474 



MINERALOGY 



A1 2 O 3 from 25 to 40, and SiO 2 , 35 to 40 per cent. ; Hexagonal ; Type, 

 Ditrigonal Polarj c = .4477; 0001 A f011 =^27 20'; roil A 10ll 

 = 46 52'; 0112 A 0112 = 25 2'; 0221 A 0221 = 7_7; Common 

 forms, c (0001), m (1010), a (1120), r (1011), o (0221), e (0112) ; 

 Other forms numerous; Twinning plane 0001, supplementary 

 twins, other twins rare; Cleavage, a and r difficult; Brittle; 

 Fracture, uneven ; H. = 7-7.6 ; G. = 2.98-3.20 ; Color, commonly 

 dark brown to black, but all shades of green, blue, red, to white ; 

 Streak, white or gray; Transparent to opaque; = 1.640; 

 = 1.622; o) - = .018; Optically ( - ). 



B.B. Generally fuses to a slag or glass, with a change in color ; 

 but some varieties fuse with difficulty, or are infusible. With 

 Turner's flux yields a boric acid flame (green). Not attacked by 

 acids. 



General description. Crystals are either long and slender, 

 parallel to the vertical axis, or short and stout, with striations on the 

 prism zone lengthwise ; combinations of the hexagonal prism of the 

 second order and the trigonal prism of the first order. Oscilla- 



FIG. 498. Tourmaline from Mesa Grande, California. The Smaller Specimen ia 

 from Acworth, New Hampshire. 



tions in growth between these two forms produce the striations 

 so characteristic. They may be deep furrows or ridged, in which 

 the cross section becomes only approximately trigonal ; such crys- 

 tals are well represented by those from Mesa Grande, California. 

 In the black variety, schorl, the polar character of the mineral is 



