Why are some minerals useful as metamorphic index minerals, while ...

[mynx]

Why are some minerals useful as metamorphic index minerals, while other minerals are not?
 
  What will be an ideal response?

Question 2

Define pressure, stress, and strain. How do they relate to one another?
 
  What will be an ideal response?

[bookworm410]

Answer to Question 1

A mineral is useful as a metamorphic index mineral if it occurs only in a restricted range of temperature and/or pressure. A mineral is not useful as a metamorphic index mineral if to occurs over a wide range of temperature and pressure.

Answer to Question 2

Pressure and stress are defined as the magnitude of a force divided by the area of the surface on which the force is applied.
Normal stress is the force applied perpendicular to a surface. Shear stress is the force applied parallel to the surface.
Pressure increases with increasing force exerted by the weight of overlying rock, at depth, and where force is equal in all three dimensions. A special type of normal stress.
Strain is the deformation of rock as a result of applied stress.