APPENDIX 

 COMMON MINERALS 



Every student of geology should be able to recognize the common 

 minerals by sight and know their approximate chemical composition. 

 In order to determine minerals without the aid of chemical tests, one 

 must depend upon their physical properties. Of these the color, 

 streak, hardness, specific gravity, and crystalline form are important. 



The color sometimes varies greatly in the same mineral, but never- 

 theless often affords a strong clue to its identity. The color of a min- 

 eral is often due to the inclusion of foreign matter, such as iron oxide 

 and organic matter, but some minerals, such as the carbonate of cop- 

 per, malachite, vary slightly. 



Each mineral has a characteristic hardness and this quality often af- 

 fords an easy means of positive identification. The scale of hardness 

 in common use is: I, Talc; 2, Gypsum; 3, Calcite; 4, Fluorite; 

 5, Apatite; 6, Orthoclase; 7, Quartz; 8, Topaz; 9, Corundum; 

 10, Diamond. Minerals with a hardness of 1 and 2 can be scratched 

 with the finger nail. If a mineral will barely scratch a copper coin, 

 it may be considered as about 3 in hardness ; if it fails to scratch 

 glass, its hardness is less than 5 ; if it scratches glass but fails to scratch 

 quartz, its hardness is between 5 and 7. A knife point is almost in- 

 dispensable in determining hardness, since with a little practice, the 

 hardness of all minerals between 1 and 6 can be readily determined. 



The streak or mark that a mineral makes on a hard white substance, 

 such as a piece of unglazed porcelain, is often important in distinguish- 

 ing between minerals. The color of the streak is the same as that of 

 the fine powder. 



When a mineral breaks or cleaves in definite directions so as to form 

 plane surfaces, it is said to have a cleavage. Since cleavage is caused 

 by the separation along and between layers of molecules, it occurs 

 only in crystals. The thin leaves of mica are formed by the splitting 

 of the mineral along cleavage planes. 



The relative weight of a mineral, or its specific gravity, is often an 

 important aid in determining a mineral by its physical properties. 



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