What is it?
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. It is one of the three allotropic varieties of mineral corresponding to the formula Al2O3 SiO2. The other two are an adalusite and kyanite. They all contain 63.2% Al2O3 and 36.8% SiO2. They are distinguished from one another by their modes of occurrence and physical and optical properties.
Sillimanite consists of chains of aluminum octahedra (green) joined by chains of alternating silica and aluminum tetrahedra (yellow and blue).
The first image shows a cross section of the structure.You can see how the tetrahedra bridge the gaps between neighboring octahedra.
The second images is slightly inclined to the cross section which showing four layers of the structure. Note how the tetrahedra that bridge the gaps between the octahedra alternate.
The final image in the picture shows the view from the side of the region which is enclosed in the box.
- Tactile properties: It is rough and abrasive
- Visual properties:
- Color: It is in grey, white, brown and yellow color.
- Lustre: It is vitreous.
- Streak: It is white.
- Diaphaneity: It is transparent to translucent
- Structural properties:
- Hardness: 6-7 mohs
- Specific gravity: 3.24
- Affinity to water: It is hydrophobic and Insoluble in water
- Heat properties:
- It has low thermal conductivity (9.10 W/m/K).
- It is resistant to dimensional changes due to heat 1.40 × 10–5/K.
- It has high specific heat capacity (124.52 J/M/K)
- It can withstand temperatures upto 1550 ºC after which it decomposes to mullite.
- Melting point: 3290 °F
- Electric properties: It does not conduct electricity.
- Combustion properties: It is non explosive and non inflammable.
- Chemical properties: It does not dissolve in acids or alkalis.