The crystal structure of ingersonite, Ca₃Mn²⁺Sb₄⁵⁺O₁₄, and its relationships with pyrochlore

Abstract

The crystal structure of ingersonite, [Ca_2.93Mn²⁺_1.06Fe²⁺_0.01][Sb⁵⁺_3.95Mg_0.05]F_0.15O_13.85, has been solved and refined in the space group P3121 [a = 7.282(2), c = 17.604(4) Å, V = 808.4(3) Å3, Z = 3] to R = 2.32% for 2219 Fo > 4σ(Fo) using MoKα X-ray data.

The structure of ingersonite is isostructural with the synthetic weberite-3T polytype and related to the pyrochlore structure type. Both ingersonite and pyrochlore structures can be described as a sequence of pairs of polyhedral layers (named M and N), stacked along [111] and [001], respectively. In terms of the cation sites, M and N layers have general formula AB₃ and A₃B, respectively, where B are the octahedral cations forming the B₂X₆ framework of the pyrochlore structure and A are the larger, interstitial cations forming eightfold polyhedra in pyrochlore.

In ingersonite, the M layers occur at z ~ 1/6, 1/2, and 5/6: The B octahedra are occupied by Sb⁵⁺ and share corners to form a pseudohexagonal tungsten bronze (HTB) motif with the A position occupied by octahedral Mn2+ at the center of the pseudohexagonal rings. N layers occur at z ~ 0, 1/3, and 2/3, with A = Ca and B = Sb⁵⁺: Isolated B octahedra share 6 edges with 6 eightfold A polyhedra, to form a continuous sheet similar to the analogous layer in pyrochlore. The stacking of successive pairs of M and N layers in ingersonite is the same as in pyrochlore. Nonetheless, a difference in the relative position between M and N layers in ingersonite and pyrochlore is observed. The crystal-chemical relationships with other pyrochlore-related minerals are outlined.