Minerals are made of elements. Elements are simple substances that cannot be broken down into any other substance. The name of an element is written down as a combination of letters called a symbol, eg: sodium is Na; chlorine is Cl. Salt, a combination of sodium and chlorine is thus written as a formula symbol NaCl. Many minerals are made up of large numbers of elements, so their formulas are complex. The most common minerals are those based on silicon and oxygen, Si04.