The chemical properties of the minerals depend on the chemical properties of the elements and compounds present in it. Moreover, the components of a mineral form a specific crystalline structure and thus result in definite chemical composition. The properties which are related directly to the chemical composition of the minerals are isomorphism, polymorphism, and pseudomorphism. A detailed examination of these properties is being conducted in the following paragraphs.
When two or more crystals have similar chemical compositions, they exist in similar crystalline form. This property is known as isomorphism. In the case of minerals, it is a phenomenon of occurrence of a group of minerals, which have the same crystal structure. These minerals show a variation in their chemical composition but their crystal structure remains almost the same. The plagioclase felspars, which are a group of triclinic minerals, provide an example of isomorphism. In these minerals, there is the continuous substitution of (Na+Si4) for (Ca+Al3) from anorthite (Al2CaO8Si2) to albite (NaAlSi2O8).
When a specific chemical compound has more than one crystalline structure. This distinct type of ability of compounds is known as polymorphism. In this case, each crystal form gives rise to a separate mineral species. Such minerals have identical chemical composition but different atomic structures (crystalline structure) are known as ”polymorphs”. For example, polymorphs of carbon are graphite and diamond, and of CaCO3 are calcite and aragonite.
If a mineral adopts the outer crystalline form of another mineral species, it is called pseudomorphism. Minerals are formed when one mineral is replaced by another without any change in the outer form of the original mineral. Thus the chemical composition and structure of a pseudomorph belongs to one mineral species whereas the crystal form corresponds to another. A common example of pseudomorph is a piece of fossil wood where wood fibers have been replaced by silica. Another example is Quartz (SiO2) after fluorite (CaF2).