Are There Clues to the Dust 'Annealing' Process in Protoplanetary Disks in IDPs?

Introduction: Silicate dust in the interstellar medium (ISM) is almost entirely amorphous [1]. However, dust at the surfaces of protoplanetary disks has crystallinities of 10-95%, and at least in some cases this crystallinity decreases with increasing distance from the central star [2]. This 'annealing' of ISM dust requires temperatures of at least 1000 K, and is clear evidence that an energetic process is operating in these disks. There is ample evidence in meteorites and IDPs for high temperature processes operating in the early Solar System. It is possible that one of these processes is also responsible for the crystallinity of dust in protoplanetary disks. Based on similarities in their IR spectra, chondritic porous interplanetary dust particles (CP-IDPs) are the best analogs available for the dust in protoplanetary disks. Here we review the properties CP-IDPs and their components to determine whether they hold clues to the process responsible for the 'annealing'. Components: Crystalline silicates are a major (~20-50 vol%) component of anhydrous IDPs and occur as single crystals ranging in size from 0.1µm to several µm, as well as in polycrystalline equilibrated aggregates (EAs) with constituent grain sizes of <0.5 µm [3]. The other major components are amorphous silicates, largely in the form of GEMS (glass with embedded metal and sulfide), and insoluble organic matter (IOM) [3]. The single crystals are typically almost pure enstatite and forsterite. The microstructure of the enstatite suggests that it formed by condensation and, at least in the case of platelets, at temperatures above the proto-orthopyroxene transition (1258 K) [4]. The single olivines are also assumed to be condensates. The enstatite/forsterite ratio varies considerably between IDPs, but forsterite seems to dominate in comets. In the equilibrium condensation sequence, pyroxene forms by reaction between olivine and SiO 2 in the gas. However, there is no evidence for this reaction in IDPs, and the enstatite probably condensed directly from the gas [4]. Whether forsterite or enstatite formed may simply have reflected the conditions (e.g., temperature) at which grain formation became possible. Crystalline silicates also occur in EAs that are a common minor (<10 vol%) component of CP IDPs. These µm-sized aggregates contain numerous grains of enstatite, pyrrhotite (Fe 1-x S), minor forsterite, and an interstitial amorphous Si-rich phase. The textures, mineralogy and mineral chemistry of EAs are