Types of Metamorphic Texture form due to varying pressure and temperature conditions. They are classified into foliated (slaty, schistose, phyllitic, gneissic) and non-foliated (granulose, mylonitic, hornfelsic) textures.
Other textures like augen, porphyroblastic, and poikilitic provide further variations. These textures help in identifying metamorphic rocks and their formation history.
Types of Metamorphic Texture:
There are two main types of metamorphic texture:
- Foliated Texture
- Non-foliated Texture
Foliated Texture:
Foliations create this texture, giving the rock a layered appearance. The parallel planes of platy (flat) minerals, mostly mica, align secondarily due to directed pressure and recrystallization, making it a directional property of metamorphic rocks.
The main types of foliated textures are given below:
Slaty texture:
This texture consists of perfect, planar, parallel foliation with slaty cleavage formed by very fine-grained platy minerals (mainly mica) in fine-grained metamorphic rocks but lacks segregation banding. Slate commonly develops this texture as a result of low-grade metamorphism.
Schistose texture:
The texture has a fine to coarse grain and features excellent parallelism and thin segregation banding of fabric elements. Schists, which result from intermediate to high-grade metamorphism, commonly develop this texture.
Phyllitic texture:
The parallel foliation (wavy and wrinkled as well) consists of fine-grained (occasionally medium-grained) platy minerals like mica and chlorite. It develops in phyllite, which forms through relatively low-grade metamorphism.
Gneissic texture:
A texture with thick compositional banding consists of alternating schistose and granulose bands that differ in both mineral composition and texture. Gneiss develops this texture well. Dark bands (schistose bands) typically form from ferromagnesian minerals, while light bands (granulose bands) form from quartz, feldspar, or carbonate minerals.
Non-foliated texture:
This texture contains no foliations and does not have parallel planes of minerals. The common types of non-foliated textures are given below:
Granulose texture:
A texture characterized by nearly Equi granular metamorphic mineral constituents (quartz, feldspar, calcite, etc.). It is formed by the recrystallization of pre-existing rocks under uniform pressure and great heat.
Mylonitic texture:
It is commonly applied to the fabric of mylonites with isolated large grains or aggregates, quartz, or feldspar enclosed in a cataclastic fine-grained matrix. It is sometimes called porphyroblast texture.
Hornfelsic texture:
A fine-grained granular or spotty texture. Sometimes, it may show a massive character. When argillaceous rocks are metamorphosed due to contact metamorphism, the carbonaceous matters are recrystallized, and these textures may be found.
Other types of texture:
Augen texture:
A texture showing evidence of shearing of the mineral crystals or grains or different movement of grains due to exceeding the clastic limit.
Porphyroblastic texture:
An uneven texture with larger crystals embedded with a smaller groundmass.
Poikiblastic texture:
An uneven texture with smaller crystals embedded with a large groundmass.
