50 OUR PHYSICAL WORLD 



Finally, the hardness of the mineral is an important aid in 

 its determination. So important is this that a very definite scale 

 of hardness has been arranged, running from the very soft 

 minerals with a hardness of "one" to the diamond with a hard- 

 ness of "ten." This scale is as follows: talc with a hardness of i ; 

 gypsum, 2 ; calcite, 3 ; fluorite, 4 ; apatite, 5 ; orthoclase feldspar, 6 ; 

 quartz, 7; topaz, 8; corundum, 9; diamond, 10. 



Minerals may be classed from the point of view of rock forma- 

 tion into essential and accessory. Quartz and orthoclase feldspar 

 are essential ingredients of granite. A rock would not be named 

 a granite unless composed largely of these two minerals. Other 

 minerals, such as mica, hornblende, etc., may be present in rela- 

 tively small quantity and the rock still be a granite. Such are 

 the accessory minerals. Essential minerals are those the pres- 

 ence of which determines the name of the rock. Accessory 

 minerals are those that may be present but need not be so neces- 

 sarily. The chief minerals that play essential roles are quartz, 

 calcite, the feldspars, mica, amphibole, pyroxene, dolomite, 

 serpentine, kaolin. These are not always essential; they may 

 at times be accessory. The accessory minerals are much more 

 numerous. Only a few of the more important can be mentioned, 

 such as magnetite, hematite, limonite, pyrite, chlorite, olivine. 



Then there is a large group of minerals which are important 

 primarily as ores of the metals used so largely in industry. 

 Some of these, as already indicated, are accessory ingredients of 

 rocks. There are magnetite, hematite, limonite, oxides of iron; 

 pyrite, a sulphide of iron; siderite, a carbonate of iron; chal- 

 copyrite and bornite, copper iron sulphides; azurite and mala- 

 chite, copper carbonates; galena, lead sulphide; sphalerite or 

 "blackjack," a sulphide of zinc; cassiterite, an oxide of tin; 

 cinnabar, mercury sulphide; pyrolusite, an oxide of manganese 

 (Fig. 28). 



Many other minerals are commercially valuable as sources of 

 chemicals needed in industry. Such are halite or rock salt; 

 borax, a borate of sodium; saltpeter, a nitrate of potash; soda 



