What rocks contain phenocrysts?
Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics. The largest crystals found in some pegmatites are often phenocrysts being significantly larger than the other minerals.
What rock types has an Aphanitic texture?
Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
What is pegmatite texture?
A pegmatitic texture is one in which the mineral grains are exceptionally large. This texture is found in intrusive rocks. The extra large size does not mean that they cooled extra slowly. Instead, the large crystals of a pegmatite formed in a magma that was extra rich in dissolved water.
What are phenocrysts made of?
A phenocryst is a conspicuous, large crystal embedded in a finer-grained matrix of smaller crystals in a porphyritic igneous rock. Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more).
What is texture describe the Inequigranular texture of igneous rock in detail?
Inequigranular texture (i) Porphyritic texture : When an igneous rock contains large crystals some minerals set in a matrix which is much finer grained or even glassy, the texture is called “Porphyritic”. The large crystals are called “Phenocrysts” and fine grained material is called “groundmass”.
What is an aphanitic igneous rock?
The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur.
What are aphanitic igneous rocks?
aphanitic; from Ancient Greek αφανης (aphanḗs) ‘invisible’) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye).
What is the difference between Phaneritic and Aphanitic rocks?
APHANITIC TEXTURE – Igneous rocks that form on the earth’s surface have very fine-grained texture because the crystals are too small to see without magnification. PHANERITIC TEXTURE – Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber.
What are pegmatite deposits?
Pegmatites are extreme igneous rocks that form during the final stage of a magma’s crystallization. They are extreme because they contain exceptionally large crystals and they sometimes contain minerals that are rarely found in other types of rocks.
Why do phenocrysts form?
How Do Phenocrysts Form? Phenocrysts, as with any other rock forms from the cooling of molten rock or magma. There are primarily two drivers that determine the crystal size of minerals within a cooled and lithified rock. One is the rate at which the magma cools and the other is chemical composition.
What is the difference between aphanitic and Phaneritic textures?
What is the difference between Aphanitic and Phaneritic textures? Crystalline rocks with mineral grains that cannot be distinguished from one another without magnification have an aphanitic igneous texture.
What kind of rock has an aphanitic texture?
Such rocks are termed ” extrusive ” rocks. Under these conditions, there is not enough time for the growth of large crystals. Basalt from surface lava flow often exhibits an aphanitic texture.
How are aphanitic rocks different from igneous rocks?
Aphanitic rock is igneous rock in which the grain or crystalline structure is too fine to be seen by the unaided eye. Such rock is formed when the material solidifies at or near the surface so that the cooling is rather rapid. What is the main difference between Phaneritic and Aphanitic rocks?
How are Phaneritic textures formed in igneous rocks?
Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth’s surface. As magma cools slowly the minerals have time to grow and form large crystals. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly.
https://www.youtube.com/watch?v=QR3yvuBdli8