CHAPTER III 



TABLE FOR THE DETERMINATION OF THE PRINCIPAL 

 ROCK-FORMING MINERALS IN SECTIONS 



I. The mineral is opaque. 

 Magnetite. Isometric. 



In crystalline outlines, rounded grains, or dustlike. 



By reflected light, bluish black in color ; p. 373. 



Chromite : Isometric. 



In crystalline outline, or rounded grains. 



By reflected light brownish and at times the very thin edges 



may appear brownish ; p. 376. 

 Ilmenite. Hexagonal. 

 Tabular, thin plates, or irregular in outline. 

 By reflected light brownish and on very thin edges brown; 



p. 346. 



Hematite. Hexagonal. 

 In thin plates, scales, and grains. 



The thin edges and scales dark red by transmitted light ; p. 343. 

 Graphite. Hexagonal. 

 In thin flakes, foliated masses, and grains. 

 By reflected light, metallic luster or dull black ; p. 284. 

 Pyrite. Isometric. 



Crystalline outlines, irregular masses, or rounded grains. 

 By reflected light yellow, metallic luster; p. 313. 

 Pyrrhotite. Hexagonal. 

 Irregular grains and masses. 

 By reflected light bronze-yellow with a metallic luster ; p. 308. 



II. Transmits light isotropically. 



A. Index of refraction below that of Canada balsam. 

 Fluorite. Isometric: n= 1.43. 



In irregular masses or rounded grains, rarely showing crys- 

 talline outlines, filling cavities or veins; Colorless, violet 

 or purple ; Cleavage cracks well developed ; Relief marked, 

 from the low refraction; p. 331. 

 Opal: Amorphous: n= 1.44. 

 2R 609 



