Variances in Pressure and Temperature

      
Pressure and temperature play an important role in the formation of minerals. Different minerals form at a variety of pressures and temperatures. There are certain minerals that prefer a low temperature and pressure environment while others prefer a very high temperature and pressure environment. In the example below, the following minerals are polymorphs of each other. This means that they are the same exact chemical formula, but they form at different temperatures and pressures.
"The phase diagram for this system  shows that each mineral is stable over a particular pressure and temperature range.  For example, andalusite forms only at pressures less than those of the “triple point”, that is, the single point in P-T space where all three phases can coexist.  Similarly, sillimanite can form only at temperatures greater than that of the triple point.  Additionally, this diagram shows that progressive heating at low pressure (P < 4000 bars)  produces first kyanite, then andalusite then sillimanite.  The transition from kyanite to andalusite is known as the andalusite isograd, while the transition from andalusite to sillimanite is called the sillimanite isograd. The phase diagram for this system  shows that each mineral is stable over a particular pressure and temperature range.  For example, andalusite forms only at pressures less than those of the “triple point”, that is, the single point in P-T space where all three phases can coexist.  Similarly, sillimanite can form only at temperatures greater than that of the triple point.  Additionally, this diagram shows that progressive heating at low pressure (P < 4000 bars)  produces first kyanite, then andalusite then sillimanite.  The transition from kyanite to andalusite is known as the andalusite isograd, while the transition from andalusite to sillimanite is called the sillimanite isograd."(Cashman, Oregon University)

phase diagram
Phase Diagram from http://serc.carleton.edu/NAGTWorkshops/mineralogy/mineral_physics.html#what
Graphite and diamond are a well known example of what differences in pressure and temperature can do. They are also polymorphs which are only made up of carbon. They are the same chemically but physically they are very different. On the Mohs hardness scale where 1 is softest and 10 is hardest, diamond is a 10 while graphite is a 1 or a 2. This is a huge difference. Graphite is harder than ones fingernail while a diamond is the hardest known mineral. At lower pressures graphite is the stable solid while diamond is only stable of a pressure of about 10^4 atm. Once the solid carbon is in the form of diamond it rarely ever will turn back to graphite. This is due to all the carbon bonds in the diamond structure. There is not enough energy for it to turn back. When this occurs is called a metastable mineral. Below is the specific pressures and temperatures for which diamond and graphite exist.
diamond phase diagram
Phase diagram from http://www.chem1.com/acad/webtext/states/changes.html

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