While chemically pure materials have a single melting point, chemical mixtures often partially melt at the solidus temperature (T or T), and fully melt at the higher liquidus temperature (T or T). The solidus is always less than or equal to the liquidus, but they need not coincide. If a gap exists between the solidus and liquidus it is called the freezing range, and within that gap, the substance consists of a mixture of solid and liquid phases (like a slurry). Such is the case, for example, with the olivine (forsterite-fayalite) system, which is common in Earth's mantle.