Geologic Processes and Mineral Changes

• Metamorphic events can range in intensity from low-grade to high-grade. For instance, mountain building is a regional metamorphic event and is impacted by extreme heat and pressure. Thus, a mountain building event is a high-grade metamorphic event. The rocks in this area are affected by extreme folding and deformation. The extent of foliation is determined not only by the parent rock, but also by the degree of metamorphism.
  
  

Formation of Foliated Textures

• Foliated rocks, such as this gneiss show a preferred parallel alignment of mineral grains.Hemera Technologies/PhotoObjects.net/Getty Images
  
  Foliated metamorphic rocks have a parallel and flat mineral arrangement, giving the rock a flaky or banded appearance. This texture forms in all types of metamorphic environments, but the resulting rock is determined by how intense the heat and pressure is. The heat and pressure can cause several different things to happen to the mineral grains within the parent rock including rotation of the mineral grains into alignment or recrystallization of minerals in a preferred direction. Compressional stress can actually squash rounded mineral grains into an oblong shape, which are also aligned in a preferred direction.
  
  

Foliated Textures

• Shiny micas stand out in schist.Jupiterimages/Photos.com/Getty Images
  
  There are three basic types of foliated textures. Each is formed from the same parent rock, shale, when increasing pressure and temperature act on the rocks. When low-grade metamorphism is applied to shale, which contains clay and mica minerals, it creates a rocky or slatey cleavage. Indeed, it is named after the rock which displays it best -- slate. This type of foliation causes sheets of the rock to split along flat, planar surfaces.
  
  Under increased pressure and temperature, the mica and chlorite in slate will grow large enough to be seen with the naked eye while still maintaining its parallel alignment. These rocks are called schist and the texture they display is termed schistocity. A schist can be identified by the shiny planar textures of the mica.
  
  Further subjecting schist to high-grade metamorphic events, minerals will migrate and separate themselves into layers. These layers give the rock a banded characteristic and is referred to as a gneissic (pronounced "nice-ick") texture. Rocks displaying this texture are also known as gneiss (pronounced "nice").
  
  

Nonfoliated Textures

• Marbles are nonfoliated metamorphic rocks created from its parent rock, limestone.John Foxx/Stockbyte/Getty Images
  
  Metamorphic rocks without mineral alignment and preferential banding are nonfoliated. Instead of splitting into layers, nonfoliated rocks will break into chunks. These textures occur in low-grade metamorphic events with minimal deformation. The parent rocks are typically composed of minerals that have equidimensional, or rounded, crystals such as calcite or quartz. For instance, limestone is a fine-grained sedimentary rock composed of calcite. When low heat and pressure are applied to it, larger interlocking crystals are allowed to form, which creates marble.