Kyanite: The Blue Mineral With Unique Characteristics

Kyanite is a striking mineral known for its brilliant blue color and unique crystal structure. As a member of the silicate mineral family, it is characterized by long, columnar crystals that typically form in blade-like shapes.

Kyanite from Barra do Salinas, Coronel Murta, Minas Gerais, Brazil (Stan Celestian)

While its vibrant blue hue is most common, kyanite can also be found in shades of green, orange, black, and colorless. Beyond its beauty, kyanite’s varying hardness and formation conditions make it a mineral of significant geological interest.

Physical Properties and Identification

  • Classification: Silicate mineral
  • Chemical Composition: Al₂SiO₅ (aluminum silicate)
  • Color: Typically blue, but can also be white, gray, green, or colorless
  • Streak: White
  • Hardness: Variable—4.5-5 parallel to the length of the crystal, 6.5-7 across the width
  • Cleavage: Perfect in two directions
  • Fracture: Splintery to uneven
  • Luster: Vitreous to pearly
  • Transparency: Transparent to translucent
  • Crystal System: Triclinic

Kyanite’s unique anisotropic hardness, which varies along different axes of the crystal, makes it both fascinating and challenging for gem cutters. Along the length of the crystal, it has a hardness of about 4.5 to 5 on the Mohs scale, while across the width, it can be as hard as 6.5 to 7.

Kyanite with Staurolite (Stan Celestian)

Formation and Geology

Kyanite forms under high-pressure, low-temperature conditions in metamorphic rocks such as schist and gneiss. It often appears alongside other metamorphic minerals like garnet and staurolite. Due to its stability under these conditions, kyanite is used as an indicator mineral to infer the pressures and temperatures that formed the host rock.

Black and blue kyanite blades

Uses and Significance

Kyanite’s properties make it valuable in several industrial applications, including the production of refractory materials and high-temperature ceramics. In gemology, its deep blue varieties are particularly prized, though its perfect cleavage and variable hardness make it a challenging material to work with.

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