High-pressure single-crystal synchrotron X-ray diffraction of kainite (KMg(SO₄) Cl 3H₂O)

Abstract

Kainite (KMg(SO₄) Cl 3H₂O) is a “mixed-salt” sulfate from the group of evaporitic minerals more soluble than Ca-sulfate hydrate and NaCl. The compressibility and structural modifications of monoclinic (sp. gr. C2/m) kainite up to a pressure of 14 GPa were studied by high-pressure single-crystal synchrotron X-ray diffraction. Kainite remains stable over the investigated pressure range and no phase transition was recognised. The bulk modulus is K₀ = 31.6 (1) GPa, with K′ fixed to 4, as obtained by fitting the P-volume data with a second-order Birch–Murnaghan EoS (BM2); instead of using a BM3 EoS, we obtained K₀ = 32.2(5) GPa, K’ =3.8 (1). The linear moduli calculated for the lattice parameters fitting the data with a BM3 EoS are for a-axis M_0a = 117(4) GPa, Mpa = 11(1), for b-axis M_0b = 113(2) GPa, Mpc = 8.6(5), and c-axis M_0c = 68.2(3) GPa, Mpc = 14(1). Structure refinements showed a strong compression of the K polyhedra and in particular K(1) and K(3) polyhedra have similar polyhedral bulk moduli: K_0K(1) = 20.8(7) GPa, K′=4.8(3); K_0K(2) = 29(1) GPa, K′=8.1(6); K_0K(3) = 26(1) GPa, K′=4.2(4). The most compressible bond distances are K(1)–Cl(2) with a shortening of about 13%, K(1)–Cl(1) with a shortening of about 10%, K(3)–Ow(6) and K(3)–O8(B) both with a shortening of 9%. S-tetrahedra are almost incompressible and Mg-octahedra bulk moduli are K_0Mg(2) = 102(4) GPa, and K_0Mg(4) = 72(1) GPa, K_0Mg(1) = 41(4) GPa K′= 8.9(1.7), and K_0Mg(3) = 65(5) GPa K′= 10(2). The strain tensor analysis indicates that the most compressible direction of the kainite monoclinic structure is oriented 29.7(2)° from the c-axis in the (0 1 0) plane. The shortening of the K(1)–K(2) distance (from 4.219(4) Å at ambient P to 3.521(7) Å at 11.9 GPa) and the different compressibilities of the octahedra/tetrahedra may explain why the stiffer direction of kainite is in the a–c plane approximatively along the direction where K(1)–K(2) and Mg(4)–Mg(3)–Mg(4) polyhedra align. This may explain the anisotropic compressional behaviour of the crystallographic axes, where c is more compressible (by tetrahedral tilting mechanism) than a and b, where cation–cation repulsion and a more rigid configuration make these directions stiffer. Following the structure modification increasing pressure a new sets of hydrogens bonds could form as oxygens and chlorine atoms get at less than 3 Å distance from the Ow.