THE AMPHIBOLES 



19 



ing silica, 40 to Sl^o ; magnesia, 10 to 23% ; alumina, 6 to 14% ; 

 lime, 10 to 13% ; ferrous and ferric oxides, 12 to 20%?. Here 

 are included the dark green, brown, and black varieties. 



The aluminous variety, common hornblende, is an original 

 and essential constituent of diorite, and of many varieties of 

 granite, gneiss, syenite, schist, andesite, and trachyte, and is 

 also present as a secondary constituent in many rocks, result- 

 ing from the molecular alteration of the augite. The non- 

 aluminous varieties occur in gneiss, crystalline limestone, and 

 other metamorphic rocks. 



By the unaided eye, or by means of blowpipe tests alone it is 

 often impossible to distinguish the minerals of this group from 

 the pyroxenes. In the thin sections this distinction is, however, 

 a matter of comparative ease. Green fibrous hornblendes may 

 result from the molecular alteration of augite, and all varieties 

 are susceptible of alteration into chloritic and ferruginous prod- 

 ucts with the separation of calcite. 



On decomposing, the amphiboles give rise to ferruginous and 

 aluminous or magnesian products. In the darker colored varie- 

 ties, the decomposition begins with hydration and the peroxida- 

 tion of the iron along lines of cleavage and fracture, whereby 

 the crystal becomes riddled with corroded areas filled with the 

 liberated iron in the form of hydrated sesquioxide. 



"When the disintegration is complete, the whole mass is con- 

 verted into an ochre-brown, earthy substance and ultimately 

 passes into a ferruginous clay. These chemical changes are indi- 

 cated in the following analysis of I. fresh, and II. decomposed 

 hornblende from Haavi on Fillejeld, Norway :^ 



Analyses op Fresh and Decomposed Hornblende 





I 





n 



Silica 



46.87 





40.82 



Aliimina 



14.81 





17.49 



Iron protoxide .... 



8.74 



Iron sesquioxide. » ♦ 



1826 



Manganese 



1.50 





2.14 



Lime 



14 91 





6.37 



Magnesia 



14.83 





9.23 



Water . 



.... 





8.00 





99 66 



100 81 



^Biscliof' s Chemical Geology, Vol. II, p. 354. 



