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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Isotope mass fractionation during evaporation of Mg2SiO4

Synthetic forsterite (Mg2SiO4) was partially evaporated in vacuum for various durations and at different temperatures. The residual charges obtained when molten Mg2SiO4 was evaporated to 12 percent of its initial mass were enriched in heavy isotopes by about 20, 30, and 15 per mil/amu for O, Mg, and Si, respectively, whereas solid forsterite evaporated to a similar residual mass fraction showed negligible fractionations. These results imply that calcium and aluminum-rich refractory inclusions in carbonaceous chondrites must have been at least partially molten in the primordial solar nebula if the observed large mass fractionation effects were caused by evaporation processes in the nebula.

Davis, Andrew M.↗
NASA NTRS1993

Evaporation kinetics of Mg2SiO4 crystals and melts from molecular dynamics simulations

Computer simulations based on the molecular dynamics (MD) technique were used to study the mechanisms and kinetics of free evaporation from crystalline and molten forsterite (i.e., Mg2SiO4) on an atomic level. The interatomic potential employed for these simulations reproduces the energetics of bonding in forsterite and in gas-phase MgO and SiO2 reasonably accurately. Results of the simulation include predicted evaporation rates, diffusion rates, and reaction mechanisms for Mg2SiO4(s or l) yields 2Mg(g) + 20(g) + SiO2(g).

Kubicki, J. D.↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 crystallizes in the orthorhombic Imma space group. The structure is three-dimensional. there are three inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with six MgO6 octahedra. There are four shorter (2.07 Å) and two longer (2.14 Å) Mg–O bond lengths. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with six MgO6 octahedra. There are a spread of Mg–O bond distances ranging from 2.06–2.12 Å. In the third Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with two equivalent MgO6 octahedra, corners with four equivalent SiO4 tetrahedra, and edges with seven MgO6 octahedra. The corner-sharing octahedral tilt angles are 16°. There are two shorter (2.04 Å) and four longer (2.15 Å) Mg–O bond lengths. Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with ten MgO6 octahedra and a cornercorner with one SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 47–61°. There are a spread of Si–O bond distances ranging from 1.65–1.72 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the second O2- site, O2- is bonded to five Mg2+ atoms to form edge-sharing OMg5 square pyramids. In the third O2- site, O2- is bonded in a trigonal planar geometry to one Mg2+ and two equivalent Si4+ atoms. In the fourth O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 is Ilmenite-like structured and crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are three inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with two equivalent SiO4 tetrahedra, edges with six MgO6 octahedra, and edges with two equivalent SiO4 tetrahedra. There are four shorter (2.02 Å) and two longer (2.17 Å) Mg–O bond lengths. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with six MgO6 octahedra. There are two shorter (2.33 Å) and four longer (2.34 Å) Mg–O bond lengths. In the third Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with two equivalent SiO4 tetrahedra, edges with six MgO6 octahedra, and edges with two equivalent SiO4 tetrahedra. There are four shorter (2.02 Å) and two longer (2.17 Å) Mg–O bond lengths. Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six MgO6 octahedra and edges with three MgO6 octahedra. The corner-sharing octahedra tilt angles range from 3–55°. There is one shorter (1.61 Å) and three longer (1.69 Å) Si–O bond length. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the third O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 is Ilmenite-like structured and crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with four equivalent MgO6 octahedra, corners with two equivalent SiO4 tetrahedra, edges with four equivalent MgO6 octahedra, and edges with two equivalent SiO4 tetrahedra. The corner-sharing octahedral tilt angles are 55°. There are two shorter (2.07 Å) and four longer (2.11 Å) Mg–O bond lengths. In the second Mg2+ site, Mg2+ is bonded to six equivalent O2- atoms to form MgO6 octahedra that share corners with twelve equivalent MgO6 octahedra and corners with six equivalent SiO4 tetrahedra. The corner-sharing octahedral tilt angles are 55°. All Mg–O bond lengths are 2.24 Å. Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six MgO6 octahedra and edges with three equivalent MgO6 octahedra. The corner-sharing octahedra tilt angles range from 54–57°. There is one shorter (1.63 Å) and three longer (1.67 Å) Si–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three equivalent Mg2+ and one Si4+ atom. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 is (La,Ba)CuO4 structured and crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Mg2+ is bonded in a 1-coordinate geometry to nine O2- atoms. There are a spread of Mg–O bond distances ranging from 1.96–2.48 Å. Si4+ is bonded to six O2- atoms to form corner-sharing SiO6 octahedra. The corner-sharing octahedral tilt angles are 0°. There is four shorter (1.75 Å) and two longer (1.79 Å) Si–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to four equivalent Mg2+ and two equivalent Si4+ atoms to form a mixture of distorted edge, face, and corner-sharing OMg4Si2 octahedra. The corner-sharing octahedra tilt angles range from 0–55°. In the second O2- site, O2- is bonded to five equivalent Mg2+ and one Si4+ atom to form distorted OMg5Si octahedra that share corners with seventeen OMg4Si2 octahedra, edges with eight equivalent OMg5Si octahedra, and faces with four equivalent OMg4Si2 octahedra. The corner-sharing octahedra tilt angles range from 0–55°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 is Ilmenite-like structured and crystallizes in the monoclinic P2/m space group. The structure is three-dimensional. there are five inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with two equivalent MgO6 octahedra, corners with two SiO4 tetrahedra, edges with five MgO6 octahedra, and edges with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 56–57°. There are a spread of Mg–O bond distances ranging from 2.05–2.14 Å. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with four equivalent MgO6 octahedra, corners with two equivalent SiO4 tetrahedra, edges with four equivalent MgO6 octahedra, and edges with two equivalent SiO4 tetrahedra. The corner-sharing octahedral tilt angles are 55°. All Mg–O bond lengths are 2.10 Å. In the third Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with twelve MgO6 octahedra and corners with six SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 55–57°. There are two shorter (2.24 Å) and four longer (2.26 Å) Mg–O bond lengths. In the fourth Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with six SiO4 tetrahedra and edges with six MgO6 octahedra. There are four shorter (2.32 Å) and two longer (2.33 Å) Mg–O bond lengths. In the fifth Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with two equivalent SiO4 tetrahedra, edges with six MgO6 octahedra, and edges with two equivalent SiO4 tetrahedra. There are four shorter (2.02 Å) and two longer (2.13 Å) Mg–O bond lengths. There are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six MgO6 octahedra and edges with three MgO6 octahedra. The corner-sharing octahedra tilt angles range from 2–55°. There are a spread of Si–O bond distances ranging from 1.62–1.69 Å. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six MgO6 octahedra and edges with three MgO6 octahedra. The corner-sharing octahedra tilt angles range from 0–56°. There is one shorter (1.62 Å) and three longer (1.68 Å) Si–O bond length. There are six inequivalent O2- sites. In the first O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the third O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the fourth O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the fifth O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom. In the sixth O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Mg2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 is Spinel structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent O2- atoms to form MgO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with six equivalent MgO6 octahedra. All Mg–O bond lengths are 2.09 Å. Si4+ is bonded to four equivalent O2- atoms to form SiO4 tetrahedra that share corners with twelve equivalent MgO6 octahedra. The corner-sharing octahedral tilt angles are 53°. All Si–O bond lengths are 1.68 Å. O2- is bonded in a rectangular see-saw-like geometry to three equivalent Mg2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 crystallizes in the orthorhombic Pbam space group. The structure is three-dimensional. Mg2+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Mg–O bond distances ranging from 2.07–2.50 Å. Si4+ is bonded to six O2- atoms to form edge-sharing SiO6 octahedra. There is two shorter (1.77 Å) and four longer (1.87 Å) Si–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 5-coordinate geometry to three equivalent Mg2+ and two equivalent Si4+ atoms. In the second O2- site, O2- is bonded in a 5-coordinate geometry to four equivalent Mg2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg2SiO4 by Materials Project

Mg2SiO4 crystallizes in the orthorhombic Cmc2_1 space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with three equivalent SiO6 octahedra, edges with two equivalent MgO6 octahedra, and edges with three equivalent SiO6 octahedra. The corner-sharing octahedra tilt angles range from 7–53°. There are a spread of Mg–O bond distances ranging from 2.02–2.12 Å. In the second Mg2+ site, Mg2+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Mg–O bond distances ranging from 2.03–2.59 Å. Si4+ is bonded to six O2- atoms to form SiO6 octahedra that share corners with three equivalent MgO6 octahedra, edges with two equivalent SiO6 octahedra, and edges with three equivalent MgO6 octahedra. The corner-sharing octahedra tilt angles range from 7–53°. There are a spread of Si–O bond distances ranging from 1.73–1.90 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to four Mg2+ and two equivalent Si4+ atoms. In the second O2- site, O2- is bonded to three Mg2+ and two equivalent Si4+ atoms to form distorted OMg3Si2 square pyramids that share corners with five equivalent OMg3Si tetrahedra, corners with two equivalent OMg4Si trigonal bipyramids, edges with two equivalent OMg3Si2 square pyramids, an edgeedge with one OMg3Si tetrahedra, and edges with three equivalent OMg4Si trigonal bipyramids. In the third O2- site, O2- is bonded to four Mg2+ and one Si4+ atom to form distorted OMg4Si trigonal bipyramids that share corners with two equivalent OMg3Si2 square pyramids, corners with five equivalent OMg3Si tetrahedra, edges with three equivalent OMg3Si2 square pyramids, an edgeedge with one OMg3Si tetrahedra, and edges with two equivalent OMg4Si trigonal bipyramids. In the fourth O2- site, O2- is bonded to three Mg2+ and one Si4+ atom to form distorted OMg3Si tetrahedra that share corners with five equivalent OMg3Si2 square pyramids, corners with two equivalent OMg3Si tetrahedra, corners with five equivalent OMg4Si trigonal bipyramids, an edgeedge with one OMg3Si2 square pyramid, and an edgeedge with one OMg4Si trigonal bipyramid.

36 MATERIALS SCIENCE↗
NASA NTRS1990

Mechanism of the gamma-beta phase transformation of Mg2SiO4 at high temperature and pressure

The results of experiments on the phase transformation of Mg2SiO4 olivine at 15 GPa pressure in a multianvil cell are reported. At this pressure and a temperature of 900 C, early formed metastable gamma-spinel transforms partially to the beta-phase. The observed microstructures, which are similar to those in shocked meteorites, show that the gamma-to-beta transformation can occur either by diffusion-controlled growth or by a martensitic mechanism, depending on how far the pressure-temperature conditions deviate from their values at phase equilibrium. The results suggest that the diffusion-controlled mechanism is most likely to operate at the beta/gamma phase boundary in the mantle, but martensitic beta-to-gamma transformation might occur in subduction zones and could reduce the shear strength of the subducting slab.

Rubie, D. C.↗
NASA NTRS1994

A crystallographic model for hydrous wadsleyite (Beta-Mg2SiO4)

Wadsleyite (beta-Mg2SiO4) is believed to be the most abundant phase in the Earth between depths of 400 and about 525km. Because of the unusual crystal chemistry, Smyth suggested that this phase might be a significant host for hydrogen in the transition zone. Indeed, of the nominally anhydrous phases believed to make up the upper mantle and transition zone none has been reported with a greater H content than wadsleyite. Young et al, report the synthesis of hydrous, Fe-bearing wadsleyite with up to 60,000 H per 10(exp 6) Si. Using ionic constraints and maximal subgroup symmetry, a hypothetical, ordered model for hydrous wadsleyite has been created and examined. The model has formula Mg7Si4O14(OH)2, has space group Pmmb, has an ordered vacancy into one of two non-equivalent M2 sites, and contains two different H positions, one on each of the non-equivalent O1 sites. Electrostatic calculations indicate that hydration would relieve the underbonding of O1 as well as the overbonding of the bridging oxygen, O2, so that the hydrous phase may be more stable than the anhydrous phase. This model makes several predictions that may be of significance for the mechanisms and amounts of H that may be stored in the transition zone of the Earth, and by which the model may be tested experimentally.

Smyth, J. R.↗
DOE OSTI2021

Using Multigrain Crystallography to Explore the Microstructural Evolution of the α-Olivine to γ-Ringwoodite Transformation and ε-Mg2SiO4 at High Pressure and Temperature

The introduction of multigrain crystallography (MGC) applied in a laser-heated diamond anvil cell (LH-DAC) using synchrotron X-rays has provided a new path to investigate the microstructural evolution of materials at extreme conditions, allowing for simultaneous investigations of phase identification, strain state determination, and orientation relations across phase transitions in a single experiment. Here, we applied this method to a sample of San Carlos olivine beginning at ambient conditions and through the α-olivine → γ-ringwoodite phase transition. At ambient temperatures, by measuring the evolution of individual Bragg reflections, olivine shows profuse angular streaking consistent with the onset of yielding at a measured stress of ~1.5 GPa, considerably lower than previously reported, which may have implications for mantle evolution. Furthermore, γ-ringwoodite phase was found to nucleate as micron to sub-micron grains imbedded with small amounts of a secondary phase at 15 GPa and 1000 °C. Using MGC, we were able to extract and refine individual crystallites of the secondary unknown phase where it was found to have a structure consistent with the ε-phase previously described in chondritic meteorites.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Hydration-driven stabilization and volume collapse of grain boundaries in Mg2SiO4 forsterite predicted by first-principles simulations

Grain boundaries in mantle minerals are of critical importance to geophysical and geochemical processes of the Earth’s interior. One of the fundamental issues is to understand how the water (H 2 O) component influences the properties of grain boundaries in silicate materials. Here, we report the results of the structure and stability of several tilt grain boundaries in Mg 2 SiO 4 forsterite over the pressure range 0 to 15 GPa using density functional theory-based first-principles simulations. Here, the results suggest greater energetic stability and hydration-driven volume collapse (negative excess volume) at zero pressure for the majority of hydrous grain boundaries relative to the anhydrous (dry) ones. All the hydrous grain boundaries become increasingly favorable at elevated pressures as the calculated hydration enthalpy systematically decreases with increasing pressure. The hydrous components at the interfacial regions are predominantly in the hydroxyl form and, to a lesser extent, in the molecular H 2 O form. Their calculated ratio ranges from 1.6 to 8.7 among the different grain boundary configurations. Our structural analysis also reveals that the hydroxyls are bound to either both Mg and Si or to Mg only. In comparison, the molecular species are bound only to Mg sites. Besides direct oxygen-hydrogen bonding, intermolecular hydrogen bonding becomes important with compression. On the basis of our results, we suggest that local atomic rearrangements caused by dissociative adsorption of water facilitate efficient compaction of the boundary interfaces, which, in turn, results in greater relative stability of hydrous grain boundaries. This means that water prefers to be incorporated within the grain boundaries over the bulk of silicate materials.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
NASA NTRS1976

The origin of chondrules - Experimental investigation of metastable liquids in the system Mg2SiO4-SiO2

Laser-melted magnesium silicate droplets were supercooled 400-750 C below their equilibrium liquidus temperatures before crystallization and their texture was compared with that of meteoritic and lunar chondrules. Crystal morphology, width and texture were studied in relation to nucleation temperature and bulk composition. It was found that the only phase to nucleate from the forsterite-enstatite normative melts was forsterite. Highly siliceous glass, about 65% SiO2 by weight, was identified interstitially to the forsterite crystals in seven of the MgSiO4 spherules and was thought to be present in all.

Blander, M.↗
NASA NTRS1982

Olivine flotation and settling experiments on the join Mg2SiO4-Fe2SiO4

Results are presented of a study of some unusual density relations between olivine and coexisting liquid in the system fosterite-fayalite. At 1 atmosphere pressure it is found that olivine floats on its coexisting liquid for intermediate compositions on this binary because of extreme partitioning of Fe into the melt phase. At 20 kilobars, the usual behavior of olivine settling is found to occur because the partitioning of Fe in the melt is reduced, aided possibly by the dissolution of CO2 in the melt from the use of a graphite container. It is determined that olivine flotation and settling are rapid in a time period of only a few hours because viscosities are slightly greater than that of paraffin oil at room temperature. Some adcumulate textures with good triple junction grain boundaries are found to be developed. Observations of differentiated magmatic systems suggest that the mechanisms by which magmas can differentiate vary considerably in the ultramafic to tholeiitic compositional range.

Herzberg, C. T.↗