Aqueous Alteration of Forsterite Glass in Planetesimals

To investigate the aqueous alteration mechanisms of forsterite glass in planetesimals, molecular dynamics calculations were performed using two-phase systems of forsterite glass and liquid water. The structures formed during aqueous alteration were classified into four phases: (i) the Mg(OH)₂ crystal-like structures, (ii) fragments consisting of Si(OH_x)₄, where x = 0–2, (iii) liquid water containing dissolved Mg and Si, and (iv) the residue of forsterite glass. The formation of phyllosilicates, such as serpentine (Mg₃Si₂O₅(OH)₄), talc (Mg₃Si₄O₁₀(OH)₂), and magnesium silicate hydrate (M–S–H), from these four phases may occur through three possible processes: (1) penetration of the Si(OH_x)₄ fragment between the layers of Mg(OH)₂, (2) adsorption of the Si(OH_x)₄ fragment on Mg(OH)₂, and (3) structural change from the residue. A similar feature of the radial distribution function for Si–Si pairs suggests that the Si(OH_x)₄ fragment is a possible origin of serpentine or talc. These results suggest that the interstitial water in mineral grains plays an important role in mineral evolution in planetesimals.