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(OHx)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(OHx)4 fragment between the layers of Mg(OH)2, (2) adsorption of the Si(OHx)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(OHx)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.