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Geology QUESTION #10647
Question 1
Kyanite is a polymorph of which mineral group (sharing the formula $\text{Al}_2\text{SiO}_5$)?
  • Sillimanite✔️
  • Cerussite
  • Siderite
  • None of these
Correct Answer Explanation
Kyanite, sillimanite, and andalusite are three polymorphs of $\text{Al}_2\text{SiO}_5$ — same chemical formula, different crystal structures.
  • Kyanite: triclinic, high pressure, low–medium temperature
  • Andalusite: orthorhombic, low pressure, low–medium temperature
  • Sillimanite: orthorhombic, high temperature
Their stability fields on P–T diagrams define metamorphic grade and are critical geothermobarometers.