Atomistic simulations of aqueous alteration processes of mafic silicates in carbonaceous chondrites

Abstract

There is clear evidence that the parent bodies of some groups of carbonaceous chondrites suffered extensive aqueous alteration. In some pristine chondrites even some minerals can be envisioned to have formed in the protoplanetary disk itself. There is clear evidence that water was available in these materials at early times, and when water alteration occurred element mobilization took place and the primordial chemistry/mineralogy changed. In general, mafic silicates were initially forming chondrules and also matrix fragments, but with time they became transformed into phyllosilicates. With the aim to have atomic-scale in-sights of these water alteration processes, in this chapter we present our first atomistic simulations related to the water alteration of silicates obtained by means of quantum chemical calculations. Results are based on the molecular simulation of the interaction of 12 water molecules with different forsteritic (Mg2SiO4) silicate surfaces of different stability and structural state. Relevant structural information (i.e., significant changes due to water interaction) and vibrational properties, including simulation of the infrared spectra, of the water/silicate interface are shown, and energetic features and thermodynamic trends of the water alteration reactions are provided. Finally, we compare such theoretical calculations with the evidence inferred from the mineralogical interpretation of the formation conditions of carbonaceous chondrites. We conclude that parent body aqueous alteration is the preferred scenario to produce the hydration features observed in these primitive meteorites.