58 



LITHOLOGICAL GEOLOGY. 



59. (7.) Scapolite. — Crystals square and eight-sided prisms, and often large 

 and cleaving imperfectly parallel to the sides of the square prism. (See fig. 38.) 

 Color, white, gray, or greenish gray, and looking much like feldspar, though 



Fig. 39. 

 .Fig. 38. 



Fig. 40. 



distinct in crystallization and cleavage. Hardness but little below that of 

 feldspar. G. = 2.6-2.75. Composition : Silica, 49.3, alumina, 27.9, lime, 22.8, 

 but usually containing some soda. 



60. (8.) Andalusite. — Occurs in whitish or grayish prismatic crystals, 

 nearly square (angle of 90° 44'), and often having the interior tessellated with 

 black (fig. 39), in which case it is usually called made, or chiastolite. Hard- 

 ness, if pure, greater than that of quartz. G. = 3.1-3.2. Before the blowpipe, 

 infusible. Composition: Silica, 37.0, alumina, 63.0. 



61. (9.) Staurotide. — In rhombic prisms of a large angle (129° 20'), often 

 having the acute edges removed so as to be six-sided. Often in crossed crystals 

 (fig. 40), whence the name, from the Greek 

 oravpog, a cross. Crystals usually thick 

 and coarse, sometimes fine lustrous. Color, 

 brown to black. A little harder than 

 quartz. G. = 3.5-3.75. Before the blow- 

 pipe, infusible. Composition : Silica, 29. 

 alumina, 53.5, peroxyd of iron, 17.2. 



62. (10.) Kyanite.— In flattened, blade- 

 like, rarely thick prisms, blades often 

 aggregated into masses. Color, sky-blue 

 to white, usually deeper blue along the 

 middle. Hardness of the extremities of the prisms as great as that of quartz. 

 G. = 3.5-3.7. Before the blowpipe, infusible. Same composition as anda- 

 lusite. Sillimanite is similar in composition, but 

 has a brownish to grayish color, a brilliant cleavage 

 in one direction, and it often runs into fibrous forms. 

 G. =3.2-3.3." 



63. (11.) Tourmaline. — In three, six, nine, or 

 twelve-sided prisms (figs. 41, 42), without vertical 

 cleavage, and when black a little pitch-like in the 

 cross-fracture. Color, commonly black ; also brown; 

 rarely green, and pink or carmine-red. As hard as 



quartz. G. = 2.9-3.3. The crystals are thick or coarse, and often penetrate 

 quartz (fig. 43) in long black prisms as large as a goose-quill ; also found in mica 



Fig. 





Fig. 42. 





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