PART II 



CHAPTER I 

 THE RELATION OF MINERALS TO THE ELEMENTS 



MINERALS, when considered from a chemical standpoint, are 

 either elements in the uncombined state or are combinations of 

 elements which have been brought together during the past ages 

 and united by chemical forces which differ in no way from the 

 same forces with which we have become acquainted in the labora- 

 tory. Some mineral molecules are simple combinations of a few 

 chemical elements ; and the same compounds are daily produced 

 in the laboratory, in the simple processes of chemical analyses, as 

 in the precipitation of calcium carbonate from a solution of a soluble 

 calcium salt with an alkali carbonate. If this is carried out at 

 room temperatures and the precipitate is allowed to stand, it will 

 crystallize, forming calcite. If the same precipitation is carried 

 out at temperatures near the boiling point of water, that is, on the 

 water bath, and allowed to crystallize in a hot solution, the form 

 aragonite will be produced. Here are two minerals, calcite and ara- 

 gonite, one crystallizing in the hexagonal system, the other in the 

 orthorhombic system. They are both calcium carbonate, and 

 chemically they are identical ; but physically they are different, and 

 as in the precipitation they are formed under different conditions, 

 they are two phases of the same chemical substance; moreover, 

 calcium carbonate directly after a rapid precipitation is amorphous, 

 another phase, and becomes crystalline only after standing. This 

 property of chemical compounds, of occurring in different physical 

 forms is known as polymorphism. Calcium carbonate is thus tri- 

 morphic. It may be amorphous, it may possess the molecule of 

 calcite, or it may possess the molecule of aragonite. Silica, SiO2, 

 is said to possess six different forms or phases. 



The various forms of a polymorphic substance will never under 

 the same conditions possess the same degree of equilibrium, and 

 one form will be more stable than the others. It is always the 



219 



