In:
Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 89, No. S01 ( 1984-11-15)
Abstract:
Structural features influencing relative enrichments, cation stabilities, and colors of vanadium‐, titanium‐, and iron‐bearing hibonite (CaAl 12 O 19 ) in meteorites are examined. These transition elements may substitute for Al 3+ ions, which occur in five different coordination sites in the hibonite crystal structure, including three distinct octahedra [Al(1), Al(3), and Al(4) positions], one tetrahedron [Al(2) position] , and an unusual trigonal bipyramid [the Al(5) position] providing fivefold coordination by oxygen ions. Mossbauer spectral measurements of terrestrial and synthetic iron‐bearing hibonites demonstrate that although Fe cations occur in four‐, five‐, and sixfold coordinations, they are relatively enriched in the trigonal bipyramidal Al(5) site, which provides the largest average Al 3+ ‐oxygen distance. Similarities with Mossbauer spectra of blue sapphires indicate that some Fe 2+ ions are also located adjacent to Ti 4+ cations in hibonite's face‐sharing Al(3) octahedra. Arguments based on ionic radius and crystal field stabilization energy criteria are used to explain the enrichment of Fe 2+ ions in the fivefold coordination Al(5) site of hibonite. Similar electronic stabilities apply also to V 3+ and Ti 3+ , but not to Cr 3+ , providing an explanation for the fractionation of vanadium into meteoritic hibonites. Three mechanisms are proposed for the blue colors of these hibonites, the visible‐region spectra of which show minima at 550 nm (blue) between two absorption bands at about 400 nm and 700 nm. One assignment of these bands is to crystal field transitions within V 3+ and Ti 3+ , respectively, which are located in the symmetry D 3 h trigonal bipyramidal Al(5) site. A second assignment of the 700 nm band is to an intense Fe 2+ → Ti 4+ intervalence transition between traces of these cations located in adjacent face‐shared Al(3) octahedra. The orange color and disappearance of the 700 nm band produced by the heat treatment of hibonite at elevated oxygen fugacities may then be the result of oxidation of either Ti 3+ to Ti 4+ or Fe 2+ to Fe 3+ . A third explanation of the blue‐orange color change involves color centers induced when primordial 26 Al decays to 26 Mg, or from trapped electrons in the lattice as a result of nonstoichiometry and structural defects in hibonite.
Type of Medium:
Online Resource
ISSN:
0148-0227
DOI:
10.1029/JB089iS01p0C313
Language:
English
Publisher:
American Geophysical Union (AGU)
Publication Date:
1984
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