What is a porphyry and how does it form?
Porphyry deposits are formed when a column of rising magma undergoes cooling in two stages: In the first stage, the magma cools slowly deep within the crust, creating the large crystal grains with a diameter of 2 millimeters or more.
What is porphyritic texture?
Porphyritic – This texture describes a rock that has well-formed crystals visible to the naked eye, called phenocrysts, set in a very fine grained or glassy matrix, called the groundmass. An aphanitic texture is developed when magma is erupted at the Earth’s surface and cools too quickly for large crystals to grow.
What does porphyry mean in earth science?
“Porphyry” is a diversity of igneous rock consisting of large-grained crystal such as quartz and feldspar scattered in a fine-grained groundmass.
What is Poikilitic texture in geology?
Poikilitic texture refers to crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals. In igneous rocks Poikilitic texture is widely used to determine order of crystallization; if one mineral is enclosed by another then the enclosed grain must have been the first to crystallize.
What does porphyry look like?
In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance. Porphyries may be aphanites or phanerites, that is, the groundmass may have microscopic crystals as in basalt, or crystals easily distinguishable with the eye, as in granite.
What is granitic composition?
The chemical composition of granite is typically 70-77% silica, 11-13% alumina, 3-5% potassium oxide, 3-5% soda, 1% lime, 2-3% total iron, and less than 1% magnesia and titania. Volcanic rock of equivalent chemical composition and mineralogy is called rhyolite.
What is the difference between porphyritic and pegmatitic textures?
Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive (the size ranges from a few centimetres to several metres). Porphyritic textures develop when conditions during cooling of a magma change relatively quickly.
How does porphyry form in igneous rocks?
Porphyritic rocks are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more.
What is Orthocumulate texture?
Adcumulates are rocks containing ~100–93% accumulated magmatic crystals in a fine-grained groundmass. Mesocumulates are rocks with between 93 and 85% accumulated minerals in a groundmass. Orthocumulates are rocks containing between 85 and 75% accumulated minerals in groundmass.
What is 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 are porphyry columns?
Porphyry is an igneous rock similar in composition to granite. Porphyry was Imperial Rome’s most prestigious stone for columns, vases, alters, busts and other objects.