Silicate minerals and Si-O glass in comet Wild 2 samples: Transmission electron microscopy

Abstract

A transmission electron microscope (TEM) study of seven comet Wild 2 samples shows that three samples consist mainly of olivine and pyroxene and four samples consist of Mg-Fe-bearing Si-O glass with minor amounts of Fe-Ni sulfide and metal. The olivine in the silicate-rich samples differs in fayalite content between the samples and shows a wide range of fayalite content within individual samples, indicating that the degree of thermal metamorphism on the comet, if any, was extremely low. One olivine grain has a high density of dislocations with Burgers vector b = [001], suggesting that the Wild 2 particles experienced hypervelocity impacts before capture. The structural type and composition of pyroxene differ between the samples and within individual samples. Both low-Ca and high-Ca pyroxenes are present. Enstatite occurs as ortho- and clinoenstatite, suggesting that the Wild 2 particles contain materials that went through distinct high-temperature and cooling histories. One silicate-rich sample exhibits a zone texture consisting of a core of low-Ca pyroxene surrounded by an inner rim of Mg-Fe-bearing Si-O-rich glass and an outer rim of melted aerogel. The texture suggests that the inner rim was formed by the mixing of melted cometary low-Ca pyroxene and melted aerogel during capture heating. The four Mg-Fe-bearing, Si-O glassrich samples show close similarities in mineralogy and texture to the inner rim of the zoned silicate-rich sample. The four samples are probably secondary products formed by interaction between melted cometary silicates and melted aerogel during the capture process.