610 MINERALOGY 



In irregular masses, grains, and filling veins, often forming the 

 cementing material in sandstones and shales; Colorless or 

 nearly so; In sections may be mistaken for leucite or so- 

 dalite; p. 369. 



Glass. Amorphous; n = 1.49; Variable. 



As the residuum of crystallization, irregular in outline, filling 

 the spaces between the crystals ; Colorless, gray, or smoky. 



Analcite. Isometric; n = 1.48. 



Rounded crystalline outlines, cloudy grains, or filling cavities 

 and veins; Cleavage cubic but not well developed; Color- 

 less ; Usually a secondary mineral after leucite, sodalite, or 

 nepheline ; p. 485. 



Sodalite group. Isometric; n = 1.48-1.50. 



In six- or eight-sided crystalline outlines or rounded grains; 

 Cleavage not well developed, often irregularly cracked and 

 fractured; forming the ground mass between crystals of 

 other species ; Usually colorless but often yellow or blue, 

 especially haiiynite and noselite ; Often filled with dustlike 

 inclusions, arranged in zones or collected at the center or 

 margin ; p. 436. 



Leucite. Isometric; n = 1.50. 



In six- or eight-sided crystalline outlines, or rounded grains ; 

 Cleavage not well developed, often irregularly fractured ; 

 inclusions when present are arranged symmetrically in 

 zones or radiating from the center ; Colorless ; Relief very 

 low; Large crystals are doubly refracting; Interference 

 color first order gray, showing polysynthetic twinning ; 

 p. 415. 

 B. Index of refraction above thai of Canada balsam. 



Spinels. Isometric; n = 1.71-1.76. 



In square or six-sided crystalline outlines or rounded grains ; 

 Cleavage not developed ; Colorless to opaque, according to the 

 species ; Relief very marked and surface very rough ; p. 371. 



Garnet. Isometric; n = 1.75-1.85. 



In crystalline outlines, rounded grains or irregular masses ; 

 Cleavage not developed ; Irregularly fractured and cracked ; 

 Relief very marked and surface rough ; Colorless, to reddish 

 of various shades, also green, brown, to nearly opaque ; p. 

 442. 



III. Transmits light anisotropically. 

 A. Uniaxial. 



