Crystal structure refinement, low- and high-temperature X-ray diffraction and Mössbauer spectroscopy study of the oxoborate ludwigite from the Iten'yurginskoe deposit

Abstract

This paper reports an investigation of the chemistry, crystal structure refinement and thermal behavior (80–1650 K) of ludwigite from the Iten'yurginskoe deposit (Eastern Chukotka, Russia). Its chemical composition was determined by electron microprobe analysis, giving an empirical formula (Mg1.70Fe2+ 0.29Mn0.01)Σ2.00(Fe3+ 0.90Al0.08Mg0.02)Σ1.00O2(BO3). A refinement of the crystal structure from single-crystal X-ray diffraction data (SCXRD) was provided for the first time for ludwigite from this deposit (R = 0.047). The structure can be described as a framework composed of [MO6] n− octahedra and isolated [BO3]3− triangles located in triangular interstices of the framework. Based on a comprehensive analysis of SCXRD and Mössbauer spectroscopy data, the M1 site is occupied by Mg, M2 and M3 by Mg and Fe2+, M4 by Fe3+, Mg and Al. There are also oxo-centered [O4M 4] n+ and [O2M 5] n+ polyhedra building up a framework with the [BO3]3− triangles located in its hexagonal interstices. No indications of magnetic ordering are found in the temperature range investigated. The Fe2+ → Fe3+ oxidation occurs above 600 K, and is accompanied by a decrease of the unit-cell parameters and subsequent incomplete solid-phase decomposition with the formation of hematite, warwickite and magnetite. The mineral melts at temperatures above 1582 K. The thermal expansion of ludwigite is slightly anisotropic, which is explained by a dense packing of the [MO6] n− octahedra as well as a virtually perpendicular orientation of the oxo-centered double chains to each other. At room temperature, maximum expansion is along the c axis (α c = 9.1 × 10−6 K−1) and minimum expansion is in the ab plane (α a = 8.6 × 10−6, α b = 7.6 × 10−6 K−1), which is due to the preferred orientation of the [BO3]3− triangles. A comparison of the thermal behavior of three oxoborates of the ludwigite group, namely azoproite (Mg,Fe2+)2(Fe3+,Ti,Mg,Al)O2(BO3), vonsenite (Fe2+,Mg)2(Fe3+,Mn2+,Sn,Al)O2(BO3) and ludwigite (Mg,Fe2+ ,Mn)2(Fe3+ ,Al,Mg)O2(BO3), is provided.