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All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Enthalpies of formation of CaAl4O7 and CaAl12O19 (hibonite) by high temperature, alkali borate solution calorimetry

Enthalpies of formation were determined for two calcium aluminate phases, CaAl4O7 and CaAl12O19, using high-temperature alkali borate solution calorimetry. The aluminates were synthesized by multiple-cycle heating and grinding stoichiometric mixtures of CaCO3 and Al2O3, and the products were characteized by X-ray diffraction and SEM microbeam analysis. The data on impurities (CaAl4O7 was found to be about 89.00 percent pure by weight and the CaAl12O19 samples about 91.48 percent pure) were used to correct the heat of solution values of the synthetic products. The enthalpies of formation, at 1063 K, from oxides, were found to be equal to -(25.6 + or - 4.7) kJ/g.f.w. for CaAl4O7 and -(33.0 + or - 9.7) kJ/g.f.w. for CaAl12O19; the respective standard enthalpies of formation from elements, at 298 K, were estimated to be -4007 + or - 5.2 kJ/g.f.w. and -10,722 + or - 12 kJ/g.f.w.

Geiger, C. A.↗
DOE OSTI2020

Materials Data on CaAl12O19 by Materials Project

CaAl12O19 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Ca2+ is bonded to twelve O2- atoms to form CaO12 cuboctahedra that share corners with six equivalent CaO12 cuboctahedra, edges with six equivalent AlO6 octahedra, edges with three equivalent AlO5 trigonal bipyramids, and faces with six equivalent AlO6 octahedra. There are six shorter (2.72 Å) and six longer (2.81 Å) Ca–O bond lengths. There are five inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to five O2- atoms to form AlO5 trigonal bipyramids that share corners with twelve AlO6 octahedra and edges with three equivalent CaO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 41–61°. There is three shorter (1.77 Å) and two longer (2.21 Å) Al–O bond length. In the second Al3+ site, Al3+ is bonded to six equivalent O2- atoms to form AlO6 octahedra that share corners with six equivalent AlO4 tetrahedra and edges with six equivalent AlO6 octahedra. All Al–O bond lengths are 1.90 Å. In the third Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with six equivalent AlO6 octahedra, corners with three equivalent AlO5 trigonal bipyramids, faces with three equivalent CaO12 cuboctahedra, and a faceface with one AlO6 octahedra. The corner-sharing octahedral tilt angles are 52°. There is three shorter (1.89 Å) and three longer (1.97 Å) Al–O bond length. In the fourth Al3+ site, Al3+ is bonded to four O2- atoms to form corner-sharing AlO4 tetrahedra. The corner-sharing octahedra tilt angles range from 53–59°. There is three shorter (1.81 Å) and one longer (1.83 Å) Al–O bond length. In the fifth Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with two equivalent AlO6 octahedra, corners with three equivalent AlO4 tetrahedra, a cornercorner with one AlO5 trigonal bipyramid, an edgeedge with one CaO12 cuboctahedra, and edges with five AlO6 octahedra. The corner-sharing octahedral tilt angles are 52°. There are a spread of Al–O bond distances ranging from 1.82–2.02 Å. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded to four Al3+ atoms to form distorted corner-sharing OAl4 tetrahedra. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Al3+ atoms. In the third O2- site, O2- is bonded in a 3-coordinate geometry to one Ca2+ and three Al3+ atoms. In the fourth O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Al3+ atoms. In the fifth O2- site, O2- is bonded in a 5-coordinate geometry to two equivalent Ca2+ and three Al3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CaAl12O19 by Materials Project

CaAl12O19 crystallizes in the hexagonal P6_3mc space group. The structure is three-dimensional. Ca2+ is bonded to twelve O2- atoms to form CaO12 cuboctahedra that share corners with six equivalent CaO12 cuboctahedra, edges with six AlO6 octahedra, edges with three equivalent AlO5 trigonal bipyramids, and faces with six AlO6 octahedra. There are a spread of Ca–O bond distances ranging from 2.71–2.81 Å. There are eight inequivalent Al3+ sites. In the first Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with six equivalent AlO6 octahedra, corners with three equivalent AlO5 trigonal bipyramids, faces with three equivalent CaO12 cuboctahedra, and a faceface with one AlO6 octahedra. The corner-sharing octahedral tilt angles are 52°. There is three shorter (1.89 Å) and three longer (1.98 Å) Al–O bond length. In the second Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with six equivalent AlO6 octahedra, corners with three equivalent AlO5 trigonal bipyramids, faces with three equivalent CaO12 cuboctahedra, and a faceface with one AlO6 octahedra. The corner-sharing octahedral tilt angles are 52°. There is three shorter (1.89 Å) and three longer (1.96 Å) Al–O bond length. In the third Al3+ site, Al3+ is bonded to four O2- atoms to form corner-sharing AlO4 tetrahedra. The corner-sharing octahedra tilt angles range from 53–59°. There is three shorter (1.81 Å) and one longer (1.83 Å) Al–O bond length. In the fourth Al3+ site, Al3+ is bonded to four O2- atoms to form corner-sharing AlO4 tetrahedra. The corner-sharing octahedra tilt angles range from 53–59°. All Al–O bond lengths are 1.82 Å. In the fifth Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with six AlO4 tetrahedra and edges with six AlO6 octahedra. There is three shorter (1.89 Å) and three longer (1.90 Å) Al–O bond length. In the sixth Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with two equivalent AlO6 octahedra, corners with three AlO4 tetrahedra, a cornercorner with one AlO5 trigonal bipyramid, an edgeedge with one CaO12 cuboctahedra, and edges with five AlO6 octahedra. The corner-sharing octahedral tilt angles are 52°. There are a spread of Al–O bond distances ranging from 1.82–2.01 Å. In the seventh Al3+ site, Al3+ is bonded to six O2- atoms to form AlO6 octahedra that share corners with two equivalent AlO6 octahedra, corners with three AlO4 tetrahedra, a cornercorner with one AlO5 trigonal bipyramid, an edgeedge with one CaO12 cuboctahedra, and edges with five AlO6 octahedra. The corner-sharing octahedral tilt angles are 52°. There are a spread of Al–O bond distances ranging from 1.82–2.03 Å. In the eighth Al3+ site, Al3+ is bonded to five O2- atoms to form distorted AlO5 trigonal bipyramids that share corners with twelve AlO6 octahedra and edges with three equivalent CaO12 cuboctahedra. The corner-sharing octahedra tilt angles range from 37–62°. There are a spread of Al–O bond distances ranging from 1.77–2.39 Å. There are nine inequivalent O2- sites. In the first O2- site, O2- is bonded in a 5-coordinate geometry to two equivalent Ca2+ and three Al3+ atoms. In the second O2- site, O2- is bonded in a 4-coordinate geometry to four Al3+ atoms. In the third O2- site, O2- is bonded in a distorted tetrahedral geometry to four Al3+ atoms. In the fourth O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Al3+ atoms. In the fifth O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Al3+ atoms. In the sixth O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Al3+ atoms. In the seventh O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Al3+ atoms. In the eighth O2- site, O2- is bonded in a 3-coordinate geometry to one Ca2+ and three Al3+ atoms. In the ninth O2- site, O2- is bonded in a 3-coordinate geometry to one Ca2+ and three Al3+ atoms.

36 MATERIALS SCIENCE↗
NASA NTRS1985

Hibonite: Crystal Chemistry and Origin of Blue Coloration in Meteoritic Assemblages

The blue color and optical spectra of hibonite, a common constituent of refractory inclusions in carbonaceous chondrites, are discussed. Because they may be manifestations of exotic cation species stabilized in unusual coordination sites in the hibonite crystalstructure. Hibonite, ideally CaAl12O19, is conducive to atomic substitution of host Ca2+ and Al3+ ions by a variety of lanthanide and first series transition elements. The latter cations are responsible for the colors of many rock-forming minerals as a result of intraelectronic or intervalence transitions. The visible-region spectra of most oxide and silicate minerals are generally well understood. Assignments of absorption bands in meteoritic hibonite optical spectra due to uncertainties of cation valencies and complexities in the crystal structure are examined. The crystal chemistry of hibonite is reviewed, Mossbauer spectral measurements of iron-bearing hibonite and electronic transitions that may be responsible for the blue coloration of meteoritic hibonites are discussed.

Burns, R. G.↗
NASA NTRS1985

Titanium isotopic anomalies in hibonites from the Murchison carbonaceous chondrite

The isotopic compositions of titanium in eight grains of hibonite (CaAl12O19) from the carbonaceous chondrite Murchison have been determined by high precision secondary ion mass spectrometry using an ion microprobe. The titanium in the hibonites varies greatly in Ti-50 from about -42 to +8 permil (relative to terrestrial) with smaller (up to 4 permil), but clearly resolvable, effects in Ti-46 and Ti-48. These results confirm the presence of widespread negative anomalies suggested by the results of Hutcheon et al. (1983) on hibonites from Murchison. The magnitude of these variations seems explicable only in terms of nucleogenic processes which produced extremely variable titanium isotopic abundances in the hibonite source materials. The hibonites evidently did not participate to the same extent as most material in the mixing and homogenisation processes that accompanied the formation and later evolution of the solar system.

Ireland, T.↗
NASA NTRS2016

Stacking Defects in Synthetic and Meteoritic Hibonites: Implications for High-Temperature Processes in the Solar Nebula

Hibonite (CaAl12O19) is a primary, highly refractory phase occurring in many Ca-Al-rich inclusions (CAIs) from different chondrite groups, except CI chondrites. Hibonite is predicted to be one of the earliest minerals to condense during cooling of the solar nebula at higher temperatures than any other major CAI mineral. Therefore, hibonite has great potential to reveal the processes and conditions of the very early, high-temperature stages of the solar nebular evolution. Previous microstructural studies of hibonite in CAIs and their Wark-Lovering (WL) rims showed the presence of numerous stacking defects in hibonite. These defects are interpreted as the modification of the stacking sequences of spinel and Ca-containing blocks within the ideal hexagonal hibonite structure, as shown by experimental studies of reaction-sintered ceramic CaO-Al2O3 compounds. We performed preliminary experiments in the CaO-Al2O3-MgO system to understand the formation processes and conditions of defect-structured hibonite found in meteorites.

Han, J.↗
NASA NTRS2016

Experimental Insights into the Origin of Defect-Structured Hibonites Found in Meteorites

Hibonite (CaAl12O19) is a primary, highly refractory phase occurring in many Ca-Al-rich inclusions (CAIs). Previous microstructural studies of hibonite in CAIs and their Wark-Lovering (WL) rims showed the presence of numerous stacking defects in hibonites. These defects are interpreted as the modification of the stacking sequences of spinel and Ca-containing blocks within the ideal hexagonal hibonite structure due to the presence of wider spinel blocks [3], as shown by experimental studies of reaction-sintered compounds in the CaO-Al2O3 system. We performed a series of experiments in the CaO-Al2O3-MgO system in order to provide additional in-sights into the formation processes and conditions of defect-structured hibonites found in meteorites.

Han. J.↗