Presolar silicates in the solar system

Presolar grains are extraterrestrial particles that predate our solar system, providing a rare glimpse into the stellar processes (e.g., nucleosynthesis, mixing, and dust condensation) and the early history of our solar system. Over 2000 presolar silicate grains have been identified since their initial discovery in interplanetary dust particles (IDPs) just over two decades ago. Research has shown that silicates are the second most abundant type of presolar grain after nanodiamonds, with concentrations of approximately 200ppm in the most primitive meteorites and up to 400ppm in anhydrous IDPs. The oxygen isotopic compositions of presolar silicates are similar to those of presolar oxides, with most grains originating from low-mass red giant or asymptotic giant branch stars with near-solar metallicity. Most of these grains are ferromagnesian silicates with high iron concentrations. TEM studies indicate that many presolar silicates have amorphous structures with heterogeneous and nonstoichiometric compositions, suggesting that the stellar environments where these grains formed were variable and rapidly changing, leading to grain condensation under nonequilibrium kinetic conditions. Presolar silicates also contain evidence for secondary processes that occurred in the solar nebula and the parent bodies of the meteorites in which they are found. Variations in abundance within individual meteorites provide insights into these secondary processes, with both thermal metamorphism and aqueous alteration leading to changes in the grains’ elemental compositions and microstructures. Studies of presolar silicates and other presolar grain types offer additional constraints on stellar environments and nucleosynthetic processes. This chapter reviews our current understanding of presolar silicate grains, which represent one key category of these particles, focusing on their identification, origins, and implications for astrophysics.