What is the difference between Porphyroblast and Porphyroclast?

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.

What is the difference between a Phenocryst and a Porphyroblast?

A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them.

What causes Granoblastic texture?

Granoblastic texture is the typical equigranular texture formed in metamorphic rocks when grains mutually adjust their boundaries in the solid state in an attempt to achieve textural equilibrium.

What minerals can be porphyroblasts?

Porphyroblast

  • Foliation.
  • Kyanite.
  • Sillimanite.
  • Quartz.
  • Garnet.
  • Ilmenite.
  • Muscovite.
  • Plagioclase.

How is porphyroblast made?

Porphyroblastic texture. Relatively large single crystals, which formed by metamorphic growth in a more fine-grained matrix, are known as porphyroblasts (from the Greek word blastos meaning growth). Porphyroblasts are a valuable source of information on local tectonic and metamorphic evolution.

Does gneiss have porphyroblast?

Dark-yellowish brown to moderate-yellowish-brown, medium-grained, granoblastic to megacrystic, mafic-rich monzogranite composed of 27 to 38 percent quartz, 28 to 39 percent orthoclase, rod and bleb perthite, microcline, and myrmekite, and 33 to 40 percent oligoclase and andesine.

What is Phaneritic and Aphanitic?

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 is granoblastic in geology?

Granoblastic is an adjective describing an anhedral phaneritic equi-granular metamorphic rock texture. Granoblastic texture is typical of quartzite, marble, charnockites and other non-foliated metamorphic rocks without porphyroblasts.

How do Foliations and banding develop?

As the minerals that form this foliation grow, they begin to break up the original beds into small pods. As the pods are compressed and extended, partly by recrystallization, they could eventually intersect again to form new compositional bands parallel to the new foliation.

How are porphyroblasts formed?

Porphyroblasts are the result of the association of one mineral having small N/G with one or more minerals having a large N/G. Garnet, feldspar, cordierite, andalusite, staurolite, and chloritoid tend to form porphyroblasts, whereas quartz, carbonates, and micas tend to be matrix-formers.

Where do porphyroblasts form?

Porphyroblasts are rare in rocks composed of one mineral but isolated large crystals may form in marble, quartzite, or rock salt giving patchy (grumous, clotted, or nodular) texture.

How is porphyroblast formed?

Porphyroblasts form by the recrystallization of existing mineral crystals during metamorphism. They are analogous to phenocrysts in igneous rock.