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The geometric effects of ^VFe²⁺ for ^VMg substitution on the crystal structures of the grandidierite-ominelite series

¹ Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
² Department of Geological Sciences, University of Maine, 5790 Bryand Research Center, Orono, Maine 04469-5790, U.S.A.

Correspondence: ^* E-mail: lgroat{at}eos.ubc.ca

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 ų (5.0%), largely as a result of expansion of the MgFe-O5, -O2, and -O6 (x2) 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 (x2) (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.

Key Words: Grandidierite • ominelite • crystal structure • substitution • borosilicates