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Vacancy infilling during the crystallization of Fe-deficient hematite; an in situ synchrotron X-ray diffraction study of non-classical crystal growth

Si Athena Chen, Peter J. Heaney, Jeffrey E. Post, Peter J. Eng and Joanne E. Stubbs
Vacancy infilling during the crystallization of Fe-deficient hematite; an in situ synchrotron X-ray diffraction study of non-classical crystal growth
American Mineralogist (September 2023) 108 (9): 1720-1731

Abstract

The crystallization of hematite from precursor ferrihydrite was studied using time-resolved, angle-dispersive synchrotron X-ray diffraction in aqueous solutions at pH 10 and 11 and at temperatures ranging from 80 to 170 degrees C. Rietveld analyses revealed a non-classical crystallization pathway involving vacancy infilling by Fe as defective hematite nanocrystals evolved. At 90 degrees C and pH 11, incipient hematite particles exhibited an Fe site occupancy as low as 0.68(2), and after 30 min, Fe occupancy plateaued at 0.84(1), achieving a metastable steady state with a composition corresponding to "hydrohematite." During crystal growth, unit-cell volume increased with an increase in Fe occupancy. The increase in Fe occupancy in hydrohematite was accomplished by deprotonation, resulting in a shortening of the long Fe-O(H) bonds and decreased distortion of the octahedral sites. Once the occupancy stabilized, the unit-cell volume contracted following further nanoparticle growth. Our study documented various synthetic routes to the formation of "hydrohematite" with an Fe vacancy of 10-20 mol% in the final product. The structure refined for synthetic hydrohematite at 90 degrees C and pH 11 closely matched that of natural hydrohematite from Salisbury, Connecticut, with a refined Fe occupancy of 0.83(2). Dry heating this natural hydrohematite generated anhydrous, stoichiometric hematite, again by continuous infilling of vacancies. The transformation initiated at 150 degrees C and was complete at 700 degrees C, and it was accompanied by the formation of a minor amorphous phase that served as a reservoir for Fe during the inoculation of the defective crystalline phase.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 108
Serial Issue: 9
Title: Vacancy infilling during the crystallization of Fe-deficient hematite; an in situ synchrotron X-ray diffraction study of non-classical crystal growth
Affiliation: Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Pages: 1720-1731
Published: 202309
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 63
Accession Number: 2023-067666
Categories: General mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: Smithsonian Institution, Department of Mineral Sciences, USA, United StatesUniversity of Chicago, Center for Advanced Radiation Sources, James Franck Institute, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2023, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2023
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