X-ray Diffraction Reveals Two Structural Transitions in Szomolnokite

Abstract

Hydrated sulfates have been identified and studied in a wide variety of environments on Earth, Mars, and the icy satellites of the solar system. The subsurface presence of hydrous sulfur-bearing phases to any extent necessitates a better understanding of their thermodynamic and elastic properties at pressure. Endmember experimental and computational data are lacking and are needed to accurately model hydrous, sulfur-bearing planetary interiors. In this work, high-pressure X-ray diffraction and synchrotron Fourier-transform infrared (FTIR) measurements were conducted on szomolnokite (FeSO₄•H₂O) and up to ~83 and 24 Gpa, respectively. This study finds a monoclinic-triclinic (C2/c to P-1) structural phase transition occurring in szomolnokite between 5.0(1) and 6.6(1) Gpa and a previously unknown triclinic-monoclinic (P-1 to P2₁) structural transition occurring between 12.7(3) and 16.8(3) Gpa. The high-pressure transition was identified by the appearance of distinct reflections in the XRD patterns that cannot be attributed to a second phase related to dissocation of the P-1 phase and is further characterized by increased H₂O-bonding within the structure. We fit 3rd order Birch-Murnaghan equations of state for each of the three phases identified in our data and refit published data to compare the elastic parameters of szomolnokite, kieserite (MgSO₄•H₂O), and blödite (Na₂Mg(SO₄)2•₄HO). At ambient pressure, szomolnokite is less compressible than blödite and more than kieserite, but by 7 Gpa both szomolnokite and kieserite have approximately the same bulk modulus, while blödite’s remains lower than both phases up to 20 Gpa. These results indicate that stability of szomolnokite’s high-pressure monoclinic phase and the retention of water within the unit cell up to pressures found in planetary deep interiors.