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 K 2 Ca 2 Cu(SO 4 ) 4 ·2H 2 O 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 C 2/ c , a = 11.734(2), b = 7.596(1), c = 10.176(1) Å, β = 124.85(1), V = 744.4(2) Å 3 , Z = 2. The crystal structure was refined to R = 0.065. The average empirical formula is K 2.31 Ca 1.99 Cu 0.88 Al 0.04 S 3.97 O 16 ·2H 2 O. Leightonite is stable up to ∼325°C, and above this temperature it decomposes into high-temperature cubic calciolangbeinite K 2 Ca 2 (SO 4 ) 3 ( P 2 1 3) and chalcocyanite CuSO 4 phases. The thermal expansion coefficients are α 11 = 8.3(9), α 22 = 3.6(4), α 33 = –4.7(5) × 10 –6 °C –1 at –180°C and α 11 = 41.6(5), α 22 =36.5(5), α 33 = –10.6(1) × 10 –6 °C –1 at 325°C).