The Barrovian sequence — chlorite, biotite, garnet, staurolite, kyanite, sillimanite — records progressively higher pressure-temperature conditions. Chlorite appears at low grade (greenschist facies); sillimanite is the highest-grade aluminosilicate polymorph. Confusing the order is a common error, especially with the biotite/garnet transition.
Question 2 True / False
Foliation in a metamorphic rock directly records the orientation of maximum compressive stress at the time of formation.
TTrue
FFalse
Answer: False
This is a classic misconception. Foliation reflects the orientation of platy minerals (like mica) that grew or rotated during deformation, but the relationship between foliation planes and the principal stress axes is geometrically complex. Foliation typically develops perpendicular to the maximum shortening direction, but strain history, mineral rotation, and subsequent deformation can all modify that relationship.
Question 3 Short Answer
How does marble differ from limestone if both consist primarily of calcite, and what process accounts for that difference?
Think about your answer, then reveal below.
Model answer: Marble forms when limestone undergoes solid-state recrystallization under heat and/or pressure. The calcite grains grow larger and interlock, obliterating original sedimentary structures like bedding and fossils. The key difference is texture — coarse interlocking crystals in marble versus finer, often layered grains in limestone — produced by metamorphic recrystallization without melting.
This tests understanding of the core concept: metamorphism transforms mineralogy and texture through recrystallization, not melting. Both rocks are calcite-dominant, so the distinction is entirely textural and structural, which reinforces what metamorphism actually does.