Chemical Composition:

* Elements: Some minerals consist of a single element, like gold (Au) or diamond (C).

* Compounds: Most minerals are compounds formed from two or more elements.

* Oxides: Contain oxygen (O) combined with another element, like hematite (Fe₂O₃).

* Sulfides: Contain sulfur (S) combined with another element, like pyrite (FeS₂).

* Sulfates: Contain sulfur (S) and oxygen (O) combined with another element, like gypsum (CaSO₄·2H₂O).

* Carbonates: Contain carbon (C) and oxygen (O) combined with another element, like calcite (CaCO₃).

* Halides: Contain halogens like chlorine (Cl) or fluorine (F), like halite (NaCl).

* Silicates: Contain silicon (Si) and oxygen (O), often with other elements, like quartz (SiO₂).

* Phosphates: Contain phosphorus (P) and oxygen (O), like apatite (Ca₅(PO₄)₃(OH,F,Cl)).

Crystal Structure:

* Crystal Lattice: Atoms in a mineral are arranged in a highly ordered, repeating pattern called a crystal lattice.

* Unit Cell: The smallest repeating unit of this pattern is called the unit cell.

* Symmetry: The arrangement of atoms in the unit cell gives a mineral its characteristic symmetry. There are seven crystal systems based on symmetry: cubic, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic, and rhombohedral.

The Mohs Hardness Scale

While not strictly a classification system, the Mohs Hardness Scale is a useful tool for identifying minerals. It ranks minerals from 1 (softest, like talc) to 10 (hardest, like diamond), based on their resistance to scratching.

Examples of Mineral Classification:

* Quartz: SiO₂ (chemical formula), hexagonal crystal system, hardness of 7.

* Halite: NaCl (chemical formula), cubic crystal system, hardness of 2.5.

* Calcite: CaCO₃ (chemical formula), rhombohedral crystal system, hardness of 3.

Note: Classifying minerals can be complex, as some minerals share similar characteristics or have variations in their chemical composition. Specialized tools and techniques are often used for accurate identification.