Everything about Porphyroblast totally explained
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 meta
pelites (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 having a
porphyroblastic texture.
Porphyroblasts don't rotate.
As they grow, the foliation is preserved in their crystal structure, which is helpful for tracking changing deformation planes.
Further Information
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