Structure and non-stoichiometry of calcium aluminates

Abstract

Lattice imaging of reaction-sintered ceramic compounds in the CaO-

Microstructural evidence for a disequilibrium condensation origin for hibonite‐spinel inclusions in the ALHA77307 CO3.0 chondrite

“…This compositional change can be due to the introduction of the (S + S + C) phase into the ideal hibonite structure. The hibonite grains in CAIs 03 and 08 show features identical to those observed by Schmid and De Jonghe () who performed an experimental study of reaction‐sintered ceramic CaO‐Al 2 O 3 compounds, supporting our interpretations of the modification of the stacking sequence in the hibonite structure. Similar microstructures were observed in hibonite from a Wark‐Lovering rim (Keller ), suggesting common formation histories for the different occurrences of hibonite.…”

“…Ideal hibonite consists of a sequence of one spinel (S) block alternating with one Ca‐containing (C) block parallel to (001), resulting in a unit cell containing two (S + C) blocks (S + C + S* + C*), where * = a rotation of the block through 180° around the c axis (Fig. a) (Schmid and De Jonghe ; Nagashima et al. ).…”

“…The C block contains Al cations in octahedral sites between the oxygen layers and in between three oxygen atoms of the central oxygen layer, giving the unit formula CaAl 6 O 11 . Unlike the (S + C + S* + C*) stacking sequence of the ideal hibonite structure, these S and C blocks can be stacked in different ratios, yielding a change in length of the c axis and unit formula (i.e., composition) (Schmid and De Jonghe ). In CAIs 03 and 08, hibonite grains show lattice fringes with predominantly 1.1 nm (002) spacings (Fig.…”

“…Projected structural views of unit cells of the hibonite (a) and (S + S + C) phase (b) parallel to the (001) plane. These were modified from Schmid and De Jonghe (). The structure of S* and C* is the blocks rotated although 180° along the c axis relative to the structure of S and C. S', SS ', and C' indicate the c axis length of individual layers.…”

Corundum–hibonite inclusions and the environments of high temperature processing in the early Solar System

“…Additionally, the observed planar defects in hibonite are an indicator of the Ca deficiency in hibonite. Stoichiometric, defect-free hibonite consists of a sequence of one spinel block alternating with one Ca-containing block, while defect-rich regions in hibonite contain extra spinel blocks (Schmid and De Jonghe, 1983;Han et al, 2015). Under Ca-depleted conditions, Al in the gas phase may have been incorporated into the hibonite as extra spinel blocks, resulting in the planar defects in ideally structured hibonite.…”