A graph showing the phases which a substance can be in. Often the axes of the graph are temperature and pressure, and there are three phases: solid, liquid and gas. In the case of minerals which can have a range of chemical formulae, the axes are concentration and temperature, and the phases will be things like different ordered states, mixtures of different states, a disordered state at high temperatures, and a liquid state at even higher temperatures. Plagioclase feldspar is a mineral with a complicated phase diagram.