Limestone is tricky when it changes. If it is completely pure and undergoes contact metamorphism, you get nice white marble. If there are impurities, other reactions will occur. Silica as a contaminant leads to wollastonite, which has calcium silicate in it. Aluminum as a contaminant will lead to anorthite in the rock, which is calcium aluminum silicate. Finally, dolomite, which is a calcium magnesium carbonate, provides a metamorphic rock that has pure calcite or calcium carbonate in it but also has periclase, which is magnesium oxide. Mix this with silica and then you get calcite and magnesium silicate, which is called forsterite.
METAMORPHIC ENVIRONMENTS You need to understand that the temperature and pressure will vary with location and circumstances for any rock undergoing a metamorphic change. There will be a magic combination of these that will create the different things you see. Geologists have created a pressure temperature grid that shows the relationship between different pressures and temperatures involved in changing rock. This is called the P-T diagram. It looks complicated but it provides a range of temperatures and pressures necessary to create the different rock types. A geologist named George Barrow lived in the 1800s and mapped out regional metamorphic zones and designed the Barrovian sequence that named rocks in order of increasing metamorphism. It starts with the chemical chlorite seen in slates and phyllites. Then it goes to biotite, also seen in phyllites but also in schists. Next is garnet, seen only in schists. The ones with higher grades of metamorphism are staurolite and kyanite, seen in schists. The highest grade is called sillimanite, seen in schists and gneisses. This fits well with where he lived in Scotland. Green coloration or greenschist rocks are seen in low pressure and temperature situations. Chlorite, epidote, and serpentine are all green so the schist will be green. These rocks are called schists because there is muscovite mica in them. There are many environments that nuance the rock types coming out of metamorphism. We talked about some of these in the beginning but now that you know these rocks 162
