Metamorphic Rocks


Staff member
Jun 3, 2020
The metamorphic rocks are formed due to the transformation of preexisting rocks under pressure and temperature. The process is called metamorphism. The rocks undergo alteration in mineral composition, structure, texture and physical properties. The chemical composition the rock is generally unchanged. The mineral composition of the resulting metamorphic rock is similar to that of the preexisting rock, which makes it possible to define the origin and the processes responsible for the metamorphism of those rocks. The preexisting rocks may be of any type: igneous, sedimentary or metamorphic.

metamorphic rocks

Classification and types of metamorphic rocks
The metamorphic rocks have a variety of classifications. The following is a simple classification for easy identification of the rocks. According to the rock structure, the metamorphic rocks can be classified as:
  • Foliated
  • Unfoliated
According to the origin or type of preexisting rock:
  • Pelitics or pelitas
  • Quartz-feldspathic
  • Mafic or Basic
  • Ultramafic or Ultrabasic
Metamorphic rocks according to rock structure
Foliated metamorphic rocks - They are all those rocks that have a foliated or banded structure, which is, a preferential alignment of the minerals in the rock. The most common examples are: Slate, phyllite, schists (micaceous shale or graphite shale), gneiss, amphibolite, mylonites and migmatite.

Unfoliated metamorphic rocks - They are those that appear massive and crystalline, that is to say that no foliation or banding of minerals is observed in the rock. The most common examples are: Hornfels or corneanas, marble, quartzite, serpentinite and granules.

Metamorphic rocks according to the protolith
Pelitas or pelitics - They are those that are rich in clay minerals and tabular silicates such as illite, montmorillonite, chlorite and Muscovite, therefore, they are enriched in aluminum and potassium oxides. Some examples of these rocks are: slate, phyllite, micaceous schists, pelitic gneiss, migmatite and sometimes impure quartzite.

Quartz-feldspar - They are those rocks that come from the metamorphism of acidic igneous rocks such as granite and rhyolite, in addition, from the rocks that derive from their erosion, such as sandstones and arches. Some examples are: quartzite, metaquarcites, quartz schists and gneiss.

Mafic or basic - They are derived from basic igneous rocks such as basalt, diabase, gabros and also from basic tuff. Examples include: green shale, blue shale, amphibolite, eclogite, mafic granules.

Ultrabasic or Ultramafic rocks - They are formed by the metamorphism of ultrabasic igneous rocks such as peridotites. The common example is the serpentine.

Metamorphic rock textures
The texture in a metamorphic rock consists in comparing the mineral constituents and describing their size, arrangement and shapes, that is, the interaction between the mineralogical constituents.

Four basic types of textures can be defined that are derived from the main texture of a metamorphic rock known as crystalloblastic:
  • Granoblastic
  • Lepidoblastic
  • Nematoblastic
  • Porphidoblastic
In the granoblastic texture, the crystals form a mosaic of grains, more or less equidimensional, with a strong tendency to hexagonal packing. On the other hand, the lepidoblastic texture is defined by intergrown and homogeneously oriented laminar minerals (micas), parallel to each other. While the nematoblastic texture, it is defined by tabular and acicular minerals (generally amphiboles and plagioclase), interspersed and oriented homogeneously, with the major axes parallel to each other. Finally, the porphyloblastic texture presents larger crystals (generally garnets) surrounded by a fine oriented matrix.

However, in the nature of metamorphism nothing happens separately, so the textures in a metamorphic rock do not occur in isolation and rather appear combined in many cases, as follows: The overall texture is described first with that of the most dominant individual texture, followed by the rest (eg porphyry-grain-lepidoblastic).

Characteristics of metamorphic rocks
The main characteristics of metamorphic rocks are to observe the occurrence or absence of foliation. When there is no foliation, the metamorphic rock presents the granoblastic texture that represents some non-foliated rocks, such as marble, quartzite, granulite.

However, the formation of a metamorphic rock is almost always due to regional metamorphism or dislocation, this causes them to appear foliated. Foliation results from the orientation of micaceous minerals (chlorite, biotite, muscovite, graphite) and elongated prismatic minerals (amphibole and plagioclase), which occur perpendicular to the main compression stress.

Uses of metamorphic rocks
  • The metamorphic rocks used in the industry are shales, marbles and gneiss, because they have commercial value.
  • The first comes from clay sediments that have undergone a low-grade metamorphic process.
  • The present slate, allows obtaining thin sheets of up to 0.5 cm that are used in the construction of houses.
  • In the case of marble, the metamorphism processes have caused a recrystallization of the original limestone crystals.
  • Some limestone accumulations have been transformed into marble by thermal metamorphism, thanks to the proximity with igneous intrusions, the industrial use of this type of rock is usually mainly ornamental.
  • However, there are cases that marble can be used to make cement.
  • Other types of metamorphic rocks, such as schists and gneiss, are also used in the ornamental industry sector.
  • Quartzite is commonly used as a track aggregate and ballast.
  • Finally, varieties of metamorphic rocks are the sources of graphite, asbestos, talc, vermiculite.

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