron varieties, barkevikite, riebeckite, arfvedr 



1- K;. 17*.). Hornbleiide-cliorite-porphyry, showing Section of a Hornblende Crystal. 



sonite and senigmatite, occur only in those igneous rocks rich in 

 alkali, as the nephelite-syenites of Greenland and Norway. Arf- 

 vedsonite is found near Magnet Cove, Arkansas. Crocidolite is a 

 fibrous soda-iron variety of varied colors, occurring near the 

 ( >range River, South Africa ; it occurs included in quartz, making a 

 very pleasing ornamental stone; when polished and mounted in 

 jewelry it is the semiprecious " tiger-eye." 



The decomposition products of the calcium and magnesium va- 

 rieties are talc, serpentine, and calcite, while those containing 

 iron in addition form hematite, limonite, or siderite, as well as 

 chlorite, epidote, and augite. Uralite is a fibrous pseudomorph 

 of hornblende after augite, having the crystalline form of augite 

 but the cleavage of hornblende; under suitable conditions this 

 process of uralitization may be reversed, and hornblende will pass 

 >ver to augite. 



