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Metamorphic Rocks
Metamorphism is when a rock partially melts at grain contact points due to an
increase in heat and/or pressure. The reactions which occur are reversible, this
is known as metastability. As rocks get buried they increase in temperature
due to the increase in geothermal gradient (heat change per km depth).
Metamorphism changes rocks but not their bulk chemistry; mineral changes
occur by new mineral growth, mineral alignment and recrystallization. These
can change characteristics such as colour, hardness and brittleness but the
overall chemistry of the rock remains the same (no elements are added or
removed). Large new formed crystals in metamorphic rocks are called
Porphyroblasts and often give metamorphic rocks their name. The amount of
metamorphism that occurs is simply the metamorphic grade i.e. Gneiss has a
higher metamorphic grade than slate or schist.
There are 3 types of metamorphic rocks:
 Thermal/contact
 Regional
 Dynamic
Thermal/ contact metamorphism
This is metamorphism on a local scale due to heat from an intruding igneous
body and creates baked margins. Spotted rock and Hornfels both form from
the contact metamorphism of shale/mudstone. Hornfels is of higher
metamorphic grade than Spotted rock. If you metamorphose Shale the bulk
chemistry is Aluminium Silicate (Al2SiO5).
Regional metamorphism
The second type of metamorphism occurs on a huge scale. It occurs due to an
increase in both heat and pressure. Heat creates recrystallization in the rock
whilst pressure creates mineral alignment known as foliation. Slate, Phyllite,
Schist, Gneiss and Migmatite are all regionally metamorphosed rocks with
increasing grade. As metamorphic grade increase new minerals form known as
index minerals such as Garnet in Schist, the appearance of the new minerals
indicate a change in metamorphic facies.
Dynamic
The final type of metamorphic rock occurs at fault lines and is the smallest
scale metamorphic rock. This occurs due to a small amount of heat and
pressure and forms rocks such as Millonite and Fault Breccia.
Different minerals are stable at different temperatures and pressures;
metastability. The degree at which a particular mineral is stable is known as its
stability field, minerals that exist outside there stability field will revert back to
their previous state or into other metamorphic minerals, for example
Andalusite can change into Silliminite and kyanite whilst also being able to
revert back to previous minerals like Mica and Feldspar. However this is
unlikely because water vapour is needed to revert them back in the retrograde reaction which is lost in the pro-grade reaction when the minerals are
metamorphosed.