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 (Mg_1.70Fe²⁺_0.29Mn_0.01)_Σ2.00(Fe³⁺_0.90Al_0.08Mg_0.02)_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 [O4M₄]ⁿ⁺ and [O2M₅]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.