Changes in crystal structures of metal oxide/ hydroxide minerals during dissolution and heating: An In-situ synchrotron X-ray diffraction study
Open Access
- Author:
- Chung, Dongyoun
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- January 11, 2022
- Committee Members:
- Peter J Heaney, Thesis Advisor/Co-Advisor
Susan Louise Brantley, Committee Member
Mark Patzkowsky, Program Head/Chair
Matthew S Fantle, Committee Member - Keywords:
- Time-resolved X-ray diffraction
Mineral dissolution
Hematite
Goethite
Thermal expansion
Heating
Rutile structure
Pyrolusite
Rutile - Abstract:
- In-situ time resolved X-ray diffraction (TRXRD) was conducted to study changes in the crystal structures of metal (hydr)oxides during aqueous dissolution and dry heating. Rietveld refinement of TRXRD data collected during the aqueous dissolution of hematite (Fe2O3) and goethite (FeOOH) showed disparate behaviors despite their similar chemistries. In hematite, refined particle sizes decreased as the Fe occupancy increased during dissolution, whereas goethite particle sizes increased as Fe occupancy decreased. We propose that the densely structured hematite dissolved through removal of surface cations, but tunnel-structured goethite allowed leaching of Fe ions into solution as H+ diffused into the structure and expanded it. The change in lattice parameters as both minerals dissolved into nanoparticles showed a strong dependency on Fe occupancy. In addition, two isostructural minerals with different chemistries, pyrolusite (MnO2) and rutile (TiO2), were dry heated to near 1000 oC and analyzed by TRXRD and Rietveld refinement. Pyrolusite exhibited two phase transitions, first to bixbyite (Mn2O3) and then to hausmannite (Mn3O4). In contrast, the rutile did not display any phase transitions. The thermal expansion coefficients, bond lengths, and bond angles were calculated for each phase as a function of temperature. The octahedral bond angles and bond lengths for pyrolusite were more distorted than those of rutile before the phase transition. These two studies demonstrate the importance of in situ TRXRD and Rietveld refinement to understand the behavior of geologically important minerals during dissolution and heating. Minor differences in structure and chemistry can strongly affect the behavior of the minerals.