Transmission electron microscopy of a refractory inclusion from the Allende meteorite: Anatomy of a pyroxene

Abstract

Abstract-A crystal ofclinopyroxene from the coarse-grained refractory inclusion Egg6 of the Allende meteorite has been studied in detail by transmission electron microscopy.The pyroxene crystal contains euhedral, dislocation-free inclusions of pure spinel MgAlz04, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high-temperature anneal, are present in the crystal. Alteration products are occasionally observed at the spinel-pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape oftubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations.The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of metasomatism.

Transmission electron microscope study of compact Type A calcium‐aluminum‐rich inclusions from CV3 chondrites: Clues to their origin

“…This is in agreement with previous studies on Type B CAIs (Barber el al., 1984; Doukhan et al, 1991). Although Doukhan et al (1991) reported some crystals with a moderate dislocation density, the total absence of mechanical twins clearly indicates that the fassaite crystals did not experience high mechanical stress. The defect-poor microstructure of the pyroxenes is in sharp contrast to the highly strained melilites.…”

“…Selected area electron diffraction analysis and HRTEM proved the absence of any topotactic orientation relationship between melilite and spinel as well as between fassaite and spinel. The crystallographic relations typical for solid-state exsolution of spinel from pyroxene are absent (Doukhan et aL, 1984(Doukhan et aL, , 1991. In the Allende CAI (A37), spinels were found to be twinned on { 1 1 1) (Fig.…”

“…So far, there are only very few TEM investigations of coarse-grained CAIs in carbonaceous chondrites (Muller, 1978;Martin and Barber, 1979;Miiller and Wlotzka, 1982;Barber et al, 1984;Doukhan et al, 1991;Keller and Buseck, 1994). Therefore, a detailed transmission electron microscope study of three compact Type A CAls from the CV3 chondrites Allende, Acfer 082, and Acfer 086 was performed in order to decipher the nature and origin of these inclusions.…”

Microstructural analysis of Wark‐Lovering rims in the Allende and Axtell CV3 chondrites: Implications for high‐temperature nebular processes

“…In comparison, there are fewer studies that have investigated the crystal chemistry and structure of CAI minerals, but these have provided valuable glimpses into their complex microstuctures (Doukhan et al. ; Greshake et al. ).…”

“…The optical microscope and electron and ion microprobes have been used extensively to study the properties of CAIs and have provided a wealth of information on their origins (e.g., Grossman 1975;Allen et al 1978;Armstrong et al 1982;MacPherson et al 1983;Kornacki and Wood 1984;Ireland 1988;Simon et al 1994;Krot et al 1995). In comparison, there are fewer studies that have investigated the crystal chemistry and structure of CAI minerals, but these have provided valuable glimpses into their complex microstuctures (Doukhan et al 1991;Greshake et al 1998). For example, Barber et al (1984) examined the microstructure of CAIs from the Allende CV3 chondrite via transmission electron microscopy (TEM) and concluded that formation occurred under conditions close to equilibrium through slow cooling and postcrystallization effects of solid-state diffusion.…”