Andalusite
Andalusite is an
aluminium nesosilicate
mineral with the chemical formula Al2SiO5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form an
X or a cross in section.
A clear variety first found in
Andalusia,
Spain can be cut into an interesting gemstone. Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of
iridescence, although the colors are actually the result of unusually strong pleochroism.
Encyclopedia
Andalusite is an
aluminium nesosilicate
mineral with the chemical formula Al
2SiO
5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form an
X or a cross in section.
A clear variety first found in
Andalusia,
Spain can be cut into an interesting gemstone. Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of
iridescence, although the colors are actually the result of unusually strong pleochroism.
Occurrence
Andalusite is a common regional metamorphic mineral which forms under high pressure and/or temperature. The minerals
kyanite and
sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found.
It was first reported from
Andalucia,
Spain in 1789.
Details
| Crystallographic system | Orthorhombic |
| Colours | Brown, green, pink, violet and red varieties |
| Lustre | |
| Hardness | 7.5 |
| Specific gravity | 3.13 - 3.17 |
| Refractive index | 1.641-1.648 |
| cleavage | 3 |
| Twinning | Lamellar |
| Pleochroism | Clear to patchy pink |
Related minerals
- Kyanite
- Sillimanite
- Chiastolite
References
See also
