CoachA porphyroblast is a large crystal that grew during metamorphism within a finer-grained groundmass; garnet, staurolite, cordierite and chloritoid form them, from millimetres to several centimetres across. A poikiloblast is a porphyroblast that contains many inclusions of other minerals. An inclusion trail is a line of aligned inclusions inside a porphyroblast, which preserves the fabric of the groundmass at the time the crystal overgrew it. A corona is a rim of one or more new minerals completely surrounding a mineral that was breaking down, because it had ceased to be stable with its neighbours: a reaction texture, frozen before the reaction was complete.
