X-ray diffraction reveals two structural transitions in szomolnokite

Olivia S. Pardo1, Vasilije V. Dobrosavljevic1, Tyler Perez1,2, Wolfgang Sturhahn1, Zhenxian Liu3, George R. Rossman1, and Jennifer M. Jackson1 

1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125-2500, U.S.A.

2Now at Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, U.S.A.

3Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, U.S.A.


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. End-member 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 (XRD) and synchrotron Fourier-transform infrared (FTIR) measurements were conducted on szomolnokite (FeSO4ˇH2O) up to ~83 and 24 GPa, respectively. This study finds a monoclinic-triclinic (C2/c to P1) structural phase transition occurring in szomolnokite between 5.0(1) and 6.6(1) GPa and a previously unknown triclinic-monoclinic (P1 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 the dissociation of the P1 phase, and it is further characterized by increased H2O bonding within the structure. We fit third-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 (MgSO4ˇH2O), and blödite (Na2Mg(SO4)2ˇ4H2O). 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 the stability of szomolnokite’s high-pressure monoclinic phase and the retention of water within the structure up to pressures found in planetary deep interiors.


Szomolnokite from the Lavender Pit, Bisbee, Cochise County, Arizona