Textures of Metamorphic Rocks

Metamorphic rock textures are the result of the physical and chemical changes that occur to a rock when it is subjected to high heat and pressure. These changes can cause the minerals in the rock to recrystallize, grow, and align in new ways. Metamorphic rock textures are classified into two broad groups: foliated and non-foliated.

Metamorphic rocks are formed by the transformation of existing rocks under high temperatures and pressures. The textures of metamorphic rocks are determined by the type of minerals present, the grain size of the minerals, and the orientation of the minerals.

Metamorphic Rock Textures

The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized.

Metamorphic Rock Textures

Metamorphic rock textures can be divided into two broad categories: foliated and non-foliated.

Foliated Metamorphic Rocks Textures

Foliated textures are characterized by the parallel alignment of mineral grains. This alignment gives the rock a banded or layered appearance. Foliated textures are common in metamorphic rocks that have been subjected to high pressure and/or directed stress.

Schistose Texture

Schistose texture is a foliated metamorphic texture that is characterized by the parallel alignment of platy or needle-like minerals. Schistose rocks are typically fine- to medium-grained and can be split into thin layers along the planes of mineral alignment. Schistose rocks are common in areas of regional metamorphism, where rocks have been subjected to high temperatures and pressures. Common schistose rocks include mica schist, amphibole schist, and quartz schist.

Common rock types are:

Schist - grains can be seen without using a microscope
Phyllite - all grains of the ground-mass are microscopic, but cleavage surfaces have a sheen caused by reflection of platy or linear minerals. Commonly corrugated.
Slate - grains are microscopic, very cleavable, usually tougher than shale.

Gneissic Texture

Gneissic texture is a foliated metamorphic texture that is characterized by the alternating bands of light and dark minerals. Gneissic rocks can be either fine- or coarse-grained and can be split into layers along the planes of mineral alignment. Gneissic rocks are common in areas of high-grade metamorphism, where rocks have been subjected to very high temperatures and pressures. Common gneissic rocks include biotite gneiss, hornblende gneiss, and augen gneiss.

Non-foliated Metamorphic Rocks Textures

Non-foliated textures are characterized by the absence of parallel alignment of mineral grains. These rocks are typically more massive and blocky than foliated rocks.

Granoblastic Texture

Granoblastic texture is a non-foliated metamorphic texture that is characterized by the interlocking of equal-sized mineral grains. Granoblastic rocks are typically medium- to coarse-grained and have a smooth, even texture. Granoblastic rocks are typically formed under high temperatures and pressures, where the mineral grains have recrystallized to form a new rock with a distinctive texture. Common granoblastic rocks include marble, quartzite, and hornfels.

Hornfelsic Texture

Grains are irregular and generally interlocking, and microscopic. Hornfelsic texture is a non-foliated metamorphic texture that is characterized by a very fine-grained, dense texture. This texture is common in metamorphic rocks that have been formed by contact metamorphism, where rocks have been heated by hot magma or lava. Rocks of this type are called “Hornfels”.

Cataclastic Texture

Cataclastic texture is a non-foliated metamorphic texture that is characterized by the presence of angular, broken fragments of minerals. Cataclastic rocks are typically formed by the dynamic metamorphism of rocks that have been subjected to high shear stress. The ground-mass is usually rock flour. This texture is common in metamorphic rocks that have been formed by dynamic metamorphism, where rocks have been subjected to high pressures and shear stresses. Common rock type is “Mylonite”.