Laboratory hydration of condensed magnesiosilica smokes with implications for hydrated silicates in IDPs and comets

Abstract

Abstract-Samples of silica-rich and MgO-rich condensed, amorphous magnesiosilica smokes were hydrated to monitor systematic mineralogical and chemical changes as a function of time and temperature controlled by their unique metastable eutectic compositions, their porous texture, and the ultrafine, nanometer grain size of all entities. At water supersaturated conditions, proto-phyllosilicates formed by spinodal-type homogeneous nucleation. Their formation and subsequent growth was entirely determined by the availability of water via pore spaces inherited from the original smokes and the textural continuity of magnesiosilica material with a mostly smectite-dehydroxylate composition. The results may have implications for the hydration of proto-CI material, the presence of rare periclase and brucite in primitive solar system bodies, and the pervasiveness of hydrated amorphous magnesiosilica dust and saponite proto-phyllosilicates in icy-protoplanets, such as comet nuclei.

Dynamic pyrometamorphism during atmospheric entry of large (˜10 micron) pyrrhotite fragments from cluster IDPs

“…However, traces of layer silicates were probably present in Halley dust (Rietmeijer et al 1989), minor amounts layer silicates are present in porous aggregate IDPs Rietmeijer and Mackinnon 1985a) and meteors with CI-like physical properties enter the Earth's atmosphere on comet-like orbits (Rietmeijer 2000) offering ample support that hydrated aggregate IDPs, cluster IDPs, and even larger aggregate debris is associated with cometary sources. Hydrocryogenic alteration in dirty-ice mixtures (Rietmeijer 1985(Rietmeijer , 2002a) and laboratory hydration experiments on amorphous MgSiO dust analogs (Rietmeijer et al 2004) provide a framework supporting the feasibility of dust hydration in cometary environments. The original subdivision of anhydrous and hydrated IDPs was based on the findings of an infrared study of 26 chondritic IDPs.…”

“…When water react with silicates in anhydrous IDP the extent of hydration will be a function of water/"rock" ratios, time and temperature of hydration as well as the ability of water to move through a porous medium. Laboratory hydration of a porous condensed MgSiO smoke demonstrated that porosity is indeed an important controlling factor during hydration (Rietmeijer et al 2004). The most likely outcome of the hydration process would be a continuous and gradual transition from anhydrous silicates (crystalline or amorphous) to partially and to fully-hydrated silicates among aggregate IDPs that can be from the same parent body or from different parent bodies.…”

“…This particular porous structure does not appear to be a gas-release feature but rather due to thermal annealing of a porous aggregate precursor as was observed developing during thermal annealing and grain coarsening of condensed porous magnesiosilica smokes . Hydration of the same smoke sample showed that hydration tends to collapse the originally anhydrous, porous smoke into a dense hydrated MgSiO-material with a disappearance of any traces of the highly porous original smoke (Rietmeijer et al 2004). This finding is consistent with the observation that hydrated IDPs tend to have a relatively uniform texture of low porosity (Bradley 1998).…”

The formation of Mg,Fe‐silicates by reactions between amorphous magnesiosilica smoke particles and metallic iron nanograins with implications for comet silicate origins

“…Vapor phase‐condensed amorphous magnesiosilica grains typically have distinct DME serpentine (Mg 3 Si 2 O 7 ) and smectite (Mg 6 Si 8 O 22 ) dehydroxylate compositions and a rare amorphous Mg 8 SiO 10 compound (Rietmeijer et al. , , , ). The absence of these nanograin compositions in the present study is consistent with the compactness of the original magnesiosilica smoke that allowed efficient chemical diffusion.…”

“…Periclase (MgO) due to brucite dehydration and DME amorphous Mg 8 SiO 10 grains (Rietmeijer et al. , ) reacting with iron melt could account for the low‐Si ferromagnesiosilica grains. Although Mg,Fe‐oxides (Fe/[Mg + Fe] <0.3) are very rare (Fig.…”