SILICATES, TITANATES, ETC. 431 



AM I'll I BOLES AND BERYL 

 AMIMHBOLES 



The aniphiboles are metasilieates of the formula, R"SiO 3 or 

 I ; I ; Bi< >,) in \\ l.ich il may be Na, K, R" = Ca, Mg, Fe, Mn, and 

 R'" = Al or Fe, or they may be mixed silicates in which there is 

 one molecule of an ortho- and one of a trisilicate ; this would yield 

 tin 1 same proportions of silicon and oxygen. Such radicals as 

 (A1,.OF 2 )", (Fe,.OF 2 ),", (A1 2 O(OH) 2 )", and (Fe 2 0(OH) 2 )" also 

 enter the molecule of the more complex varieties. They are dis- 

 tributed through the orthorhombic, monoclinic, and triclinic 

 systems as are the pyroxenes, but the orthorhombic varieties are 

 very unimportant and not rock-forming minerals. They differ 

 from the pyroxenes, with which the simpler forms are dimorphous, 

 in that magnesia or its equivalent enters the molecule in greater 

 proportions, in amphibole, in amounts of three or four to one of 

 calrium, while in pyroxene they are in the proportion of one to one. 

 They all cleave at an angle approximately 124, while in pyroxenes 

 it is 87. In crystalline habit they are elongated or fibrous, with 

 the exception of the basaltic hornblende, which occurs usually in 

 equidimensional crystals. 



The following table will show the crystalline and optical rela- 

 tions of the various amphiboles. 



TREMOLITE 



Tremolite. CaMg 3 (SiO 3 ) 4 ; CaO = 13.45, MgO =^28.83, 

 SiO 2 = 57.72 ; Monoclinic ; Type, Digonal Equatorial ; a : b : c = 

 .5415 : 1 : .2986 ; P = 74 48' = 100 A 001 ; 100 A 110 = 27 54' ; 

 m,m' = 55 49; 001 A 101 = 24 4'; 001 A 011 = 15 46'; 

 Common forms, c (001), a (100), b (010), m (110), r (Oil), t (101) ; 

 Twinning plane 100, contact twins, also lamellar parallel to the 

 base; Cleavage, prismatic (124 30'), a and b at times distinct; 

 Brittle ; Fracture, uneven ; H. = 5-6 ; G. = 2.9-3.4 ; Color, gray, 

 white, brown, or nearly black, according to composition; Streak, 

 white to gray; Luster, vitreous to pearly; Subtranslucent to 

 opaque. For the optical properties see table, page 432. 



B.B. Like the pyroxenes in every respect, from which they 

 must be distinguished by their physical and optical properties. 



General Description. Crystals are elongated or fibrous paral- 

 lel to the vertical axis. In basaltic hornblende the habit is short 



