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

• Published in: Physics and chemistry of minerals Vol. 45; no. 8; pp. 727 - 743
• Main Authors: Nazzareni, S., Comodi, P., Hanfland, M.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.09.2018; Springer Nature B.V
• Subjects: Bulk modulus; Cations; Chlorine; Compressibility; Compressive strength; Crystallography; Crystallography and Scattering Methods; Crystals; Earth and Environmental Science; Earth Sciences; Geochemistry; Lattice parameters; Mathematical analysis; Mineral Resources; Mineralogy; Monoclinic lattice; Original Paper; Phase transitions; Polyhedra; Single crystals; Sodium chloride; Strain analysis; Sulfates; Synchrotron radiation; Tensor analysis; Tensors; Tetrahedra; X-ray diffraction; Brines; Chlorine; Evaporitic minerals; Mg sulfate; High pressure; Crystal structure; Phase transition
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-018-0958-x

Kainite (KMg(SO 4 ) Cl 3H 2 O) 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. C 2/ 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 K 0  = 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 K 0  = 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 0 a  = 117(4) GPa, Mpa = 11(1), for b -axis M 0b  = 113(2) GPa, Mpc = 8.6(5), and c -axis M 0 c  = 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.