Kyanite – A Polymorph with Differential Hardness
Gemmologists like kyanite, lapidaries do not! It is a gemstones that appears on most Professional Gemmologist’s theory or practical exams. Give an example of a gemstone that shows differential hardness? Name a polymorph of andalusite and sillimanite? Name a gemstone that has a bladed habit? The answer to all those questions is kyanite!
Kyanite is a Polymorph of Andalusite and Sillimanite
A polymorph sounds like a character from an X-men movie or one of those Transformer toys that is a truck and then you change it into a robot. In gemmology a polymorph is a gemstone that has the same chemical composition as another gemstone, but has a different crystal structure. Kyanite is a member of the aluminosilicate series and has a chemical composition of Al2SiO5. Two other gemstones with the same chemical composition and members of the aluminosilicate series are andalusite and sillimanite. Andalusite and sillimanite are called polymorphs of kyanite because they have the same chemical composition as Kyanite.
Why do andalusite, sillimanite and kyanite all have different crystal structures?
As you can see from the diagram when the temperature and pressure are both high the mineral formed with the chemical composition of Al2SiO5 is sillimanite. If the pressure is high, but the temperature is not as high, then kyanite is formed. When the temperature is high, but the pressure is not high, then andalusite is formed.
All three gemstones have the same chemical composition, but their internal crystalline structure is different due to the conditions when they were formed.
Bladed Kyanite has Differential Hardness
Many crystalline materials have physical and optical properties that differ depending on the direction. Think of tourmaline crystals that can be a lovely green in one direction and almost black in another. Kyanite has a bladed habit, this means that the crystals of kyanite in their natural state are elongated and flattened like a blade of grass. Kyanite is very unique in that it has a wide variation in hardness in the same crystal face. 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. Other minerals usually have variable hardness on different crystal faces due to a different concentration and orientation of the atoms in the structure.
Differential Hardness Makes Kyanite Very Difficult to Cut
Kyanite is used for purposes other than as a gemstone. Kyanite is widely used in the manufacture of glass, burner tips, spark plugs, heating elements and high voltage electrical insulators and in the ceramic industry.
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I plan to introduce different gemstones alphabetically. I will start with “A” (Ammolite) and see where it leads me. Leave a comment and let me know if you have a favourite gemstone that starts with the letter “L”. Take the poll! Can you name a reference to a gemstone in a song, movie or book?