In geochemistry, a trace element is one whose concentration is less than 1000 ppm or 0.1% of a rock's composition. The term is used mainly in igneous petrology. Trace elements will be either compatible with a liquid or solid phase. If compatible with a mineral, it will be incorporated into a solid phase (e.g., nickel's compatibility with olivine). If it is incompatible with any existing mineral phase it will remain in the liquid magma phase. The measurement of this ratio is known as the partition coefficient. Trace elements can be substituted for network-forming ions in mineral structures. Trace elements that are not essential to a mineral's defined composition will not appear in the chemical formula of that mineral.