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.
How do you identify porphyroblast?
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
What is the difference between Porphyroblastic texture and Blastoporphyritic texture?
Porphyroclastic: a cataclastic texture characterized by the presence of large relict mineral grains set in a matrix of smaller crushed grains. Blastoporphyritic: a relict texture in a cataclastic metamorphic rock in which traces of an original porphyritic texture remain.
What’s gneissic texture?
The “gneissic texture” refers to the segregation of light and dark minerals. It is indicative of high-grade metamorphism where the temperature is high enough, say 600-700 °C, so that enough ion migration occurs to segregate the minerals.
Whats the definition of foliated?
Definition of foliated 1 : composed of or separable into layers a foliated rock. 2 : ornamented with foils or a leaf design.
What minerals form porphyroblasts?
Garnet commonly forms porphyroblasts. It is a silicate mineral which is often used in gemstones.
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. Fenite is mostly alkalic feldspar, with some aegirine, subordinate alkali-hornblende, and accessory sphene and apatite.
What is the definition of metasomatism quizlet?
Metasomatism is a type of metamorphism in which a rock’s bulk composition changes only because of fluid transport of chemical substances into or out of the rock.
What is Gneissose structure?
Gneissose structure This is a banded structure in which the minerals have been segregated into discontinuous bands, each of which is dominated by one or two minerals. These bands range in thickness from 1 mm to several centimeters.
What is a porphyroclastic texture?
Porphyroclastic texture Porphyroclasts are single crystals of a size exceeding the mean grain size in the surrounding matrix and typical for mylonites. They are relic structures of a more coarse-grained original fabric. Common minerals that form porphyroclasts are feldspar, garnet, muscovite, hornblende and pyroxenes.
What is a porphyroblast?
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
What is the meaning of porphyria?
the uneven granular texture of a rock caused by the presence of large crystals, or porphyroblasts, in a fine-grained groundmass.
Where do porphyritic textures occur in igneous rocks?
Porphyritic textures occur in coarse, medium and fine-grained igneous rocks. Usually the larger crystals, known as phenocrysts, formed earlier in the crystallisation sequence of the magma.