The crystal structure of sillimanite, refined using single-crystal neutron diffraction data measured at room temperature yielded a = 7.479(1), b = 7.670(2), c = 5.769(1) Å, space group Pbnm. The atomic parameters were refined by full-matrix least-squares techniques. The final agreement R(F2) is 0.037 based on 857 reflections. The Al-O and Si-O bond lengths and O-Si-O and O-Al-O bond angles have estimated standard deviations of ≤ 0.001 Å and 0.06° and agree well with those based on X-ray diffraction experiments. Refinement of the neutron scattering lengths, b, show Al and Si to be ordered within the tetrahedral sites and the minor Fe present in the structure to occupy the octahedral site.

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