What is metamorphic grade, and present an example of how a rock ...

[SGallaher96]

What is metamorphic grade, and present an example of how a rock reaches its metamorphic grade? What will be an ideal response?

Question 2

What are index minerals, and what are they used for? Give an example of minerals that would be
  found in a low-grade, intermediate, and high-grade metamorphosed shale. What will be an ideal response?

[djofnc]

Answer to Question 1

Metamorphic grade is a term that characterizes the degree to which a rock has undergone metamorphic
change. When shale is metamorphosed, chlorite first begins to crystallize at about 200C. If the
metamorphism stops there, the rock will be low grade. If temperatures and pressures increase, new
minerals replace chlorite, and if temperatures exceed 500C, sillimanite will appear.

Answer to Question 2

Index minerals are minerals that are known to form only within specific temperature and pressure
ranges. Their presence allows geologists to establish the metamorphic grade of a rock because the
highest-grade mineral indicates the temperature and pressure the rock was at for some time. Low-grade
minerals include chlorite, muscovite, and biotite; intermediate include muscovite, biotite, garnet,
staurolite, and kyanite; and sillimanite indicates high grade