THE CRYSTAL STRUCTURE OF INGERSONITE, Ca3Mn2+Sb54+O14, AND ITS RELATIONSHIPS WITH PYROCHLORE

Abstract

The crystal structure of ingersonite, [Ca2.93 Mn1.062+Fe0.01] [Sb3.955+Mg0.05]F0.15 O13.85, 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 F0> 4σ(F0) 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 comers 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 observed. The crystal-chemical relationships with other pyrochlore-related minerals are outlined.