High-temperature behavior of synthetic analogues of scottyite BaCu2Si2O7 and colinowensite BaCuSi2O6
Shapenkov S. V., Zolotarev A. A. jr., Zhitova E. S.., Krivovichev S. V., Krzhizhanovskaya M. G.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 2. P. 125-134
Analogues of two minerals: scottyite BaCu2Si2O7 and colinowensite BaCu2Si2O6, have been synthesized by hydrothermal method. According to the single crystal X-ray diffraction, synthetic analogue of scottyite is orthorhombic, space group Pnma, a = 6.8805(15), b = 13.190(3), c = 6.8903(17) Å; analogue of colinowensite is tetragonal, I41/acd, a = 9.9811(2), c = 22.3369(6) Å. The high-temperature (HT) X-ray powder diffraction study has shown that both phases are stable within the range of temperature from 26 to 900 °C. Calculated thermal expansion coefficients for scottyite are αа = 910 6, αb = 510-6, αс =810-6 °С-1 at T = 200 °C; αа = 1010-6, αb = 610-6, αс = 1210-6 °С at T = 400 °C; αа = 1010-6, αb = 810-6, αс = 1610-6 °С-1 at T = 600 °C, and the main HT deformation of the structure is associated with straightening of chains, built by copper squares, along the c axis. Calculated thermal expansion coefficients for colinowensite are αа = 1010-6 °С-1, αс = -510-6 °С-1 at T = 200 °C; αа = 910-6 °С-1, αс = 510-6 °С-1 at T = 400 °C; αа = 910-6 °С-1, αс = 1510-6 °С-1 at T = 600 °C. The thermal behavior of colinowensite is characterized by contraction of c parameter in the 25—400 °C range of temperature, while in the region 400—900 °C the phase becomes expanded. This high-temperature behavior is interpreted as a stretching of barium polyhedral chains.
Key words: scottyite, colinowensite, crystal structure, X-ray diffraction, thermal expansion.