When we say that a mineral has a definite chemical composition, it means that the mineral is made up of a specific set of elements in fixed proportions. The chemical composition of a mineral is determined by its atomic structure and bonding. The elements that make up a mineral are present in a specific ratio, and this ratio is characteristic of that particular mineral. For example, the mineral halite (NaCl) is always composed of one atom of sodium and one atom of chlorine. The chemical composition of a mineral can be expressed using a chemical formula, which shows the proportions of the different elements present. For example, the chemical formula for halite is NaCl. The definite chemical composition of minerals is one of the key properties used to identify and classify them.