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High-pressure structural behavior of ingersonite, Ca₃Mn²⁺Sb₄⁵⁺O₁₄: An in-situ single-crystal X-ray study

¹ Dipartimento di Scienze della Terra, Università di Perugia, Piazza dell’Università, I-06100 Perugia, Italy
² Dipartimento di Scienze della Terra, Università di Firenze, via La Pira 4, I-50121 Firenze, Italy
³ Museo di Storia Naturale, sezione di Mineralogia, Università di Firenze, via La Pira 4, I-50121 Firenze, Italy

Correspondence: ^* E-mail: zanazzi{at}unipg.it

An in-situ, high-pressure, single-crystal X-ray diffraction study has been carried out at room temperature up to 7.42 GPa on a crystal of ingersonite, ideally Ca₃Mn²⁺Sb₄⁵⁺O₁₄, from the type material. Ingersonite is isostructural with the synthetic weberite-3T polytype and related to the pyrochlore structure-type. Owing to the P range investigated and the quality of data, a second-order Birch-Murnaghan Equation of State (EoS) is the best approximation to describe the ingersonite volume evolution with P. The refined EoS parameters are V₀ = 810.6(1) ų and K₀ = 154.5(2.4) GPa. The behavior of ingersonite with pressure is almost isotropic and the decrease of the unit-cell volume is mainly due to the kinking of the polyhedra rather than their volume decrease. The overall mean distances are quite constant, indicating virtually no compressibility of both the A and B polyhedra in the P range investigated. However, some geometrical changes in the pyrochlore-like AB₃ layer can be observed and compared with those observed in synthetic pyrochlore compounds. The largest change is observed for the z atomic coordinate of the O2 atom. Using an anion-centered polyhedral description, O2 is the only O atom that is asymmetrically located in an octahedral interstice, this feature being the most remarkable difference between the structure of ingersonite (i.e., weberite-3T type, space group P3₁21) and that of zirconolite-3T (pyrochlore structure type, space group P3₁21), where all the O atoms occupy the tetrahedral interstices of a cubic A₂B₂ array. With the increase of pressure, the O2 atom migrates from the A₄B₂ octahedral cavity toward the adjacent AB₃ tetrahedral cavity, suggesting that a transition from weberite-3T to zirconolite-3T structure type could occur at pressures higher than 11 GPa.

Key Words: Ingersonite • high pressure • compressibility • crystal structure • phase transition