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saved searches - My saved topics - My newsletter A porphyroblast
is a large mineral crystal in a metamorphic rock which has grown
within the finer grained groundmass as a result of static
recrystallisation in the rock. Therefore porphyroblasts are
almost always nice euhedral crystals. The most common
porphyroblasts in metapelites (metamorphosed mudstones and
siltstones) are garnets and staurolites, which stand out in well
foliated metapelites (such as schists) against the platy mica
matrix. A similar type of crystal is a phenocryst, a large
crystal in an igneous rock. Porphyroblasts are often confused
with porphyroclasts, which can also be large outstanding
crystals, but which are older than the matrix of the rock. A
rock which has many porphyroblasts is described as ha