Some compounds can combine in more than one crystal form. Diamond and graphite are perhaps the most familiar example. We call such minerals polymorphs.
We call a change from one polymorph to another, a phase change.
The stability of minerals is the tendency of the mineral to remain unchanged. For example, water ice is stable at temperatures below 0°C (32°F), but put it in a warmer area, and it melts (changes from solid to liquid).
The stability of a mineral is related to the environment.
Several environmental factors affect mineral stability:
Geologists can use more subtle differences in rock mineralogy, in conjunction with laboratory experiments, to estimate the temperature and pressure at which a rock formed, or was later heated.
Laboratory "maps" of mineral stability are called phase diagrams.
Time is an important factor in discussing phase changes. Some chemical reactions, such as an explosion, are vigorous and fast, others are very slow.
The study of the rates of mineral reactions is called kinetics.
Often minerals take very long times to complete phase transitions. Thus, in our short lifetimes we may not see the transformations. The kinetics of natural reactions makes studying mineral formation a challenge - we can't set up an experiment to run millions of years!
Sometimes the reactions can be changed by the presence of other chemicals, such as water. For example, rust forms on metal faster in the presence of salt - that's why many automobiles used to rust very quickly in regions with harsh winters - salt on the roads catalyzed the oxidation (rusting) of the metal.