THE EARTH'S ROCK FOUNDATIONS 



6l 



KEY TO COMMON ROCK-FORMING MINERALS Continued 



Easily scratched 

 with a knife 



Scratched with a 

 knife with dif- 

 ficulty 



Scratched by 

 quartz but not 

 with a knife 



Galena, 2.5 



Serpentine, 2.5-4.0 



Calcite, 3 



Sphalerite, 3.5-4.0 

 (Zinc blende) 



Chalcopyrite, 3.5-4.0 

 (Copper pyrite) 



Dolomite, 3.5-4.0 

 (Pearl spar) 



Mica, see above 

 Limonite, 5.0-5.5 



Pyroxene or 

 Augite, 5-6 



Amphibole or 



Hornblendes, 5-6 



^Hematite, 5.5-6.5 



Lead gray, streak same. Metal- 

 lic luster. Very heavy; cleaves 

 in cubes. 



Color, shades of green. Luster 

 greasy, waxy, or earthy. Feels 

 smooth or greasy. Compact and 

 amorphous, making a rock of 

 the same name. 



Many colors, streak white to 

 gray. Always cleaves into 

 rhombs. Effervesces in dilute 

 acid. 



Yellow, red, brown, black. 

 Luster resinous when yellow. 

 Perfect cleavage. Brittle. 



Brass yellow, often tarnished, 

 then showing iridescence. 

 Streak green-black. Softer than 

 pyrite. 



White, gray, green, black. 



Streak white. Transparent to 



translucent. Crystals curved 

 like saddles. 



Dark brown, streak yellowish 

 brown. Of ten fibrous; if earthy, 

 color is yellow. In cubical 

 crystals. 



Green to black. Fracture un- 

 even to conchoidal. Usually in 

 short, thick, eight-sided prisms. 

 Cleavage poor; faces meet at 90. 



Brown, green, or black, darker 

 than augite. Fracture as above. 

 Luster pearly on cleavage faces. 

 Crystals long, slender, six-sided, 

 faces finely cross-striate. Cleav- 

 age faces meet at 125. 



Cherry red to iron black; streak 

 red. Metallic luster, massive or 

 fibrous or scaly. 



