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Crystal structure determination of orthorhombic variscite2O and its derivative AlPO4 structure at high temperature

  • Matteo Ardit ORCID logo , Brian L. Phillips and David L. Bish
Published/Copyright: July 2, 2022
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American Mineralogist
From the journal American Mineralogist Volume 107 Issue 7

Abstract

Variscite [Al(PO4)·2H2O] is an uncommon secondary phosphate mineral but is important in a variety of environmental and technological applications. It exists in at least one monoclinic (metavariscite) and two orthorhombic polymorphs (“Lucin-type” and “Messbach-type”), but the fine-grained nature of the “Messbach-type” variscite has hampered the determination of its crystal structure. The crystal structure of the latter from Tooele County, Utah, was solved and refined using laboratory powder X‑ray diffraction (XRD) data, charge-flipping, and the Rietveld method. Both variscite modifications belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms. Topological analysis reveals that the two structures are polytypes. Based on our results and our structural interpretations, we refer to “Lucin-type” variscite as variscite1O and the “Messbach-type” as variscite2O, to be consistent with modern polytype terminology. The similarity of these two structures suggests that 1O-2O interstratifications may exist in nature, which is consistent with observed broadening of diffraction peaks of the Tooele material. 31P and 27Al MAS/NMR measurements are consistent with the XRD-determined crystal structure, and they show distinct signals for each of the two independent P and Al positions in variscite2O.

High-temperature XRD, thermal analyses, and NMR measurements were applied to study the nature of the transformation of variscite2O to a derivative AlPO4 structure above 473 K. Charge-flipping analysis showed that the crystal structure of the new anhydrous AlPO4 phase (AlPO4-var2O in analogy to its parent structure) can be described as a 3D framework of alternating AlO4 and PO4 tetrahedra linked by bridging O atoms. Thermogravimetric analyses revealed almost complete dehydration above ~450 K, and NMR results were consistent with tetrahedral Al and P atoms.

Keywords: Variscite; X-ray powder diffraction; charge-flipping; nuclear magnetic resonance; polytypism; AlPO4

Funding statement: M.A. acknowledges financial support by the grant from the Italian Ministry of Education (MIUR) through the project “Dipartimenti di Eccellenza 2018–2022.”

Acknowledgments

We thank Laura Chelazzi (CRIST—Centro di Servizi di Cristallografia Strutturale, University of Firenze) for XRF analysis. We thank Christian L. Lengauer and an anonymous reviewer for constructive reviews.

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Received: 2021-04-27
Accepted: 2021-07-07
Published Online: 2022-07-02
Published in Print: 2022-07-26

© 2022 Mineralogical Society of America

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https://doi.org/10.2138/am-2022-8119
Keywords for this article
Variscite; X-ray powder diffraction; charge-flipping; nuclear magnetic resonance; polytypism; AlPO4

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. Mineral evolution heralds a new era for mineralogy
  3. MSA Review
  4. Pauling’s rules for oxide-based minerals: A re-examination based on quantum mechanical constraints and modern applications of bond-valence theory to Earth materials
  5. A cotunnite-type new high-pressure phase of Fe2S
  6. Density determination of liquid iron-nickel-sulfur at high pressure
  7. On the paragenetic modes of minerals: A mineral evolution perspective
  8. Lumping and splitting: Toward a classification of mineral natural kinds
  9. Thermal expansion of minerals in the amphibole supergroup
  10. A multi-faceted experimental study on the dynamic behavior of MgSiO3 glass in the Earth’s deep interior
  11. Origin of β-cristobalite in Libyan Desert Glass: The hottest naturally occurring silica polymorph?
  12. Time-resolved Raman and luminescence spectroscopy of synthetic REE-doped hydroxylapatites and natural apatites
  13. Reexamination of the structure of opal-A: A combined study of synchrotron X-ray diffraction and pair distribution function analysis
  14. A first-principles study of water in wadsleyite and ringwoodite: Implication for the 520 km discontinuity
  15. Inclusions in calcite phantom crystals suggest role of clay minerals in dolomite formation
  16. Crystal-chemical reinvestigation of probertite, CaNa[B5O7(OH)4]·3H2O, a mineral commodity of boron
  17. Crystal structure determination of orthorhombic variscite2O and its derivative AlPO4 structure at high temperature
  18. Transformation of Fe-bearing minerals from Dongsheng sandstone-type uranium deposit, Ordos Basin, north-central China: Implications for ore genesis
  19. Vaterite in a decrepitated diamond-bearing inclusion in zircon from a stromatic migmatite in the Chinese Sulu ultrahigh-pressure metamorphic belt
  20. Oxygen diffusion in garnet: Experimental calibration and implications for timescales of metamorphic processes and retention of primary O isotopic signatures
  21. Oxidation state of iron and Fe-Mg partitioning between olivine and basaltic martian melts
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