We determined for the first time the crystal structures of high-pressure K-cymrite (KAlSi 3 O 8 •H 2 O) and its dehydrated form kokchetavite (KAlSi 3 O 8) using single crystal X-ray diffraction. The structure of K-cymrite has been successfully refined in the hexagonal space group (P6/mmm, a = 5.3361(3) Å, c = 7.7081(7) Å, V = 190.08(3) Å 3 , R1 = 0.036 for 127 unique observed reflections) and it is in complete agreement with previous powder X-ray diffraction models. In contrast, kokchetavite shows superstructural reflections, suggesting new values of unit cell and space group P6/mcc (a = 10.5757(3) Å, c = 15.6404 (6) Å, V = 1514.94(10) Å 3 , R1 = 0.068 for 1455 unique observed reflections). During dehydration, single-crystal grains of Kcymrite transform into single-crystal grains of kokchetavite. The latter questions a previous interpretation of kokchetavite crystals in mineral inclusions as a product of direct crystallization from fluid/melt. The Raman spectrum of K-cymrite shows a strong polarization dependence, which is important in the identification of the mineral inclusions.

The influence of temperature on fluorescence spectra used for pressure calibration in a diamond anvil cell (i.e., that of ruby) was always considered as an undesirable phenomenon decreasing the accuracy of pressure measurement in high-temperature experiments. Consequently, nearly temperature-independent fluorescence lines of such pressure calibrants as Sm2+:SrB4O7 and Sm3+:YAG were proposed as the best suited for pressure measurement. We analyzed pressure- and temperature-induced shifts of eight fluorescence lines in Sm2+:SrB4O7 spectrum, extending previous calibrations to 60 GPa and 580 °C, and demonstrated that both pressure and temperature in a “high-pressure + high-temperature” experiment can be reliably estimated by analysis of shifts of two fluorescence lines with different sensitivity to pressure and temperature. We tested the proposed method in an experiment with simultaneous pressure and temperature (up to 7 GPa and 400 °C) and proved that the temperature may be successfully estimated together with pressure from a single Sm2+:SrB4O7 spectrum with an e.s.d. of 8 °C.

A pressure shift of Raman band of liquid water (HO) may be an important tool for measuring residual pressures in mineral inclusions, in situ barometry in high-pressure cells, and as an indicator of pressure-induced structural transitions in HO. However, there was no consensus as to how the broad and asymmetric water Raman band should be quantitatively described, which has led to fundamental inconsistencies between reported data. In order to overcome this issue, we measured Raman spectra of HO in situ up to 1.2 GPa using a diamond anvil cell, and use them to test different approaches proposed for the description of the water Raman band. We found that the most physically meaningful description of water Raman band is the decomposition into a linear background and three Gaussian components, associated with differently H-bonded HO molecules. Two of these components demonstrate a pronounced anomaly in pressure shift near 0.4 GPa, supporting ideas of structural transition in HO at this pressure. The most convenient approach for pressure calibration is the use of "a linear background + one Gaussian" decomposition (the pressure can be measured using the formula P (GPa) = -0.0317(3)·Δν (cm), where Δν represents the difference between the position of water Raman band, fitted as a single Gaussian, in measured spectrum and spectrum at ambient pressure).