BISILICATES. 



249 



differs from that mineral in lustre, specific gravity, and 

 greater fusibility. 



Ohs. From Uto, Sweden ; also from Elba {Castor or Cas- 

 tor it e). 



Amphibole. — Hornblende. 



Monoclinic. I A 7=124° 30'. Cleavage perfect parallel 

 with I. Often in long, slender, flat rhombic 

 prisms (fig. 2), breaking easily transversely; 

 also often in 6-sided prisms, with oblique 

 extremities. Frequently columnar, with a 

 bladed structure ; long fibrous, the fibres 

 coarse or fine and often like flax, with a 

 pearly or silky lustre ; also lamellar ; also 

 granular, either coarse or fine. 



Colors from white to black, passing 

 through bluish-green, grayish-green, green, 

 and brownish-green shades, to black. Lus- 

 tre vitreous, with the cleavage face inclining to pearly. 

 Xearly transparent to opaque. H. = 5-6. G. = 2*9-3'4. 



Composition. R 3 Si, as for pyroxene. R may corre- 

 spond to two or more of the basic elements Mg, Ca, Fe, Mn, 

 Xa 2 , Kj, the first three being most common. Aluminum 

 is very often present in amphibole, replacing a portion 

 of the silicon. The blowpipe characters are like those of 

 pyroxene. It fuses, but the fusibility varies indefinitely, 

 being easiest in the black varieties. 



Biff. It is distinguished by the very ready cleavages pa- 

 rallel to a prism of 124|- , while pyroxene cleaves at nearly 

 a right angle (87° 5'). 



This species, like pyroxene, has numerous varieties, dif- 

 fering much in external appearance, and arising from the 

 same causes — isomorphism and crystallization. 



The following are the most important varieties : 



I. LIGHT-COLORED VARIETIES. 



Tremolite, Grammatite. Tremolite comprises the white 

 and grayish crystallizations which usually occur in blades 

 or long crystals penetrating the gangue or aggregated into 

 coarse columnar forms. Sometimes nearly transparent. 

 G. =2*9. Formula (Ca, Mg) 3 Si = Silica 57*70, magnesia 

 28'85, lime 13 *45 = 100. The name is from the foreign lo- 

 cality, Tremola, in Switzerland. 



Actinolite. The light-green varieties. It is a magnesium- 



