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The electron microprobe compositions and crystal structure of seven members of the grandidierite- ominelite (MgAl 3 BSiO 9 -Fe 2+ Al 3 BSiO 9 ) series with X = (Fe 2+  + Mn + Zn)/(Fe 2+  + 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 Fe 3+  from the electron microprobe analyses gave 0.04.0.06 Fe 3+  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 Å 3  (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 SiO 4  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 BO 3  triangles appear to behave as relatively invariant units in the crystal structure.

The geometric effects of V Fe 2+ for V Mg substitution on the crystal structures of the grandidierite-ominelite series