The electron microprobe compositions and crystal structure of seven members of the grandidierite-ominelite (MgAl3BSiO9–Fe2+Al3BSiO9) series with X = (Fe2+ + Mn + Zn)/(Fe2+ + Mn + Zn + Mg) ranging from 0.00 to 0.52 were studied to determine the geometric effects of Fe substitution for Mg on the crystal structures. Calculating Fe3+ from the electron microprobe analyses gave 0.04–0.06 Fe3+ apfu, but such small amounts at the Al sites could not be detected in the refinements. Regression equations derived from single-crystal X-ray diffraction data show that b increases by 0.18 Å from X = 0–1. The crystal structure refinements show that the most significant changes involve the (Mg,Fe2+)O5 polyhedron, which increases in volume by 0.36 Å3 (5.0%), largely as a result of expansion of the MgFe-O5, -O2, and -O6 (×2) bond distances, which increase by 0.09 (4.4%), 0.06, and 0.04 Å, respectively. Other significant changes include increasing O1-MgFe-O2 (3.44°) and -Al3-O5a angles (1.9°) and a decreasing O6-MgFe-O6b (−2.20°) angle. Significant increases are also seen in the lengths of the O1–O2 (0.13 Å) and O6-O5a (×2) (0.09 Å) edges. The SiO4 tetrahedra appear to respond to changes in the surrounding polyhedra by changing O-Si-O angles such that the tetrahedral angle variance and mean tetrahedral quadratic elongation increase with X. The BO3 triangles appear to behave as relatively invariant units in the crystal structure.

Regression equations obtained from the MgFe-O bond distances were used to determine a radius for VFe2+ of 0.70 Å. Although our samples show little Mn, the presence of Mn2+ at the MgFe site would be expected to cause more distortion than an equivalent amount of Fe2+. Substitution of Zn likely would have little effect. The presence of Cr3+ at any of the Al sites would be expected to increase the size of the coordination sphere, but the substitution of P5+ for Si at the Si sites would most likely decrease the Si-O bond distances.

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