publisher: Elsevier articletitle: Modal mineralogy of CI and CI-like chondrites by X-ray diffraction journaltitle: Geochimica et Cosmochimica Acta articlelink: http://dx.doi.org/10.1016/j.gca.2015.05.038 content_type: article copyright: Copyright © 2015 The Authors. Published by Elsevier Ltd

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO 2 -bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation.

The CI and CI-like chondrites provide a record of aqueous alteration in the early solar system. However, the CI-like chondrites differ in having also experienced a late stage period of thermal metamorphism. In order to constrain the nature and extent of the aqueous and thermal alteration, we have investigated the bulk mineralogy and abundance of H 2 O in the CI and CI-like chondrites using thermogravimetric analysis and infrared spectroscopy. The CI chondrites Ivuna and Orgueil show significant mass loss (28.5-31.8 wt.%) upon heating to 1000°C due to dehydration and dehydroxylation of abundant phyllosilicates and Fe-(oxy)hydroxides and the decomposition of Fe-sulphides, carbonates and organics. Infrared spectra for Ivuna and Orgueil have a prominent 3-μm feature due to bound −OH/H 2 O in phyllosilicates and Fe-(oxy)hydroxides and only a minor 11-μm feature from anhydrous silicates. These characteristics are consistent with previous studies indicating that the CI chondrites underwent near-complete aqueous alteration. Similarities in the total abundance of H 2 O and 3 μm/11 μm ratio suggest that there is no difference in the relative degree of hydration experienced by Ivuna and Orgueil. In contrast, the CI-like chondrites Y-82162 and Y-980115 show lower mass loss (13.8-18.8 wt.%) and contain >50 % less H 2 O than the CI chondrites. The 3-μm feature is almost absent from spectra of Y-82162 and Y-980115 but the 11-μm feature is intense. The CI-like chondrites experienced thermal metamorphism at temperatures >500°C that initially caused dehydration and dehydroxylation of phyllosilicates before partial recrystallization back into anhydrous silicates. The surfaces of many C-type asteroids were probably heated through impact metamorphism and/or solar radiation, so thermally altered carbonaceous chondrites are likely good analogues for samples that will be returned by the Hayabusa-2 and OSIRIS-REx missions.

Abstract-The CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water-rich asteroids. We show that CM1 chondrites contain abundant (84-91 vol%) phyllosilicate, plus olivine (4-8 vol%), magnetite (2-3 vol%), Fe-sulfide (<5 vol%), and calcite (<2 vol%). The CM1/2 chondrites contain phyllosilicate (71-88 vol%), olivine (4-20 vol%), enstatite (2-6 vol%), magnetite (2-3 vol%), Fe-sulfides (1-2 vol%), and calcite (~1 vol%). As aqueous alteration progressed, the abundance of Mg-serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM1/2 and CM1 chondrites are often depleted relative to the CM2s. The modal data support the model, whereby metal and Fe-rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg-rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM1 chondrites experienced increased alteration due to elevated temperatures (>120°C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that "CM1-like" asteroids, as has been proposed for Bennu-target for the OSIRISREx mission-will have a high abundance of Mg-rich phyllosilicates and Fe-oxides, but be depleted in calcite.

The highly hydrated, petrologic type 1 CM and CI carbonaceous chondrites likely derived from primitive, water-rich asteroids, two of which are the targets for JAXA's Hayabusa2 and NASA's OSIRIS-REx missions. We have collected visible and near-infrared (VNIR) and mid infrared (MIR) reflectance spectra from well-characterized CM1/2, CM1, and CI1 chondrites and identified trends related to their mineralogy and degree of secondary processing. The spectral slope between 0.65 and 1.05 lm decreases with increasing total phyllosilicate abundance and increasing magnetite abundance, both of which are associated with more extensive aqueous alteration. Furthermore, features at 3 lm shift from centers near 2.80 lm in the intermediately altered CM1/2 chondrites to near 2.73 lm in the highly altered CM1 chondrites. The Christiansen features (CF) and the transparency features shift to shorter wavelengths as the phyllosilicate composition of the meteorites becomes more Mg-rich, which occurs as aqueous alteration proceeds. Spectra also show a feature near 6 lm, which is related to the presence of phyllosilicates, but is not a reliable parameter for estimating the degree of aqueous alteration. The observed trends can be used to estimate the surface mineralogy and the degree of aqueous alteration in remote observations of asteroids. For example, (1) Ceres has a sharp feature near 2.72 lm, which is similar in both position and shape to the same feature in the spectra of the highly altered CM1 MIL 05137, suggesting abundant Mg-rich phyllosilicates on the surface. Notably, both OSIRIS-REx and Hayabusa2 have onboard instruments which cover the VNIR and MIR wavelength ranges, so the results presented here will help in corroborating initial results from Bennu and Ryugu. Raymond 2011). Currently, JAXA's Hayabusa2 is at the Cg-type asteroid (162173) Ryugu, and NASA's Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx, Lauretta et al. 2017) is at the B-type asteroid (101955) Bennu. Both missions aim to collect and return samples from the surfaces of their target asteroids. The returned samples will have geological context (provided by the remote sensing instruments on the spacecraft) and will not be subject to any terrestrial contamination, providing an unprecedented opportunity to study the formation and

Interstellar dust (ISD) from the local interstellar medium (LISM) streams into the solar system from approximately the direction of the constellation Ophiuchus. Prior to the return of the NASA Stardust spacecraft (1) no recognizable samples of this interstellar dust were available for laboratory study. Thus, our understanding of the properties of contemporary ISD has been derived primarily from astronomical observations of the ISM, including optical properties of the ISD and remote spectroscopy of the gas composition (2-4), and from in situ measurements by the dust analyzers on the Cassini, Ulysses and Galileo spacecraft (5-7). The canonical picture of ISD is that it is dominated by ~0.2 µm diameter (8) amorphous silicate grains, with or without carbonaceous mantles. However, the inferred properties of the particles, including size distribution, density and composition are heavily model dependent.

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