HORNBI.KNDE. 



153 



less modified; M : M = 124° 30. Cleavage perfec 

 allel with the sides of the prism. Of- ] 



ten in long slender flat rhombic prisms, 

 (fig. 3) breaking easily transversely ; also 

 4, 6, and 8 sided prisms with oblique ex- 

 tremities, e : e= 148° 30'. Occurs also 

 frequently columnar, with a bladed struc- 

 ture ; often fibrous, the fibers coarse or 

 fine and frequently like flax, with a pearly 

 or silky luster ; also lamellar ; also granu- 

 Jar, either coarse or fine ; generally firmly 

 compact ; rarely friable. 



Colors from white to black passing through bluish green, 

 grayish green, green, and brownish green shades, to black. 

 Luster vitreous, with the cleavage face inclining to pearly. 

 Nearly transparent to opaque. H = 5 — 6. Gr = 2*9 — 3.4. 

 Varieties and Composition. This species, like pyroxene, 

 has numerous varieties, differing much in external appeal 

 ance, and arising from the same causes — isomorphism and 

 crystallization. Alumina enters into the constitution of some 

 and replaces part of the other ingredients. The following 

 are the most important : 



1. Light Colored Varieties. 



Tremolite, Grammatite. Tremolite comprises the white, 

 grayish, and light greenish slender crystallizations, usually 

 in blades or long crystals, penetrating the gangue or aggre- 

 gated into coarse columnar forms. Sometimes nearly trans- 

 parent. Gr=2*93. The name is from the foreign locality, 

 Tremola in Switzerland. 



Actinolite. The light green varieties. Glassy actinolite 

 includes the bright glassy crystals, of a rich green color, usu- 

 ally long and slender (fig. 3) and penetrating the gangue 

 like tremolite. Radiated actinolite includes olive green 

 masses, consisting of aggregations of coarse acicular fibers, 

 ladiating or divergent. Asbestiform actinolite resembles the 

 radiated, but the fibers are more delicate. Massive actino- 

 lite consists of angular grains instead of fibers. Gr = 3*02 

 — 3*03. The name actinolite alludes to the radiated struc- 



What is the crystallization of hornblende ? What are common forms? 

 What is said of the columnar and fibrous varieties? What are its col- 

 ors? On what do the characters of its varieties depend? What is tre- 

 molite? what actinolite? Mention the characters of the varieties of 

 actinolite ? 



