The Guide to Rocks and Minerals

In focus: Kyanite

Chemical Composition: Al2SiO5
Group: Silicate
Colour: Blue, White, Gray, Green, Black
Streak: white
Hardness: 5.5-7
Density: 3.5-3.7
Luster: Vitreous
Cleavage: Perfect x1
Temperature resistance: high

Kyanite is an anhydrous aluminosilicate mineral Its name comes from the Greek 'kyanos' meaning 'blue'. Most Kyanites are indeed blue but some Kyanite specimens may be white, transparent, or even brown.

Kyanite has two polymorphs: Andalusite and Sillimanite. A polymorph is a mineral that shares the same chemistry with anotherbut has a different crystal structure. While all three minerals share the same chemical composition (Al₂SiO₅) they differ considerably in their crystal structures. Whereas in Andalusite, the Al octahedral chains are also linked by alternating Al octahedra and Si tetrahedra in a 3-D lattice and in Sillimanite, these Al octahedral chains alternate with Si tetrahedra; in Kyanite, the octahedra in the Al octahedral chains are more ordered and more compact than in Sillimanite or Andalusite. Thus Kyanite is the high pressure polymorph.

Kyanite is usually found in aluminium-rich metamorphic rocks that were formed out of sedimentary rocks with a high clay content. It is usually associated with its two polymorphs but also quartz, staurolite, micas and garnets. Studies have shown that each of the polymorphs forms under very specific temperature/pressure conditions. Relative to each other, Kyanite forms in a lower temperature/higher pressure environment; Andalusite forms in a lower temperature/lower pressure environment, and Sillimanite forms in a higher temperature/higher pressure environment.

Mineral uses

used to make a variety of refractory materials (materials which withstand high temperatures) e.g.. refractories for the production of steel
refractories to produce glass and heat-resistant ceramics
to make spark plugs
because it does not conduct electricity, it is also used in the manufacture of electric insulators
as a gemstone (to a limited extent)
Kyanite is also a collector's mineral

One characteristic feature of Kyanite is an uneven hardness across the crystal structure. The hardness of Kyanite is approximately 4.5 when scratched parallel to the long axis of the crystal and approximately 6.5 when scratched perpendicular to or across the long axis. The phenomenon is called anisotropism. Although anisotropism is not unique to Kyanite, because of the significant discrepancy in hardness, in the case of Kyanite it can be considered an identifying characteristic.

_______________________________
Home	\|	Shopping	\|	Database