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Paper: Microcrystals and Amorphous Material in Comets and Primitive Meteorites: Keys to Understanding Processes in the Early Solar System
Volume: 341, Chondrites and the Protoplanetary Disk
Page: 675
Authors: Nuth, J.A., III.; Brearley, A.J.; Scott, E.R.D.
Abstract: Silicate dust entering the solar nebula was overwhelmingly amorphous based on studies of the current interstellar grain population. Moreover, no global high temperature event could have crystallized the dust in the nebula without also destroying the very fragile, noble-gas-rich, carbonaceous presolar grains found in all unaltered primitive meteorites, though more localized heating events undoubtedly occurred. Fine-grained, crystalline silicates in comets, meteorites and IDPs are therefore products of relatively localized thermal and/or aqueous processes and their presence can potentially be used to constrain the metamorphic events that occurred both in the nebula prior to the formation of primitive planetesimals as well as during subsequent parent body processing. As examples, the presence of crystalline magnesium silicates in comets requires both grain annealing and transport out to the nebular environment where comets form (50-200 AU), whereas in primitive chondrite matrices, crystalline Mg-silicates probably formed by evaporation, recondensation and annealing in the inner nebula. Nevertheless, the presence in both IDPs and chondrite matrices of Mn-rich forsterite and enstatite grains with structures indicative of cooling in days shows that identical thermal processes formed some magnesium silicates in comets and asteroids. The bulk of the amorphous material in primitive chondrite matrices may have formed by evaporation and condensation during high temperature chondrule-forming events. Crystalline FeO-rich silicates, such as olivine and phyllosilicates, formed by secondary processes such as thermal metamorphism and aqueous alteration on asteroidal parent bodies and are not the products of primary nebular processes. We explore several such processes and their wider implications for the chemistry of protostellar systems in this chapter.

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