Composition of system - aluminous silicates.

Three common polymorphs of Al2SiO5 are common index minerals. Their stability space is shown in the underlying P-T diagram/map.

• andalusite
• kyanite
• sillimanite

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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