Structure and Properties of Water in a New Model of the 10-& Aring; Phase: Classical and Ab Initio Atomistic Computational Modeling

The 10-& Aring; phase is an important member of the family of dense hydrous magnesium silicates (DHMSs) that play a major role in the water budget in the Earth's upper mantle. Its nominal composition is usually written as Mg3Si4O10(OH)(2).xH(2)O, and its structure is often described as layers of talc with some amount of water present in the interlayer space. However, its actual structure and composition and the detailed mechanisms of retaining H2O molecules within the mineral are not yet sufficiently known. In particular, more recent spectroscopic and diffraction data indicate the presence of Si vacancies in the tetrahedral silicate sheets of the 10-& Aring; phase leading to the formation of Q(2)-type Si sites terminated by silanol groups. These silanols are, in turn, hydrogen bonded to interlayer H2O molecules. Here, we use classical and ab initio molecular dynamics (MD) simulations to compare the structures and properties of ideal and defect models of the 10-& Aring; phase under ambient conditions. For classical MD simulations, the most recent modification of the ClayFF force field is used, which can accurately account for the bending of Mg-O-H and Si-O-H angles in the mineral layers, including the structural defects. The crystal lattice parameters, elastic constants, structure, and dynamics of the interlayer hydrogen bonding network for the model 10-& Aring; phase are calculated and compared with available experimental data. The results demonstrate that the inclusion of Si vacancies leads to better agreement with crystallographic data, elastic constants, and bulk and shear moduli compared to a simpler model based on the idealized talc structure. The results also clearly illustrate the importance of the explicit inclusion of Mg-O-H and Si-O-H angular bending terms for accurate modeling of the 10-& Aring; phase. In particular, the properly constrained orientation of the silanol groups promotes the formation of strong hydrogen bonds with the interlayer H2O molecules.

Automated summary

The 10Å phase is an important component in the Earth's upper mantle water budget, and its structure and hydrous mechanisms are not fully understood. Recent spectroscopy and diffraction data reveal the presence of Si vacancies and silanol groups in the mineral. Molecular dynamics simulations show that the inclusion of Si vacancies leads to improved agreement with experimental data, indicating the significance of Si vacancies in the structure and dynamics of the 10Å phase.