| Summary: | We investigate the composition of C-type asteroids and comets providing insights with the combined petrographic and spectroscopic study of micrometeorites (MMs) (TAM5.29, TAM5.30, TAM18c.11 and TAM18c.13) and laboratory experiments on carbonaceous chondrites (CCs): FRO95002, FRO99040 and FRO90006 (CO3 CCs), MCY14001 (CM2) and DaG521 (CV3). The composition of C-type asteroids is similar to that of CCs. The IR reflectance spectra of C-type asteroids show some unresolved features, in particular the 3 μm band is difficult to attribute to a specific compound and the origin of the organic matter indicated by the 3.4-3.5 μm bands is still debated. The main processes suggested to create these features are low-T aqueous alteration, hydrothermalism and cryovolcanism. TAM5.29 is dominated by a matrix of Fe-olivines and clasts of andradite surrounded by diopside-jarosite derived from hydrothermal alteration at T~250°C. A second episode of alteration at T<100°C resulted in the formation of iddingsite. TAM5.29 mineralogy is unique among the MMs and is also different from the known CV CCs representing a newly described alteration environment on the CV parent body. TAM18c.11 and TAM18c.13 have a high Antarctic alteration with widespread replacement with jarosite. Phyllosilicates are recognisable along with small metal alloys and CH, SH and NH functional groups are ubiquitous. TAM5.30 is a CO-like MM and records a processing history characterised by an initial episode of low-temperature CM-like aqueous alteration, which later transitioned into higher temperature (~300°C) CV-like metasomatic alteration. The 3.15 μm band of TAM5.29 is found scattered around the matrix, thus related to Fe-rich hydrous phases. An isolated spot gave spectra characterised by a 3.04 μm band that always appears together with the 3.4 μm and 3.5 μm bands of the aliphatic organics, indicating a relation between the 3.04 μm band and the organic matter. In TAM18c.11 the 3 μm is found at 3.05-3.06 μm in close relationship with ammonium-jarosite. Spectra of ...
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