METAMORPHIC ROCKS

Metamorphic Textures And Structures

Some very common Metamorphic Textures and Structures are discussed briefly as follows.

Metamorphic Structures

Definite mechanical conditions and recrystallization determine the metamorphic structures. Five major types of metamorphic structures have been recognised as follows:

(i) Cataclastic structure:

This type of structure is produced under stress and in absence of high temperature, whereby rocks are subjected to shearing and fragmentation. Only the durable mineral partly survive the crushing force and the less durable ones are powdered. Thus, when resistant minerals and rock fragments stand out in a pseudo porphyritic manner in the finer materials, it is known as ‘porphyroclastic structure.’ Phenocrysts are called ‘porphyroclasts. Argillaceous rocks develop salty cleavage, harder rocks may be shattered and crushed forming crush breccia and crush conglomerate. When the rocks are highly crushed into fine grained rocks, they are known as mylonites. Since these structures are formed due to cataclasis, they are, as a whole, known as cataclastic structure.

(ii) Maculose structure:

This type of structure is produced by thermal metamorphism of argillaceous rocks like shales. Here, larger crystals of andalusite, cordierite and biotite are sometimes well developed giving a spotted appearance to the rocks. The well-developed crystals are known as ‘porphyroblasts’ with increasing degree of metamorphism, the spotted slates pass into extremely fine grained granular rock known as Hornfels.

Maculose Structure

(iii) Schistose structure:

This type of structure is when the platy or flaky minerals like the micas and other inequidimensional minerals show a preferred orientation along parallel planes, under the effect of the stress dominating during metamorphism. The longer directions are parallel to the direction of maximum stress. Schistosity is the property or tendency of a foliated rock, whereby it can be readily split along foliation plane.

Schistose Structure

 

(iv) Granulose structure:

This is found in the rocks composed of equidimensional minerals like quartz, feldspar and pyroxenes. They are formed by the recrystallization of pre-existing rocks, under uniform pressure and great heat. The typical texture is coarsely granoblastic. These structures are also known as ‘saccharoidal.

Quartzites and marbles are typical examples of this structure.

Granulose Structure

(v) Gneissose structure:

It is a banded structure due to alteration of schistose (dark coloured) and granulose (light coloured) bands and is produced by highest grade of metamorphism, typically by regional metamorphism. The bands differ from ore another in colour, texture and mineral composition. Gneisses typically show this type of structure.

Gneissose Structure

Metamorphic Textures

The texture of a metamorphic rock is both a description of its constituent minerals along with their arrangement and size. There are two types of Textures; namely:

(A)Foliated Textures

  1. Slaty Texture – This texture is caused by the parallel orientation of microscopic grains. The name for the rock with this texture is slate, and the rock is characterized by a tendency to separate along parallel planes. This feature is a property known as slaty cleavage. 
  2. Phyllitic Texture – This texture is formed by the parallel arrangement of platy minerals, usually micas. The parallelism is often crenulated. The predominance of micaceous minerals imparts a sheen to the hand specimens. A rock with a phyllitic texture is called a phyllite.
  3. Schistose Texture This is a foliated texture resulting from the subparallel to parallel orientation of platy minerals such as chlorite or micas. Other common minerals present are quartz and amphiholes. A schistose texture lies between the parallel platy appearance of phyllite and the distinct banding of gneissic texture. The average grain size of the minerals is generally smaller than in a gneiss. A rock with schistose texture is called a schist.
  4. Gneissic Texture: This is a coarsely foliated texture in which the minerals have been segregated into discontinuous hands, each of which is dominated by one or two minerals. These bands range in thickness from 1 mm to several centimetres. The individual mineral grains are macroscopic and impart a striped appearance to a hand specimen. Light-colored bands commonly contain quartz and feldspar and the dark bands are commonly composed of hornblende and biotite. A rock with a gneissic texture is called gneiss.

(B) Non- Foliated Texture

Granoblastic Texture: The metamorphic rocks with no visible preferred orientation of mineral grains have a non-foliated granoblastic texture. Non-foliated rocks commonly contain equidimensional grains of a single mineral such as quartz, calcite, or dolomite.

Examples of such rocks are quartzite, formed from a quartz sandstone, and marble, formed from a limestone or dolomite and the third non-foliated rock is hornfels identified by its dense, fine grained, hard, blocky or splintery texture composed of several silicate minerals

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