High temperature and/or pressure conditions within Earth’s crust alter the crystalline (atomic) structures of minerals without involving a liquid, or magma, phase. Rearrangement into new and different crystalline structures leads to the formation of new minerals.

Audio Transcript 
These two cases show minerals having formed in a metamorphic environment. Metamorphism means rock materials, and the minerals they contain, having been changed in form and composition. High temperature and pressure conditions, such as those prevailing at great depths within the Earth's crust, particularly during times of mountain-building episodes, or when large bodies of magma, which simply are hot masses of liquefied rock, are able to recrystallize the original rock and mineral materials. 

Recrystallization is the process whereby minerals with new and different crystalline structures are produced without involving the liquid phase. These newly formed minerals may have either the same chemical composition as the original materials, but now with different crystalline structures, or else they may have totally different chemical compositions from the original minerals. Seliminite, kyanite, andalusite, storalite, almandine garnet, epidote, and talc are all minerals characteristically formed in a metamorphic environment. 

From experiments conducted in laboratories, it is known that certain minerals form under quite well-defined pressure/temperature conditions. Chlorite, for example typically develops when the metamorphic grade is not that intense. Other minerals, such as almandine garnet, staurolite, and kyanite, develop when pressure/temperature conditions are relatively higher. And a mineral such seliminite indicates a metamorphic grade that was very, very high. 

Information from these mineralogical barometers and thermometers, so to speak, provides the student of the Earth with a reasonably clear understanding about the physical conditions that once prevailed in a particular region where metamorphic rocks now occur at the surface. In the remote geological past, however, these rock materials were once buried deeply, possibly within the root system of a rising mountain chain.