Map of phase changes for shales
Composition of system - aluminous silicates.
Three common polymorphs of Al2SiO5 are common
index minerals. Their stability space is shown in the underlying
While this map is incomplete it does show the
approximately correct position of the phase change boundaries.
You can use it to answer the following questions.
- Which of the two paths shown probably characterizes
movement through crust with a higher geothermal gradient and
which with a lower geothermal gradient?
- How might the retrograde and prograde paths
differ and how does this explain why mineral assemblages from
great depth are exposed at the earth's surface (i.e. why doesn't
the retrograde path cause them to change to minerals stable at
the lower P and T they experience upon ascent)?
- If you were to consider weathering as a metamorphic
reaction would it be a prograde or retrograde one?
- If you were to consider diagenesis and lithification
as a metamorphic reaction would it be prograde or retrograde?
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