A reminder of mineralogy
Natural minerals

A mineral is a homogeneous substance resulting from natural physico-chemical reactions and characterized by its crystallo-chemical properties. This very general definition does not exclude liquids or gases found on or in the earth's crust. However, under normal conditions of pressure and temperature, minerals are usually solid. They come in two different states:

• the crystalline state, characterized by the regular repetition of an elementary pattern of atoms, ions or molecules;

• the amorphous state, characterized by total disorder in the distribution of constituents.

A mineral is said to be homogeneous when it has the same physical, structural and chemical properties at all points. It should be noted, however, that there are a few exceptions where hardness, for example, varies...