Proton-containing defects at forsterite {010} tilt grain boundaries and stepped surfaces
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Nora H. De Leeuw
,
Stephen C. Parker
Abstract
Atomistic simulation techniques are used to investigate the effect of proton-containing defects on the structure and stabilities of a range of grain boundaries of forsterite. We study two series of stepped {010} tilt boundaries that are at 90° to each other: one with the {100} plane as step wall and the other with the {001} plane. Each series consist of several grain boundaries with increasing terrace area (i.e., decreasing boundary angle). The ratios of boundary and surface energies γb/γs and γb/γ{010} with boundary angle show maxima at a boundary angle ϕ = ~30° and minima at ϕ = 0° and ~60°. The adhesion energies of the two series show a minimum at low boundary angle (ϕ = 20-30°), indicating that there is an optimum size for the {010} terrace area, where the relative stabilities of grain boundary and related surface make separation of the boundary into the free surfaces energetically least expensive.
Dissociative adsorption of water molecules in the bulk crystal is preferred at the M2 site, but is an endothermic process with a calculated hydration energy of +119 kJ/mol. Hydration of the grain boundaries on the other hand is energetically favorable with hydration energies tending toward -80 kJ/mol compared with a surface hydration energy at the planar {010} surface of -90 kJ/mol. We also investigated the adsorption of protons at cation defects, by modeling the process of replacement of Mg ions in the bulk and along the grain boundaries by two H+ ions each. Replacement of Mg2+ in the bulk crystal occurs preferentially at the M1 site and is calculated to be exothermic with a replacement energy of -78 kJ/mol; Mg2+ replacement along both series of grain boundaries is also an exothermic process. The replacement energies tend to a constant value of approximately -210 kJ/mol with increasing terrace area. (cf., the calculated value for the planar {010} surface of -230.4 kJ/mol).
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
- Measurement of crystal size distributions
- Pressure dependence of the solubility of Ar and Kr in melts of the system SiO2-NaAlSi2O6
- Solubility behavior of water in haploandesitic melts at high pressure and high temperature
- Proton-containing defects at forsterite {010} tilt grain boundaries and stepped surfaces
- Incorporation of Fe3+ into forsterite and wadsleyite
- Molecular dynamics simulation of Al/Si-ordered plagioclase feldspar
- Cation ordering and structural variations with temperature in MgAl2O4 spinel: An X-ray single-crystal study
- Aluminium coordination in tektites: A XANES study
- Crystal structure of Cr-mullite
- Structure of synthetic 2-line ferrihydrite by electron nanodiffraction
- Transmission electron microscopy study of gaudefroyite, Ca8Mn6 3+[(BO3)6(CO3)2O6]
- Nano- to micro-scale decompression products in ultrahigh-pressure phengite: HRTEM and AEM study, and some petrological implications
- New insights into the mechanism for chloritization of biotite using polytype analysis
- The dissolution of hectorite: In-situ, real-time observations using atomic force microscopy
- Quantification of minor phases in growth kinetics experiments with powder X-ray diffraction
- Illite-smectite structural changes during metamorphism in black Cambrian Alum shales from the Baltic area
- The tremolite-actinolite-ferro–actinolite series: Systematic relationships among cell parameters, composition, optical properties, and habit, and evidence of discontinuities
- Cordierite I: The coordination of Fe2+
- Cordierite II: The role of CO2 and H2O
- Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak batholith (central Colorado)
- The crystal structure of TlAlSiO4: The role of inert pairs in exclusion of Tl from silicate minerals
- The structure of agrinierite: a Sr-containing uranyl oxide hydrate mineral
- The crystal structure of namibite, Cu(BiO)2VO4(OH), and revision of its symmetry
- The crystal structure of pararobertsite and its relationship to mitridatite
- Description and crystal structure of cabalzarite Ca(Mg,Al,Fe)2(AsO4)2(H2O,OH)2, a new mineral of the tsumcorite group
- Tegengrenite, a new, rhombohedral spinel-related Sb mineral from the Jakobsberg Fe-Mn deposit, Värmland, Sweden
