If a gap exists between the solidus and liquidus, then within that gap, the substance is not stable as either a solid or a liquid; such is the case, for example, with the olivine (forsterite-fayalite) system.
If a gap exists between the solidus and liquidus, then within that gap, the substance consists of solid and liquid phases simultaneously (like a "slurry"). Such is the case, for example, with the olivine (forsterite-fayalite) system.