The crystal structure of magnesium-aluminium ludwigite, Mg2.11Al0.31Fe0.53Ti0.05Sb0.01BO5, a combined single crystal X-ray and HREM study

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Abstract
The structure of a magnesium-aluminium ludwigite, Mg 2.11 -Al 0.31 Fe 0.53 Ti 0.05 Sb 0.01 BO 5 , from Fresno County, California, has been determined by single crystal X-ray diffraction and electron microscopy (HREM) techniques. The HREM studies verified the ludwigite structure type of the present specimen and revealed some structural defects common to other members of the pinakiolite family. The space group is Pbam , Z = 4, with the unit cell dimensions a = 9.2324(8) Å, b = 12.2247(13) Å, and c = 2.9972(3) Å. The structure has been refined versus 1015 reflections with sin( θ )/ λ ≤ 0.9 Å −1 to an R value of 0.018. The structural model yields a composition (Mg,Al) 2.411(3) (Fe,Ti) 0.589(3) BO 5 in excellent agreement with the composition found ( cf . above) by energy dispersive X-ray analysis (EDX) on the same crystal that was used in the X-ray study. The structure obtained is well ordered and has a metal ion charge distribution in agreement with that found in other members of the pinakiolite family. The scheme of the cation ordering in the different octahedra was derived from i.a. estimated bond valence distributions. This scheme gives bond valences for all the oxygen atoms within the range 1.91 to 2.04 valence units.