191 



Hardness. 



The " hai'dness " of minerals is very variable, and is of great value in 

 their determination. It is tested by the readiness with which the mineral 

 under examination can be scratched by other substances. 



A series of ten minerals have been chosen as types, and arranged in the 

 order of their hardness. These minerals are : — 1. Talc. 2. Selenite. 

 3. Calcite. 4. Fluor. 5. Apatite. 6. Orthoclase. 7. Quartz. 8. Topaz. 

 9. Corundum (sapphire, ruby). 10. Diamond. 



Each mineral in this " scale of hardness " will scratch all that precede, 

 and can be scratched by all that follow it. By testing an unknown mineral 

 with the different members of the scale, its place in the series can be 

 readily determined. Thus, a piece of galena will scratch selenite (So. 2), 

 and can be easily scratched by calcite (No. 3) ; in this case Ave should 

 describe the hardness as between 2 and 3. As another instance, the 

 sap-pldre and the ruhy have a hardness of 9 ; unless the mineral will 

 scratch toj)az (No. 8), it cannot be sapphire nor ruby. 



The following rough tests will be found very useful : — 



1. A good knife or file will scratch all minerals that have a hardness 



of 6 or less. 



2. A bronze coin can be scratclied by all minerals having a hardness 



above 3 "5. 



3. The thumb-nail scratches all minerals having a hardness of 2 '5 or less. 



Tenacity. 

 Minerals behave very differently when cut or pulverised. 

 The following are the terms used : — 



1. Malleability (when pieces can be flattened by hammering) : copper, 



gold, platinum. 



2. Sectility (when the mineral can be cut) : copper pyrites, copper- 



glance, silver chloride. 



3. Flexibility (when a flake can be bent without breaking) : talc, 



selenite. 



4. Elasticity (when the flake resumes its original position after 



bending) : mica. 



5. Brittleness: most minerals, e.g., iron pyrites. 



Gold and platinum are found characteristically flattened in alluvial drift. 

 Elasticity is a very characteristic property of mica. 



