What mineral can be porphyroblasts?
Garnet
Garnet commonly forms porphyroblasts. It is a silicate mineral which is often used in gemstones.
What metamorphic rock has porphyroblasts?
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.
What is a Porphyroblastic texture?
the uneven granular texture of a rock caused by the presence of large crystals, or porphyroblasts, in a fine-grained groundmass. Unlike phenocrysts, porphyroblasts form during recrystallization in the course of metasomatism of solid rock.
What is a Phenocrysts in geology?
A relatively large crystal embedded in a finer-grained or glassy igneous rock. The presence of phenocrysts gives the rock a porphyritic texture (see illustration). Phenocrysts are represented most commonly by feldspar, quartz, biotite, hornblende, pyroxene and olivine.
How do porphyroblasts form?
A large crystal that is surrounded by a finer-grained matrix in a metamorphic rock. Porphyroblasts form by the recrystallization of existing mineral crystals during metamorphism. They are analogous to phenocrysts in igneous rock.
What is Fenitization?
A quartzo-feldspathic rock that has been altered by alkali metasomatism at the contact of a carbonatite intrusive complex. The process is called fenitization.
What do you mean by metasomatism?
Metasomatism refers to the process whereby a preexisting igneous, sedimentary, or metamorphic rock undergoes compositional and mineralogical transformations associated with chemical reactions triggered by the reaction of fluids (so-called metasomatic agents), which invade the protolith.
Which is the best description of a porphyroblast?
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
When do porphyroblasts grow in a metamorphic rock?
In metamorphic rocks that experience deformation during metamorphism, porphyroblasts may grow before, during, or after the phase of deformation recorded by the matrix minerals.
What kind of rock is a porphyroclast made of?
A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals. Porphyroclasts are fragments of the original rock before dynamic recrystallisation or cataclasis produced the groundmass.
Why are porphyroblastic rocks described as poikiloblastic?
If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as poikiloblastic. This observation can help interpret deformation history. A rock which has many porphyroblasts is described as having a porphyroblastic texture .