Geology And Geophysics

Compositional and Mineralogical changes that Occur in Metamorphic Rock



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Metamorphic rocks are rocks that have been changed by heat, pressure, chemicals, or any combination thereof. The original rocks are transformed into new types of rocks. Igneous, sedimentary, and older metamorphic rocks can undergo metamorphism. The changes the rocks experience are in texture and composition. Some rocks when subjected to metamorphism do not change composition, like sandstone and limestone, because their original chemistry is too simple. Those rocks will experience changes in texture. Shale, on the other hand, is more complex and can experiences both types of change.

The compositional, or mineralogical, changes that take place when metamorphic rocks are formed are determined by the pressure and temperature reached, the composition of fluid present in the process, and what the original rock was made of. Temperature not only causes chemical bonds to break and reform, but also causes the formation of crystals. Crystals are more thermodynamically stable than grainy rocks, meaning they can undergo more heat. The compositional changes resulting from pressure are also chemical reactions where the minerals in the rocks are trying to become more stable. The chemical make-up will realign to create different structures made up of the same elements. If a fluid is present, containing new minerals, they will be incorporated into the chemical reactions when they seep into the pores of the rocks undergoing reactions. The latter is the only way new elements will enter the rock, otherwise the composition only experiences changes in chemical structures. A simple change in structure can result in something as subtle as a shift in color from the parent rock to the new metamorphic one.

There are a number of minerals that are found commonly in metamorphic rocks (e.g. calcite, garnet, staurolite, sillimanite, talc, kyanite). Kyanite (Al2SiO5) occurs strictly in rocks exposed to extreme pressure. It is most often found in schist, a metamorphism of basalt, shale, or slate. Asbestos, which is associated with lung disease and mesothelioma, is a product of metamorphism. It is a fibrous mineral found in serpentinite, or “snake rock,” so named because the asbestos tends to form veiny lines running through the rock. 

Studying the composition of metamorphic rocks can shed light on the composition of the parent rocks and also where and at what temperature the transformation took place. This is important to geologists because it sheds light on changes that have occurred in the earth over time. The same type of parent rock can form a variety of types of metamorphic rocks, depending upon the conditions. Low temperatures and pressures will form slate and phyllite, which contain chlorite, muscovite, and biotite. Greater pressure makes schist, often with garnet and staurolite. More extreme pressures and temperatures result in gneiss and migmatite, containing sillimanite.

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