Mylonite

Classification：Metamorphic Rock

The rock formed by strong crushing and plastic deformation is formed in the high temperature shear zone of the deep crust.

Mylonite particles are very thin and banded distributed dynamic metamorphic rocks. Most of the minerals in the rock can't be identified by the naked eye. The original coarse-grained rock (granite, etc.) is broken into powder (gouge) by strong directional pressure, and then cemented to form hard rock, and there is no much change in mineral composition with the original rock. It is mainly distributed in the reverse fault and the translational fault zone. Low strength, easy to cause leakage and formation of weak intercalation are unfavorable for stability of rock mass.

Basic features:
Mylonite is composed of matrix and speckle. The matrix is mainly composed of quartz and mica mineral particles, dense and hard, and mineral particles can not be identified by naked eyes. Most of the broken spots are hard minerals such as feldspar. In the rock, secondary foliation, lineation and other plastic flow structures develop.
Different minerals in Mylonite often have different morphologies and deformation characteristics. Quartz is relatively easy to occur plastic deformation, often rectangular, or small recrystallized particles around the patch, forming a typical structure of Mylonite -- core mantle structure. The ribbon (ribbon) structure of quartz is common. The fining of quartz and other minerals is mainly through the dislocation creep mechanism of the lattice, which is a dynamic recrystallization effect under stress. The hard minerals, such as feldspar, are large in general particles and often appear with residual spots, often characterized by brittle fracture, and the phenomena of wave form extinction and mechanical double crystals can be formed by dislocation slip. 

The Mylonite series of rocks are divided into two categories:
(1) Mylonite, the rock of fine ductile shear deformation is divided into initial Mylonite, Mylonite and super Mylonite according to its ductile matrix content, and the matrix is 10 ~ 50% and 50 ~ 90% and 90 ~ 100% respectively. It is mainly composed of layered silicate minerals, such as mica, chlorite and so on. Mylonite is called thousand grain Mylonite, or referred to as "thousand mylonite".
(2) the coarse-grained ductile deformable rock characterized by grain growth, and its grain ductile deformation structure has been completely or almost all replaced by recrystallized grain, and the strain effect in grain is eliminated because of recrystallization in Mylonite. Asymmetric larger eyeball phenocrysts or residual phenocrysts are embedded in granite ductile matrix. The residual Mylonite belts are often accompanied by metasomatism, mixing and even granitization.

Structure:
Mylonite often has stripes and laminar structures, and the formation of bands and laminae is caused by mineral composition, color and particle size. Mylonite is also common in some new minerals, such as chlorite and sericite, muscovite, epidote, talc and serpentine. These minerals are regularly aligned, resulting in more obvious stripe structure.

Form：
Mylonite is dense and hard, mainly composed of granite and quartz sandstone, and some new minerals associated with it, such as chlorite, sericite, serpentine and so on.
Mylonite, the rock of fine ductile shear deformation is divided into initial Mylonite, Mylonite and super Mylonite according to its ductile matrix content, and each matrix accounts for 10 ~ 50%, 50 ~ 90%, 90 ~ 100%. Mainly composed of layered silicate minerals, such as mica and chlorite, Mylonite is called thousand Mylonite.
The residual Mylonite, with coarse grain ductile deformation characterized by grain growth, has been replaced by the recrystallized grain in all or almost all grain ductile deformation structures, and the strain effect in grain is eliminated because of recrystallization. Asymmetric larger eyeball phenocrysts or residual phenocrysts are embedded in the ductile matrix of granitic texture. The residual Mylonite belts are often accompanied by metasomatism, mixing and granitization.

Formation:
Mylonite is often formed from granitic rocks and sandstone. Therefore, the main mineral components are quartz and feldspar, which are often compressed and elongated, and the quartz grains can also appear in the wavy extinction bands of parallel optical axes. In grinding matrix, there are sometimes larger quartz and feldspar single grains (or debris), or eyeballs formed by the two. In the body of the eyeball, we also see the bending of the wave extinction and cleavage twinning.