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Sillimanite [Al2SiO5]
Structure Orthorhombic
Space Group Pbnm
(No. 62)
a=7.4883 Å, b=7.6808 Å, c=5.7774 Å
a=b=g=90.00
Z=4
Atomic Positional Parameters
Al1 4a 0.0000 0.0000 0.0000
Al2 4c 0.1417 0.3449 0.2500
Si1 4c 0.1533 0.3402 0.7500
O1 4c 0.3605 0.4094 0.7500
O2 4c 0.3569 0.4341 0.2500
O3 4c 0.4763 0.0015 0.7500
O4 8d 0.1252 0.2230 0.5145
Reference
JK Winter and S Ghose (1979) American Mineralogist, 64, 573-586
Mineral Chemistry
Sillimanite is one of the three aluminium silicate polymorphs,
the other two being andalusite and kyanite.
Formed at high temperatures (above 500oC)
in metamorphosed rocks, sillimanite has the simplest crystal structure of
the three polymorphs. Aluminiums are found in both tetrahedral and octahedral
coordination with oxygen. Silicons are tetrahedrally coordinated.
The CHIME figure shows aluminiums as grey spheres, silicons as yellow spheres, and oxygens as red spheres.