Every mineral formed is characterized by a chemical composition. Each may be classified as either a chemical compound or simply as an element. A chemical compound is formed when two or more elements are found present in a chemical combination. There are also some elements that commonly occur uncombined. These are the elements: carbon, gold, platinum, and silver. A crystallized carbon for example could turn out to be a diamond. The elements, oxygen and silicon, when combined, will form quartz, a rock crystal. The indicated chemical composition will be SiO2, silicon dioxide. Many gem minerals are characterized by exactly complex chemical formulas.

Some chemical substances have the same chemical composition but may occur in different modifications with distinct chemical and physical properties. The chemical substance is said to be dimorphous if it occurs in two distinct modifications. It is trimorphous if in three modifications. It is polymorphous if in many modifications.

Minerals are classified under different division with regard to chemical composition. These are elements, sulfides, oxides and hydroxides, carbonates, sulfates, phosphates, and silicates.

Elements

The elemental group has three divisions. These are the metals and inter-metallic elements, non-metals, and semi-metals. Natural alloys are also included like the natural alloys of gold and silver. The same is true with nitrides, phosphides, silicides and carbides, which are usually common natural constituents of few rare meteorites.

Sulfides

The sulfide class members form an economically important metal ores like copper, lead, and silver. They commonly originate in an igneous environment, with dark colored streak or black streak. They appear to be of metallic luster and are considered opaque. Most sulfides are generally sectile with high densities. Hardness appears to be soft to average. Few members of this class tend to break the mold and they sometimes appear vitreous and transparent.

Oxides and Hydroxides

This mineral class is of large diversity. It includes metallic minerals. Sometimes, they are of black appearance but others turn out to be very colorful. Some minerals are quite hard while others appear to be quite so soft. Valuable metals can be extracted from their ore formations. Their common occurrences have the appearance of the precipitates close to the surface of the earth, their inclusion in oxidation products made in weathering zone of the near surface layer, their appearance as accessory mineral present in igneous rocks formed at earth’s crust and mantle.

Carbonates

Carbonates are known to be one of the common constituents of the near-outer layer crust of the earth. They are often found together with borates and other related nitrates. Carbonates are considered as both chemically-related and structurally-related group. The triangle of this anionic unit has either a residing boron, carbon, or nitrogen atom at the center. The oxygen atoms can be found sitting at the three corners of triangle. A trigonal symmetry is shown by the three-fold symmetry of this triangle.

Typical carbonates tend to originate with exception of carbonate igneous intrusions, in sedimentary and oxidizing environments. They appear to be transparent faintly colored with white streak. Their density is average above average. They are known to have possible solubility and at some degree of acidic solution. With regard to cleavage, carbonates appear to be soft and almost good to perfect. This is a common characteristic of all group members because of their common chemistry sharing.

Carbonates are the resultant of the setting and accumulation of the shells of dead planktonic life on the sea floor, after which they are deposited in marine setting. Carbonates can also be found in karst regions and in evaporitic settings.

Sulfates

The mineral sulfates are considered important and are known to include specimens that are very attractive and very interesting. They show vitreous luster, with average hardness. Their density occurs to be average above average. They commonly form in metamorphic zones, in evaporitic setting where slow evaporation of waters with high saline content occur, in secondary oxidation product of sulfides, and in hydrothermal veins systems.

Phosphates

This mineral class includes any mineral with tetrahedral unit AO2 where A may appear to be vanadium, antimony, phosphorus, or arsenic. Apatite is considered the most common phosphate. Apatite can be found in bones and teeth of many animals. Phosphates show vitreous to dull luster. They appear to be strongly colored when examined under a polarizing microscope. Their density is average above average, showing an average hardness range 4 to 7. When viewed under polarizing petrographic microscope, low index of refraction is shown unless ions like lead are present.

Silicates

This class is considered the largest group with silicon and oxygen composition and other additional ions. Silicates are found to be almost 30% of all minerals content. Some geologists made estimation that silicate constitutes almost 90% of the Earth’s crust. These minerals appear to be tetrahedron in form and belong to the anionic group. Their occurrence is of great abundance just like oxygen.