Spectra for certain comets show the presence of crystalline silicate dust grains believed to have been incorporated during comet formation. While grain crystallization is widely assumed to result from the thermal annealing of precursor amorphous grains, the physical processes behind the silicate amorphous-to-crystalline transition are poorly understood. This makes it difficult to place constraints on the evolutionary histories of both grains and comets, and consequently, on the nebular conditions in which they formed. It has, therefore, become necessary to study this process in the laboratory using simulated grain materials. In this paper, we discuss recent results from laboratory investigations into a basic amorphous MgSiO3 silicate annealed in the region of 1000 K. Our object is not to model the behavior of dust grains per se, but to study the underlying process of crystallization and separate the physics of the material from the astrophysics of dust grains. In our experiments, we bring together spectroscopic measurements made in the infrared with the high resolution structural probing capabilities of synchrotron X-ray powder diffraction. The combined use of these complementary techniques provides insights into the crystallization process that would not be easily obtained if each was used in isolation. In particular, we focus on the extent to which the identification of certain spectral features attributed to crystalline phases extends to the physical structure of the grain material itself. Specifically, we have identified several key features in the way amorphous MgSiO3 behaves when annealed. Rather than crystallize directly to enstatite (MgSiO3) structures, in crystallographic terms, amorphous MgSiO3 can enter a mixed phase of crystalline forsterite (Mg2SiO4) and SiO2-rich amorphous silicate where structural evolution appears to stall. Spectroscopically, the evolution of the 10 micrometer band does not appear to correlate directly with structural evolution, and therefore, may be a poor indicator of the degree of crystallinity. Indeed, certain features in this band may not be indicators of crystal type. However, the 20 micrometer band is found to be a good indicator of crystal structure. We suggest that forsterite forms from the ordering of pre-existing regions rich in SiO4 and that this phase separation is aided by a dehydrogenation processes that results in the evolutionary stall. The implications of this work regarding future observations of comets are discussed.

Two types of pallasites can be distinguished on the basis of the grain shape of olivine (rounded or angular). It has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body. In order to characterize the kinetics of rounding of olivine grains in an Fe-Ni matrix, we carried out a series of annealing experiments using a mixture of olivine and Fe-Ni powder. We were able to reproduce, at a miniature scale, the range of textures in pallasites. The rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 micrometers, even though the experiments were performed below the solidus of the Fe-Ni metal. For instance, grains 14 m in diameter became nearly spherical within 7 days at 1400 degrees C. For the morphological analysis of olivine grains, we used two independent techniques: the "critical diameter method" and the "Gaussian diffusion-resample method," a new technique specially developed for our study. Both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice, not by surface diffusion along the olivine-metal boundaries. We used our experimental data to estimate the time scales required for the development of olivine-metal textures in natural pallasites. We determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals: 100 years to produce rounded olivine grains 0.1 mm in radius at 1300-1400C.

Fassaite is a major component of Ca-Al-rich inclusions (CAIs) of Types B and degrees C that crystallized from liquids. In contrast, this mineral is rarely reported in Type A inclusions and has been much less studied. In this paper, we report highly Ti-, Al-enriched fassaite that occurs as rims on perovskite in two compact Type A inclusions from the Ningqiang meteorite. In addition, one of the inclusions contains an euhedral grain of Sc-fassaite (16.4 wt% Sc2O3) isolated in melilite. The occurrence and mineral chemistry of the fassaite rims can be explained by a reaction of pre-existing perovskite with CAI melts. Hence, such rims may serve as an indicator for partial melting of Type A inclusions. The Sc-fassaite is probably a relict grain. A third spherical CAI contains several euhedral grains of V-fassaite (4.8-5.4 wt% V2O3) enclosed in a melilite fragment. The high V content of fassaite cannot be related to any Fremdlinge, magnetite, or metallic Fe-Ni, because these phases are absent in the inclusion. In the same CAI, other fassaites intergrow with spinel and minor perovskite, filling voids inside of the melilite and space adjacent to the Wark-Lovering rim. The fassaite intergrown with spinel is almost V-free. The coexistence of two types of fassaite suggests that this CAI has not been completely melted.

The 50,000 year old, 1.8 km diameter Lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock (the 65 million year old Deccan Traps). The composition of the Lonar basalts is similar to martian basaltic meteorites, which establishes Lonar as an excellent analogue for similarly sized craters on the surface of Mars. Samples from cores drilled into the Lonar crater floor show that there are basaltic impact breccias that have been altered by post-impact hydrothermal processes to produce an assemblage of secondary alteration minerals. Microprobe data and X-ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite, with minor celadonite, and carbonate. Thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130 degrees C and 200 degrees C. By comparing the Lonar alteration assemblage with alteration at other terrestrial craters, we conclude that the Lonar crater represents a lower size limit for impact-induced hydrothermal activity. Based on these results, we suggest that similarly sized craters on Mars have the potential to form hydrothermal systems, as long as liquid water was present on or near the martian surface. Furthermore, the Fe-rich alteration minerals produced by post-impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil.

Hydrogen isotope ratios of organic compounds in carbonaceous chondrites provide critical information about their origins and evolutionary history. However, because many of these compounds are obtained by aqueous extraction, the degree of hydrogen-deuterium (H/D) exchange that occurs during the process needs to be quantitatively evaluated. This study uses compound- specific hydrogen isotopic analysis to quantify the H/D exchange during aqueous extraction. Three common meteoritic dicarboxylic acids (succinic, glutaric, and 2-methyl glutaric acids) were refluxed under conditions simulating the extraction process. Changes in delta-D values of the dicarboxylic acids were measured following the reflux experiments. A pseudo-first order rate law was used to model the H/D exchange rates which were then used to calculate the isotope exchange resulting from aqueous extraction. The degree of H/D exchange varies as a result of differences in molecular structure, the alkalinity of the extraction solution and presence/absence of meteorite powder. However, our model indicates that succinic, glutaric, and 2-methyl glutaric acids with a delta-D of 1800 would experience isotope changes of 38 ppm, 10 ppm, and 6 ppm, respectively during the extraction process. Therefore, the overall change in delta-D values of the dicarboxylic acids during the aqueous extraction process is negligible. We also demonstrate that H/D exchange occurs on the chiral alpha-carbon in 2-methyl glutaric acid. The results suggest that the racemic mixture of 2-methyl glutaric acid in the Tagish Lake meteorite could result from post-synthesis aqueous alteration. The approach employed in this study can also be used to quantify H/D exchange for other important meteoritic compounds such as amino acids.