One woman's quest for geological understanding!

Kyanite

This is one of my favourite pieces – especially because it also contains  garnet, black tourmaline and mica!  It was a gift from my mother a few years ago, so no idea where it was collected…

  • Chemical Formula: Al2SiO5
  • Crystal symmetry: Triclinic
  • Crystal habit: Columnar; fibrous; bladed
  • Crystal system:Triclinic pinacoidal
  • Twinning: Lamellar on {100}
  • Cleavage: [100] perfect [010] imperfect with 79° angle between
  • Fracture: Splintery
  • Tenacity: Brittle
  • Mohs scale hardness: 4.5-5 parallel to one axis, 6.5-7 perpendicular to that axis
  • Luster: Vitreous to pearly
  • Streak: White
  • Pleochroism: Trichroic, colorless to pale blue to blue

Kyanite is one of  which are useful indicators of the conditions of metamorphism in metamorphic rocks.   (A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals.)

The Al2SiO5 polymorphs are useful index minerals because they are very common in aluminous metamorphic rocks.  The presence of one of the polymorphs gives a broad indication of the pressure-temperature regime that the metamorphic rock attained.

Kyanite is formed at high-pressures and low-temperatures.

Kyanite Kyanite

(The coexistence of two polymorphs in a rock provides more specific pressure-temperature information because they are restricted to a line in pressure-temperature space.  The coexistence of all three polymorphs defines a specific pressure and temperature of metamorphism.)

 

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