260 MINERALOGY 



1. Talc 1.13 6. Orthoclase ... 191 



2. Gypsum 12.03 7. Quartz .... 254 



3. Calcite 15.3 8. Topaz .... 459 



4. Fluorite 37.3 9. Sapphire . . . 1,000 



5. Apatite 53.5 10. Diamond . . . 140,000 



The specimens used for the scale should be crystalline, cleavable, 

 and as nearly transparent as possible. 



In testing a mineral for hardness, a sharp point of the mineral 

 in the scale or test mineral is pressed firmly on a smooth surface 

 of the mineral to be tested and drawn across with a quick move- 

 ment. The harder the mineral the more pressure will be re- 

 quired to make the scratch. Care must also be taken not to mis- 

 take the mark left by a soft mineral on a harder surface as a 

 scratch. This mark, like a chalk mark, may be easily rubbed 

 off, while a scratch may be tested by drawing the finger nail across 

 it. Minerals of the same hardness will scratch each other when 

 tested in this way. 



In ordinary cases, as the determination of hardness for use with 

 the blowpipe tables, hardness above or below 3, 5, and 7 is an im- 

 portant point. If a mineral will scratch a copper coin, it may be 

 considered as above 3 in hardness, and failing to scratch glass 

 it would be below 5 ; failing to scratch quartz and scratching glass, 

 it would be between 5 and 7 in hardness. With experience the 

 approximate hardness of a specimen may be determined by the 

 ease or difficulty with which it is cut or scratched by an old knife- 

 blade or file. When the knife fails to have any effect on the speci- 

 men, it is above 6 in hardness. In testing the hardness of a min- 

 eral, care must always be taken that it is as pure as possible and 

 free from decomposition products, remembering that impurities, 

 as sand, etc., will cause a soft mineral to appear much harder than 

 in reality it is, and by decomposition minerals which when unaltered 

 are very hard, as corundum or andalusite, will appear softer. 



Specific gravity = G, is the expression in figures of the ratio of 

 the weight of unit volume of the substance to the weight of unit 

 volume of water at 4 C. A mineral in which G = 2.65 (quartz) 

 will weigh 2.65 gm. per cubic centimeter, since one cubic centimeter 

 of water weighs one gram. Knowing the specific gravity of any 

 mineral or rock, it is an easy matter to determine the weight of any 

 cubic amount, as a yard or a foot. For the identification of min- 

 erals, the specific gravity is determined approximately to the second 



