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DOE OSTI2020

Materials Data on MgSiO4 by Materials Project

MgSiO4 is beta Vanadium nitride-derived structured and crystallizes in the monoclinic P2/m space group. The structure is three-dimensional. Mg is bonded to six O atoms to form MgO6 octahedra that share corners with eight equivalent SiO6 octahedra and edges with two equivalent MgO6 octahedra. The corner-sharing octahedra tilt angles range from 45–51°. There are four shorter (2.03 Å) and two longer (2.14 Å) Mg–O bond lengths. Si is bonded to six O atoms to form SiO6 octahedra that share corners with eight equivalent MgO6 octahedra and edges with two equivalent SiO6 octahedra. The corner-sharing octahedra tilt angles range from 45–51°. There is two shorter (1.69 Å) and four longer (1.85 Å) Si–O bond length. There are two inequivalent O sites. In the first O site, O is bonded in a distorted trigonal planar geometry to one Mg and two equivalent Si atoms. In the second O site, O is bonded in a distorted T-shaped geometry to two equivalent Mg and one Si atom.

36 MATERIALS SCIENCE↗

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 NTRS2009

Late Reduction Textures in Almahata Sitta Ureilite

The Almahata Sitta ureilite, derived from asteroid 2008 TC3, consists of many individual fragments recovered from the Nubian dessert strewn field [1]. Like most ureilites, it contains abundant carbon and exhibits examples of disequilibrium textures that record a late reduction event accompanied by rapid cooling (tens of degC/h) from high temperatures (1150-1300 C). Variations in Fe/Mg of silicate minerals are accompanied by variations in Fe/Mn, indicating loss of Fe into metal [2]. In coarser-grained fragments of Almahata Sitta, olivine exhibits irregular high mg# rims in contact with networks of interstitial metal 5- 20 microns in typical thickness. This is a common ureilite texture thought to be driven by the reaction of graphite to a CO gas phase and the concurrent reduction of FeO in olivine to Fe metal, with excess silica going primarily into pyroxene (2MgFeSiO4 + C approaches MgSiO4 + MgSiO3 + 2Fe + CO) [3, see also 4,5,6]. Other fragments of Almahata Sitta exhibit anomalous textures such as fine grain size, high porosity, and abundant graphite. Within these fragments pyroxene locally exhibits high-mg# rims in contact with metal and a discreet silica phase, suggesting that the reduction mechanism MgFeSi2O6 + C approaches MgSiO3 + Fe + SiO2 + CO. Metals in Almahata Sitta are particularly unaltered in comparison to ureilite finds. Variations in minor and trace element composition of this metal might partly result from localized dilution as iron is supplied by reduction of silicates.

Herrin, J. S.↗