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How are porphyroblasts formed?
[ pôr-fîr′ə-blăst′ ] 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 process of metamorphism creates porphyroblasts?
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. 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 minerals are found in schist?
Most schists are composed largely of platy minerals such as muscovite, chlorite, talc, sericite, biotite, and graphite; feldspar and quartz are much less abundant in schist than in gneiss.
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.
Where is mica schist formed?
How did it form? Mica schist is a metamorphic rock that formed through the intense heat and pressure that were generated when Africa and North America slammed together to create Pangaea, about 275 million years ago.
What causes a rock to sparkle?
Mica minerals make some rocks sparkle! They sparkle because light is reflected on their flat surfaces, which are where the mineral breaks along its plane of cleavage. These minerals break so easily along their cleavage that some crystals have broken into many thin layers that look like the pages of a little book.
What type of metamorphic rock is schist?
Schist is medium grade metamorphic rock, formed by the metamorphosis of mudstone / shale, or some types of igneous rock, to a higher degree than slate, i.e. it has been subjected to higher temperatures and pressures.
What minerals are in phenocrysts?
Phenocrysts comprise <20% of the ZAC obsidian and <35% of the PR38 pumice, and are dominantly alkali feldspar, with subordinate plagioclase (only in PR38), clinopyroxene, biotite, and magnetite, in order of decreasing abundance.
What minerals are likely to form phenocrysts in a basalt?
Mineral content – groundmass generally of pyroxene ( augite), plagioclase and olivine, possibly with minor glass; if porphyritic the phenocrysts will be any of olivine, pyroxene or plagioclase.
What kind of crystals are porphyroblasts made of?
Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. 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.
Can a porphyroblast be a poikiloblastic mineral?
Porphyroblasts are often confused with porphyroclasts, which can also be large outstanding crystals, but which are older than the matrix of the rock. If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as poikiloblastic.
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.
Where can you find garnet in a porphyroblast?
An almandine – garnet growing as a porphyroblast in a quartzitic gneiss. The garnet measures 3 cm. Location: Paraíba, Brazil. Dark-coloured porphyroblasts of garnet in mica schist at Syros, Greece. A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix.