Gold is transported in high-temperature chloride-bearing hydrothermal fluids in the form of AuCl₂⁻. The stability of this complex has been extensively studied, but there is still considerable disagreement between available experimental data on the temperature region 300–500 °C. To solve this problem, we measured the solubility of gold in HCl/NaCl fluids (NaCl concentration varied from 0.1 to 3 mol·(kg·H₂O)⁻¹) at 450 °C and pressures from 500 to 1500 bar (1 bar = 10⁵ Pa). The experiments were performed using a batch autoclave method at contrasting redox conditions: in reduced experiments hydrogen was added to the autoclave, and in oxidized experiments the redox state was controlled by the aqueous SO₂/SO₃ buffer. Hydrogen pressure in the autoclaves was measured after the experiments in the reduced system. The gold solubility constant, Au_(cr) + HCl°_(aq) + Cl⁻ = AuCl₂⁻ + 0.5 H₂°_(aq), was determined for the experimental T-P parameters as log K_s° = −4.77 ± 0.07 (500 bar), −5.11 ± 0.08 (1000 bar), and −5.43 ± 0.09 (1500 bar). These data, together with values from the literature for temperatures from 25 to 1000 °C, were fitted to the simple equation log K_s° = 4.302 − 7304∙T(K)⁻¹ − 4.77∙log d(w) + 11080∙(log d(w))∙T(K)⁻¹ − 6.94 × 10⁶∙(log d(w)) T(K)⁻², where d(w) is the pure water density. This equation can be used together with the extended Debye–Hückel equation for activity coefficients to calculate gold solubility at pressures up to 5000 bar at fluid chlorinities at least up to 30 wt %. The speciation of gold in natural chloride-bearing fluids is discussed.

We have studied and optimized conditions for spontaneous flux growth of TbCr₃(BO₃)₄ crystals. Phase relations in the pseudoternary system TbCr₃(BO₃)₄–K₂Mo₃O₁₀–B₂O₃ have been studied in the temperature range 900–1130°C and the single-phase terbium chromium borate crystallization field has been mapped out. It has been shown that increasing the TbCr₃(BO₃)₄ content of the starting high-temperature solution leads to a rhombohedral-to-monoclinic phase transition. Using K₂Mo₃O₁₀-based high-temperature solutions, we have grown single-phase TbCr₃(BO₃)₄ single crystals or crystals in which the rhombohedral phase (sp. gr. R32) significantly prevails over the monoclinic phase (sp. gr. C2/c). The grown crystals have been characterized by X-ray diffraction techniques, IR spectroscopy, and magnetic measurements.

The temperature dependence of the magnetic susceptibility χ (T) of the GdCr₃(BO₃)₄ single crystal was studied in the temperature range of 2–300 K for two orientations of the external magnetic field: H ǁ c and H ± c. It was established that gadolinium chromium borate is ordered antiferromagnetically at T_N = 7 K. It was found that, at T > 10 K, the magnetic properties of the crystal are isotropic, which is characteristic of the pure spin states of Cr³⁺ and Gd³⁺ ions. It was demonstrated that the magnetic susceptibility χ(T) of the GdCr₃(BO₃)₄ compound at T > T_N is well described by a one-dimensional spin model for a system of ferromagnetic chains of Cr³⁺ ions with ferromagnetic exchange 2J/k = −13.6 K and antiferromagnetic interchain interaction of 2J′/k = + 0.72 K. The gadolinium subsystem is antiferromagnetically biased due to the exchange f-d-interaction, which gives ${\theta }_{\text{Gd}}=-2.8\mathrm{K}$ . It was assumed that the GdCr₃(BO₃)₄ compound is an antiferromagnet with “easy-plane” type anisotropy in the magnetically ordered state.

We present an extensive study of the luminescence characteristics of Mn impurity ions in a YAl₃(BO₃)₄:Mn crystal, in combination with X-ray fluorescence analysis and determination of the valence state of Mn by XANES (X-ray absorption near-edge structure) spectroscopy. The valences of manganese Mn²⁺(d⁵) and Mn³⁺(d⁴) were determined by the XANES and high-resolution optical spectroscopy methods shown to be complementary. We observe the R₁ and R₂ luminescence and absorption lines characteristic of the ²E ↔ ⁴A₂ transitions in d³ ions (such as Mn⁴⁺ and Cr³⁺) and show that they arise due to uncontrolled admixture of Cr³⁺ ions. A broad luminescent band in the green part of the spectrum is attributed to transitions in Mn²⁺. Narrow zero-phonon infrared luminescence lines near 1060 nm (9400 cm⁻¹) and 760 nm (13,160 cm⁻¹) are associated with spin-forbidden transitions in Mn³⁺: ¹T₂ → ³T₁ (between excited triplets) and ¹T₂ → ⁵E (to the ground state). Spin-allowed ⁵T₂ → ⁵E Mn³⁺ transitions show up as a broad band in the orange region of the spectrum. Using the data of optical spectroscopy and Tanabe–Sugano diagrams we estimated the crystal-field parameter Dq and Racah parameter B for Mn³⁺ in YAB:Mn as Dq = 1785 cm⁻¹ and B = 800 cm⁻¹. Our work can serve as a basis for further study of YAB:Mn for the purposes of luminescent thermometry, as well as other applications.