Hydrogen distribution in chondrodite: A first-principles calculation

In synthetic hydroxyl-chondrodite and hydroxyl-clinohumite, two hydrogen sites (H1 and H2) are known. However, hydrogen occupying only H1 site has been reported for natural F- and Ti-rich chondrodite and clinohumite. In order to understand hydrogen distribution in these sites, stability of hydrogens at these sites in chondrodite is studied by first-principles density functional theory calculation. For hydroxyl-chondrodite, lowest energy was found when both H1 and H2 sites are half occupied by hydrogens. In this configuration, strong H1-H1 repulsion is avoided, and nearly straight hydrogen bond of O-H1...O-H2 is realized. When a half of OH^- is replaced with F^- or replaced with O^2- in the case of Ti-containing systems, hydrogen occupying H1 site is always favored over hydrogen in H2 site so as to form nearly straight hydrogen bond of O-H1...F^- (or O^2-). Our calculations are fully consistent with the observed structures in both synthetic and natural chondrodite, and revealed that hydrogen distribution in humite group minerals is mostly governed by hydrogen bonding at H1 site.