Metamorphic Rock Identification

Metamorphic Rocks

Metamorphic rocks are rocks that have undergone a change from their original form due to changes in temperature, pressure or chemical alteration. The classification of metamorphic rocks is based on the minerals that are present and the temperature and pressure at which these minerals form. Determination of this information is not easily accomplished in this lab. Therefore, a simplified system is used based on texture and composition.


Texture is divided into two groups. Foliated textures show a distinct planar character. This means that the minerals in the rock are all aligned with each other. This planar character can be flat like a piece of slate or folded. Non-foliated textures have minerals that are not aligned. Essentially, the minerals are randomly oriented.


Foliated textures show four types of foliation. Slaty cleavage is composed of platy minerals that are too small to see. Typically, these rocks split along parallel, planar surfaces. Phyllitic foliation is composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye. The larger size gives the foliation a slighly shiny appearance. Schistose foliation is composed of larger minerals which are visible to the unaided eye. Platy minerals tend to dominate. Gneissic banding is the easiest of the foliations to recognize. It is composed of alternating bands of dark and light minerals.


Non-foliated textures are identified by their lack of planar character. Further identification of non-foliated rocks is dependent on the composition of the minerals or components in the rock. Anthracite coal is similar to bituminous coal. Both are black in color , and is composed of carbon. Anthracite coal is generally shiny in appearance and breaks with a conchoidal fracture (broken glass also shows this type of fracture). Metaconglomerate is composed of pebbles and gravel that have been flattened due to directed pressure. Quartzite is composed of quartz sand grains. Quartz has a hardness of 7, which makes it difficult to scratch. Marble is composed of calcite and will readily react to a small drop of HCl.

Metamorphic Rock Identification Chart
Foliated slaty mica Regional Mudstone Slate
phyllitic quartz, mica, chlorite Regional Mudstone Phyllite
schistose mica, quartz Regional Slate Schist
schistose amphibole, plagioclase Regional Basalt or Gabbro Amphibolite
gneissic banding  feldspar, mica, quartz Regional Schist Gneiss
 Non-Foliated   carbon Contact or Regional Bituminous Coal Anthracite Coal
  quartz, rock fragments Contact or Regional Conglomerate Metaconglomerate
  calcite Contact or Regional Limestone Marble
  quartz Contact or Regional Sandstone Quartzite

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