Oxoborates of the ludwigite group: Natural and mineral-like compounds as prospective materials

Research subject. Natural oxoborates of the ludwigite group, including azoproite, ludwigite, and vonsenite. Their empirical formulas based on five oxygen atoms have the following form: azoproite (Mg 1.81 Fe 2+ 0.19 ) ∑2.00 (Fe 3+ 0.36 Ti 0.26 Mg 0.26 Al 0.12 ) ∑1.00 O 2 (BO 3 ), ludwigite (Mg 1.69 Fe 2+ 0.30 Mn 2+ 0.01 ) Σ 2.00 (Fe 3+ 0.90 Al 0.07 Mg 0.02 Sn 0.01 ) Σ 1.00 O 2 (BO 3 ) and vonsenite (Fe 2+ 1.86 Mg 0.13 ) ∑1.99 (Fe 3+ 0.92 Mn 2+ 0.05 Sn 4+ 0.02 Al 0.02 ) ∑1.01 O 2 (BO 3 ). Aim. To establish the relationship between the composition, crystal structure, and thermal behavior (293–1373 K) of the minerals. Materials and methods. Ludwigite was collected at the Iten’yurginskoe tin skarn deposit; vonsenite was collected at the Titovskoe magnesium-skarn boron deposit; azoproite was collected at magnesian skarns of the Tazheran alkaline massif. The methods of single crystal X-ray diffraction, energy dispersive X-ray spectroscopy, high-temperature X-ray diffraction, Mössbauer spectroscopy, and thermal analysis were used. Results. Low-charge cations (Fe 2+ , Fe 2.5+ , Mg 2+ ) tend to occupy the M (1)– M (3) sites, and high-charge cations (Fe 3+ , Al 3+ , Ti 4+ , Sn 4+ ) generally occupy the M (4) site. Azoproite is characterized by the highest melting temperature T m > 1650 K. Due to the low Fe 2+ content, azoproite does not undergo solid-phase decomposition across the investigated temperature range. The melting point of ludwigite exceeds 1582 K, which is due to the high Mg content; as a result of the Fe 2+ → Fe 3+ oxidation, it gradually decomposes with the formation of hematite, warwickite, and magnetite. The temperatures of oxidation and solid-phase decomposition in the Fe 2+ -rich vonsenite are approximately 100 K lower than those in ludwigite. The melting point of vonsenite is 1571 K. All the minerals are characterized by a weak degree of thermal expansion anisotropy. The main contribution to the thermal expansion anisotropy is due to the preferred orientation of the BO 3 3– triangles. Conclusions. The thermal properties of the oxoborates depend on their chemical composition. It was established that T m increases with an increase in the Mg and Ti 4+ content, and decreases with an increase in the Fe 2+ content. The Fe 2+ → Fe 3+ oxidation is observed when the FeO component in the minerals exceeds 10 wt %, which leads to the solid-phase decomposition starting at temperatures of about 500–600 K. The values of the 293K α V volume thermal expansion of ludwigite and azoproite are comparable, while the largest values were observed for vonsenite. This is associated with the largest average bond lengths, primarily those of 6 .