Holdaway, M. J. Department of Geological Sciences, Southern Methodist University, Dallas, Texas.
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A nesosilicate mineral, Al2SiO5, crystallizing in the triclinic system and occurring in metamorphic rocks. It is essentially a pure phase, but minor amounts of iron (Fe3+), chromium (Cr3+), and titanium (Ti4+) may substitute for aluminum (Al). The structure of kyanite is based on cubic close-packed oxygens (O). Ten percent of the tetrahedral (fourfold) interstices are filled with silicon (Si), and 40 percent of the octahedral (sixfold) interstices are filled with Al. The Al, with O at the corners, occurs in zigzag edge-sharing chains of Al-O octahedra. Si-O tetrahedra share corners with Al-O octahedra along the sides (Fig. 1). This structure is about 10 percent denser than that of the other two Al2SiO5 polymorphs, sillimanite and andalusite, making kyanite the high-pressure polymorph.
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