Metamorphic Textures

There are three basic types of metamorphic textures, both of which form in a response to minerals trying to become stable under metamorphic conditions.

 

Foliated texture

The foliated texture developes as the minerals try to realign themselves due do compressional or shear stress. This can occur in a few ways:

1. Rotation of platy and/or elongated minerals
   
   
   
   
2. Changing equidimensional grains into elongated shapes that are aligned
   
   
   
   
3. Recrystallization of minerals in the direction of preferred orientation

Here is a link to an animation explaining these three processes: https://courses.nus.edu.sg/course/geosnk/internet/animations/resources/anim/FoliationProcesses_GL.html

As the degree of metamorphism increases, so does the amount of foliation. Low grade metamorphism produces just enough foliation to see it; slightly more, like with a phyllite, and the layers become more pronounced and platey minerals like micas start to become large enough to be seen with the naked eye. Minerals in medium grade metamorphic conditions are much larger and usually can be easily identified. Garnets, amphiboles, and kyanite form porphyroblasts (see below) in schists. Finally, high grade metamorphism produces a texture called compositional banding, where minerals recrystalize into layers, alternating between mafic minerals in one layer, and felsic ones in another.

Compositional banding can form in one of three processes:

1. Original layering in the protolith. (not common)
2. Extensive high T shearing. This occurs during shearing or folding of the protolith. Mafic minerals in the protolith strech and flatten parallel to the shearing direction. This creates alternating layers of mafic and felsic minerals in the gneiss.
3. Solid-state chemical differentiation. Minerals start to dissolve, and the ions migrate and congregate with similar ions. Thus, felsic minerals will grow in one location, mafic ones in another, creating bands of felsic and mafic minerals.

 

Nonfoliated texture

This texture contains equidimensional crystals, and often resembles a coarse-grained igneous rock. Because all of the minerals are the same, deformation is minimal.

 

Porphyroblastic textures

Large grains, called porphyroblasts, are surrounded by a fine-grained matrix of other minerals. Porphyroblasts are typically garnet, staurolite, and/or andalusite.