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Substitution of Ti (super 3+) and Ti (super 4+) in hibonite (CaAl (sub 12) O (sub 19) )

Patricia M. Doyle, Paul F. Schofield, Andrew J. Berry, Andrew M. Walker and Kevin S. Knight
Substitution of Ti (super 3+) and Ti (super 4+) in hibonite (CaAl (sub 12) O (sub 19) )
American Mineralogist (July 2014) 99 (7): 1369-1382


The structures of eight synthetic samples of hibonite, with variable Ti oxidation state and Ti concentration (2.4-15.9 wt% TiO (sub 2) ) that span the range reported for natural hibonite found in meteorites, were determined by Rietveld refinements of neutron powder diffraction data. Ti (super 3+) was found to exclusively occupy the octahedral face-sharing M4 site irrespective of the presence or absence of Ti (super 4+) . Ti (super 4+) partitions between the trigonal bipyramidal M2 site and the M4 site. The ratio (Ti (super 4+) on M2):(Ti (super 4+) on M4) appears to be constant for all the samples, with an average of 0.18(2) irrespective of the concentrations of Ti (super 3+) and Ti (super 4+) . These substitutional sites were shown to be the most stable configurations for Ti in hibonite from calculations using density functional theory, although the predicted preference of Ti (super 4+) for M4 over M2 is not as strong as is observed. This is attributed to the different Ti contents of the experimental and calculated structures and suggests that the Ti site occupancies might change between these concentrations. Furthermore, it is shown that Ti has a preference to occupy neighboring M4 sites such that Ti-Ti interactions occur with stabilization energies of 83 kJ/mol for Ti (super 3+) -Ti (super 3+) and at least 15 kJ/mol for Ti (super 4+) -Ti (super 4+) . Features in optical spectroscopy and electron spin resonance data from meteoritic and synthetic hibonites that have been used to infer Ti (super 3+) /Ti (super 4+) are shown to actually derive from these Ti-Ti interactions. The amount of Ti (super 4+) in hibonite can be determined from the unit-cell parameters if Sigma Ti is determined independently. Ti (super 3+) /Ti (super 4+) in hibonite may record the oxygen fugacity (f (sub O2) ) of the early solar nebula, however, the existence of Ti (super 3+) -Ti (super 3+) and Ti (super 4+) -Ti (super 4+) interactions and the potential for Ti (super 4+) -Ti (super 3+) interactions need to be considered when interpreting spectroscopic data in terms of Ti valence state and f (sub O2) . Hibonite as a single-mineral oxybarometer must be used with caution due to the potential role of crystal chemistry (including Ti-Ti interactions) to stabilize Ti oxidation states independently of f (sub O (sub 2) ) .

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 99
Serial Issue: 7
Title: Substitution of Ti (super 3+) and Ti (super 4+) in hibonite (CaAl (sub 12) O (sub 19) )
Affiliation: Imperial College London, Department of Earth Science and Engineering, London, United Kingdom
Pages: 1369-1382
Published: 201407
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 73
Accession Number: 2014-058172
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: Natural History Museum, GBR, United KingdomUniversity of Bristol, GBR, United KingdomRutherford Appleton Laboratory, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201431
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