612 MINERALOGY 



b. Double refraction greater than that of quartz. 



Scapolite; Tetragonal; n = 1.55-1.58; co = .013- 

 .035. 



Fibrous aggregates, rounded grains or rods; Cleavage 

 prismatic ; Colorless ; Not pleochroic ; Interference 

 colors, bright, higher first and lower second orders; 

 Optically negative ( ) ;' p. 453. 



Calcite; Hexagonal; n = 3.57; <o = .172. 



Irregular masses and grains, often showing twinning 

 lamella; Cleavage well developed, rhombohedral, 

 74 55'; Relief varies greatly with the direction of 

 the section ; Colorless or pale ; Not pleochroic ; Inter- 

 ference colors pale of high orders ; Optically ( ) ; 

 p. 379. 



Tourmaline; Hexagonal; n = 1.65; o> = .017- 

 .034. 



Lath-shaped, hexagonal or trigonal outlines, fibrous 

 aggregates or rounded grains ; Cleavage none ; Color, 

 greenish to dark brown, or pale; Pleochroism very 

 marked, increasing with the depth of color of the 

 section; Relief marked; Interference colors, upper 

 first and lower second orders; Optically negative 

 (-); p. 473. 



Zircon; Tetragonal; n = 1.96; - o> = .044-.062. 



In short square prisms or rounded grains ; Cleavage not 

 well developed ; Nearly always colorless ; Relief very 

 marked; Not pleochroic; Interference colors bril- 

 liant high order; Optically positive (+); p. 456. 



Rutile; Tetragonal; n = 1.76; - o> = .287. 



Short square or long acicular prisms or rounded grains ; 

 Cleavage not marked; Colorless, brown, or red; 

 Relief very strong; Not pleochroic; Interference 

 colors brilliant high order, but not recognizable in 

 deeply colored sections; Optically positive (+) ; 

 p. 349. 



Biotite may at times appear uniaxial, see p. 615. 

 B. Biaxial. 



1. Index of refraction less than that of Canada balsam. 



a. Double refraction that of quartz or less. Not pleochroic. 



Zeolites; n = 1.48 - 1.53; Double refraction .003-.012. 



In acicular or fibrous aggregates filling cavities or veins. 



