Ginga V. A., Siidra O. I., Ugolkov V. L., Bubnova R. S.
Refinement of the crystal structure and features of the thermal behavior of volborthite Cu3V2O7(OH)2 ⋅ 2H2O from the Tyuya-Muyun deposit, Kyrgyzstan
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 5. P. 115-133
https://doi.org/10.31857/S0869605521050063
Full text is available on eLIBRARY.RU
Language: English
Abstract
Thermal analysis, microprobe chemical analysis, IR spectroscopy, high-temperature X-ray diffraction and single-crystal X-ray diffraction analysis were performed for volborthite Cu3V2O7(OH)2 ⋅ 2H2O from the Tyuya-Muyun deposit (Fergana Valley, Kyrgyzstan). The crystal structure of volborthite was refined in C2/m (a = 10.617(3), b = 5.884(1), c = 7.204(2) Å, β = 94.559(5)°) to R1 = 0.054 for 536 independent reflections with |Fo| ≥ 4σF. Empirical formula of volborthite calculated on the basis of 5 cations per formula unit is (Cu2.75, Zn0.19, Ni0.06)Σ3.0 V1.99O7(OH)1.975 ⋅ 2H2O. Thermal analysis and powder X-ray diffraction studies were performed in the range 30–780 °C. Volborthite is stable in the temperature range 30–180 °C and expands anisotropically. For the temperature range of 30–180 °C, expansion is observed along the bisector of the β angle, while compression is observed in the perpendicular direction along the bisector of the acute angle. Volborthite starts to dehydrate at 182 °C. The character of the thermal expansion of volborthite changes in such a way that compression is observed along the bisector of the β angle, while expansion is observed in the perpendicular direction along the bisector of the acute angle. After dehydration, volborthite gradually transforms into a number of anhydrous copper vanadates: ziesite Cu2V2O7, synthetic phase Cu3V2O8, macbirneyite Cu3(VO4)2, stoiberite [Cu5O2](VO4)2, fingerite Cu3[Cu4O2](VO4)6, blossite α-Cu2V2O7.
Keywords: volborthite, thermal analysis, thermal expansion, high-temperature X-ray diffraction, crystal structure, Tyuya-Muyun deposit
