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Metamorphic rocks are rocks that were originally formed under a particular set of temperatures, pressures, and chemical conditions and then have been "remade" under a different set of these same conditions. Some examples are schists, gneiss, marble, and quartzite.
Metamorphic Minerals: Metamorphic minerals include kyanite, sillimanite, staurolite, and garnet. Other minerals may be found in metamorphic rocks including olivine, pyrooxenes, amphilboles, micas, feldspars, and quartz but these minerals may or may not be a result of the metamorphism.
Recrystallization: The particle size recrystallizes in metamorphic rocks due to different temperatures and pressures that causes changes in the original crystal structure. An example would be the small crystals of limestone changing to the large crystals of marble.
Foliation: Foliation is the term used for layering in metamorphic rocks. When force is applied to a rock in only one direction it causes the minerals within the rocks to grow crystals with their long axis perpendicular to the direction of the force. When the rock is viewed it is said to show bands where the recrystallized minerals have grown. Examples are tourmaline and mica. Slate is an example of a fine grained foliated metamorphic rock.
Solid State Reaction: A solid state reaction occurs when two different minerals next to each other react to form a new mineral when subjected to high temperatures. As the two minerals are super-heated, they migrate across their respective boundaries forming a new mineral at the boundary. The diffusion across the boundary continues until both of the original minerals are "used up" forming the new mineral.
Metasomatism: The change of a rocks chemical composition by the introduction of outside chemicals. These chemicals are sometimes introduced by the action of water. One important point to remember is that much of the mineralization of a particular rock is a direct result of minerals being brought into the mix by water.
Contact Metamorphism: This is the process whereby a magma body such as granite intrudes into surrounding rock and changes the chemical composition of the rocks at the intrusive boundary. One interesting fact of this type of chemical change is when granite intrudes into a limestone country rock. In this situation, metallic ore bodies tend to form such as gold and silver. Many gold mines are located at or near contact points of granite and limestone. One such area is the Fenner Hills near Needles California.
Regional Metamorphism: When there is a large introduction of magma across many square miles the resulting metamorphism is called a regional metamorphism or regional change. When this happens the overlaying rock will recrystallize with a new chemical composition. The original chemical composition may not be identifiable.
References:Introduction to Physical Geology by Millet and A Guide to Field Indentification of Rocks and Minerals by Charles A. Sorrel.
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