The Behavior of the Crystal Structure of Leightonite K2Ca2Cu(SO4)4⋅2H2O in the Temperature Interval from –180 to 325°C

This paper presents the results of mineralogical and crystal chemical studies of leightonite K2Ca2Cu(SO4)4·2H2O from the Kamchatka Peninsula (Russia), including an investigation of its thermal behavior in a wide temperature range (–180 ≤ T ≤ 325°C). The following methods were used: energy dispersive X-ray spectral analysis, single-crystal and powder X-ray diffraction, and in situ powder X-ray diffraction at negative and high temperatures). The temperature range of the existence of the mineral are established, the main values of the thermal expansion tensor are calculated, and a structural interpretation of thermal expansion is given. The mineral crystallizes in the monoclinic crystal system, space group C2/c, a = 11.734(2), b = 7.596(1), c = 10.176(1) Å, β = 124.85(1), V = 744.4(2) Å3, Z = 2. The crystal structure was refined to R = 0.065. The average empirical formula is K2.31Ca1.99Cu0.88Al0.04S3.97O16·2H2O. Leightonite is stable up to ∼325°C, and above this temperature it decomposes into high-temperature cubic calciolangbeinite K2Ca2(SO4)3 (P213) and chalcocyanite CuSO4 phases. The thermal expansion coefficients are α11 = 8.3(9), α22 = 3.6(4), α33 = –4.7(5) × 10–6°C–1 at –180°C and α11 = 41.6(5), α22 =36.5(5), α33 = –10.6(1) × 10–6°C–1 at 325°C).