6o 



OUR PHYSICAL WORLD 



nail. Chlorite gives its green tinge to many igneous rocks known 

 commonly as green stones, and to some schists and slates. 



Serpentine is usually massive, sometimes fibrous like asbestos 

 (chrysolite). It is green in color, occasionally so dark as to be 

 nearly black. It has a greasy feel, a waxy luster (pearly in the 

 fibrous sorts), and is quite soft (2.5-3). It is not only a common 

 accessory mineral in many igneous and metamorphic rocks but 

 also forms great bodies of rock itself. 



Talc is readily recognized by its softness. It makes a light 

 streak even on cloth. It is usually white to green. It is some- 

 what laminated like mica, but the flakes are inelastic. It has 

 a distinctly greasy feel. 



Such, then, are the common rock-forming minerals. The 

 student should be familiar with them before he goes on to a study 

 of the rocks. The following tabulation will serve to give the 

 characteristics in condensed form. 



KEY TO COMMON ROCK-FORMING MINERALS 



'Chalk, 0.5-2.5 White to gray, dull, crumbles in 



fingers, no earthy odor when 

 breathed upon, effervesces with 

 acid. 



So soft they can 

 be scratched with < 

 the finger nail 



Chlorite, 1.5-4.0 



Gypsum, 1.5-2.0 



Kaolin, 0.5-2.5 



Mica, 2.2-5.0 



A green mineral of pearly to 

 vitreous luster with greasy feel- 

 ing. It usually occurs in grains 

 or scales in basic rocks. 



Many colors, streak always 

 white. Massive (alabastine), 

 fibrous (satin spar), foliated (if 

 transparent called selenite). 



Many colors, streak like color. 

 Feels greasy. Strong clay odor 

 when breathed on. Dull to 

 pearly luster; brittle. 



Perfect cleavage; very thin 

 elastic scales can be obtained. 

 The black sort is biotite; the 

 colorless, gray, or pale green, 

 muscovite. 



