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

Kainite (KMg(SO4) 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. C2/m) kainite up to a pressure of 14GPa 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-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)degrees from the c-axis in the (0 1 0) plane. The shortening of the K(1)-K(2) distance (from 4.219(4) angstrom at ambient P to 3.521(7) angstrom 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 angstrom distance from the Ow.