What rocks are porphyritic?
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all types of igneous rocks can display some degree of porphyritic texture.
Which rock can have porphyritic texture?
A porphyritic texture is developed when magma that has been slowly cooling and crystallising within the Earth’s crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly. This texture is characteristic of most volcanic rocks.
Where is porphyritic rock found?
When the ground mass is composed of small crystals of feldspar and quartz, the rock is called granite-porphyry. Both are of the composition of granite, and both are found in the elvan dikes of the tin district in Cornwall. Quartz- porphyry is common in the Porcupine gold camp, Ontario.
What minerals are in porphyritic?
Porphyritic Granite
Type | Igneous Rock |
---|---|
Chemical Composition | Felsic |
Color | Predominately pink |
Mineral Composition | Potassium Feldspar, Quartz, Sodium Plagioclase, Biotite, Hornblende |
Miscellaneous | Potassium Feldspar Phenocrysts in a phaneritic (coarse-grained) groundmass |
Is basalt a porphyritic?
Basalt is mainly made of two minerals: Plagioclase feldspar and pyroxene. Basalt has several textural varieties such as glassy, massive, porphyritic, vesicular, scoriaceous.
Is obsidian porphyritic?
Few obsidians are entirely vitreous; usually they have small crystals of felspar, quartz, biotite or iron oxides, and when these are numerous the rock is called a porphyritic obsidian (or hyalo-liparite).
What is an example of porphyritic texture?
Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals. Examples of porphyritic rocks are: andesite porphyry and rhyolite porphyry.
What rocks are Phaneritic?
Phaneritic: any coarse-grained igneous rock, often intrusive, usually formed as a result of a longer cooling history (ex. granite, gabbro). Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass).
Is andesite a porphyritic?
Andesite most commonly denotes fine-grained, usually porphyritic rocks; in composition these correspond roughly to the intrusive igneous rock diorite and consist essentially of andesine (a plagioclase feldspar) and one or more ferromagnesian minerals, such as pyroxene or biotite.
Is granite an aphanitic?
Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture.
Is granite an igneous rock?
Granite is an igneous rock composed of mostly two minerals: quartz and feldspar. It is an intrusive rock, meaning that it crystallized from magma that cooled far below the earth’s surface.
Is basalt a phaneritic?
The individual crystals in phaneritic texture are readily visible to the unaided eye. Figure 4.1. 1: Basalt is a classic fine-grained (aphanitic) extrusive igneous rock. This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine.
Is obsidian a phaneritic?
Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks.
Is gabbro a porphyritic?
Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclase oikocrysts have grown earlier than the groundmass minerals.
Is diorite a porphyritic?
Diorite is occasionally porphyritic. It usually contains enough mafic minerals to be dark in appearance. Orbicular diorite shows alternating concentric growth bands of plagioclase and amphibole surrounding a nucleus, within a diorite porphyry matrix.
Is obsidian igneous?
Rondi: Everyone, meet Obsidian , an igneous rock that from melted rock, or magma. Obsidian is an “extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an “intrusive” rock.
Is marble an igneous?
Metamorphic rock is an igneous or sedimentary rock type. The original rock is subjected to high pressure and heat, causing a profound physical and/or chemical change (this process is called metamorphism). Marble (e.g. Bianco di Carrara) and quartzite are mostly metamorphic rocks.
Is olivine a phaneritic?
MAFIC ROCKS GABBRO – phaneritic (large crystals) of mafic minerals: olivine, pyroxene, etc.
What is porphyritic basalt?
Porphyrite or porphyritic basalt is characterized by obvious crystals, usually of plagioclase, which is often white or tan in color. These crystals are typically interpreted as phases that formed before eruption, where magma was being stored (a “Magma Chamber).
Can granite be porphyritic?
Porphyritic texture is quite common in granites, as well as in lavas erupted from volcanoes.