168 FIRST YEAR COURSE IN GENERAL SCIENCE 



pound of two or more elements. The five most abundant 

 elements in minerals are oxygen, silicon, aluminum, iron, 

 and calcium. Many others exist in small quantity. 



Minerals differ in color, hardness, solubility, and fusibility, 

 and in the form of their crystals. A crystal is a body of 

 definite form, having a number of flat, lustrous surfaces. 

 Crystals are formed from the cooling of a melted substance 

 or the evaporation of a solution. Every mineral has its own 

 form of crystal. 



189. Ores. An ore is a mineral which is valued for the 

 metal that can be obtained from it. Iron ore is a compound 



of iron and other elements, 

 one of which is often oxygen. 

 Many ores, as those of sil- 

 ver, lead, and zinc, are com- 

 pounds of sulphur and the 

 metal. Most minerals con- 

 tain a small amount of 

 metal, but minerals are not 

 classed as ores unless it is 

 profitable to separate the 

 metal from the other ele- 

 ments with which it is com- 

 bined. 



190. Reduction of Ores. 

 - By reduction of an ore 

 is meant the separation of 

 the metal from its com- 

 pounds and from the rock 

 with which the compounds are mixed. The compound is 

 said to be reduced, and the material which produces the 

 separation of the metal is called the reducing agent. 



If chemical solvents are to be used to remove the metal, 

 the ore is first crushed under huge weights or hammers, called 

 stamps. It is then submitted to the action of chemicals 



FIG. 83. QUARTZ CRYSTALS 



When solutions of minerals evapo- 

 rate slowly, or when melted substances 

 cool slowly, crystals of various sizes 

 form. Whatever the size, the faces are 

 planes. In crystals of a given mineral, 

 corresponding faces always make the 

 same angle with each other. 



