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) ų, Z = 2. The crystal structure was refined to R = 0.065. The average empirical formula is K_2.31Ca_1.99Cu_0.88Al_0.04S_3.97O₁₆·2H₂O. 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).