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Andalusite [Al2SiO5]
Structure Orthorhombic
Space Group Pnnm
(No. 58)
a=7.7980 Å, b=7.9031 Å, c=5.5566 Å
a=b=g=90.00
Z=4
Atomic Positional Parameters
O4 8h 0.2305 0.1339 0.2394
Al1 4e 0.0000 0.0000 0.2419
Al2 4g 0.3705 0.1391 0.5000
O1 4g 0.4246 0.3629 0.0000
O2 4g 0.1030 0.4003 0.0000
O3 4g 0.4233 0.3629 0.5000
Si1 4g 0.2460 0.2520 0.0000
Reference
JK Winter and S Ghose (1979) American Mineralogist, 64, 573-586
Mineral Chemistry
The structure of andalusite consists of chains of edge-linked AlO6 octahedra running parallel to the z-axis. These chains are linked by Si coordinated by four oxygens, and Al (unusually) coordinated by five oxygens. Andalusite may be found in argillaceous rocks of contact aureoles surrounding igneous intrusions. Kyanite and sillimanite - Al2SiO5 polymorphs - and andalusite may all be found in aluminous metamorphic rocks depending on the pressure-temperature conditions of formation.
The CHIME figure shows aluminiums as grey spheres, silicons as yellow spheres, and oxygens as red spheres.