TlllO PRINCIPAL ROCK IMIIMINC MINERALS 611 



1. Index of refraction less than that of ( ';m:nl;i baNam. 



a. Double refraction c([ii:tl to or loss than that of quart/; 



not pleochroic. 1 



Nephelite. Hexagonal; n = 1.54; <o = .005. 

 In nearly square or hexagonal outline, irregular masses, 

 and rounded grains; Colorless, gray, greenish, bluish 

 or brown; Cleavage prismatic and basal, though not 

 well developed in sections ; Relief very flat; Inter- 

 ference color low first order gray; Optically negative 



(-); P. 440. 



Tridymite; Hexagonal; n = 1.477; - <o = .002. 



In hexagonal plates and scaly aggregates; Colorless 

 and transparent ; The surface appears rough from the 

 low refraction ; Interference color very low gray of 

 the first order ; Optically positive (+) ; p. 361. 



Quartz; Hexagonal; n = 1.547; <* = .009. 



Irregular grains and angular masses ; No cleavage or 

 relief; Interference colors first order gray to yellow; 

 Optically positive (+) ; p. 352. 



2. Index of refraction greater than that of Canada balsam, 

 o. Double refraction less than that of quartz. 



Apatite; Hexagonal; n = 1.635; a) = .004. 



Hexagonal sections, prismatic elongated sections, or 

 rounded grains; Cleavage not developed, elongated 

 sections show transverse parting; Colorless; Not 

 pleochroic ; Relief not marked ; Interference colors 

 first order gray ; Optically negative ( ) ; p. 508. 



Vesuvianite; Tetragonal; n = 1.72; <o - = .006. 



In short square prisms, grains, or irregular masses ; Cleav- 

 age not developed ; Colorless, yellow, green, brown, 

 or blue ; Relief marked, surface rough ; Slightly pleo- 

 chroic; Interference color first order gray, often 

 anomalously high; Optically negative ( ); p. 455. 



Corundum; Hexagonal; n = 1.766; to = .009. 



Crystalline outlines, elongated parallel to 6, grains or 

 irregular masses ; Rhombohedral parting ; Colorless, 

 red, or blue ; Pleochroic only in highly colored sec- 

 tions ; Relief high ; Interference color first order 

 gray to yellow; Optically negative ( ) ; p. 341. 



1 Chlorite, which is pleochroic, may appear uniaxial ; p. 352. 



