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NASA NTRS1994

MgSiO3-FeSiO3-Al2O3 in the Earth's lower mantle: Perovskite and garnet at 1200 km depth

Natural pyroxene and garnet starting material are used to study the effects of joint Fe and Al substitution into MgSiO3 perovskite at approxmiately 50 GPa. Garnet is found to coexist with perovskite in samples containing both Fe and Al to pressures occurring deep into the lower mantel (approximately 1200 km depth). The volume of the perovskite unit cell is V(sub o(Angstrom(exp 3)) = 162.59 + 5.95x(sub FeSiO3) + 10.80x(sub Al2O3) with aluminum causing a significant increase in the distortion from the ideal cubic cell. On the basis of a proposed extension of the MgSiO3-Al2O3 high-pressure phase diagram toward FeSiO3, Fe is shown to partition preferentially into the garnet phase. The stability of garnet deep into the lower mantel may hinder the penetration of subducted slabs below the transition zone.

O'Neill, Bridget↗

DOE OSTI2020

Materials Data on FeSiO3 by Materials Project

FeSiO3 is Esseneite-derived structured and crystallizes in the triclinic P-1 space group. The structure is three-dimensional. there are nine inequivalent Fe2+ sites. In the first Fe2+ site, Fe2+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with five SiO4 tetrahedra, edges with three FeO6 octahedra, and an edgeedge with one SiO4 tetrahedra. There are a spread of Fe–O bond distances ranging from 2.12–2.29 Å. In the second Fe2+ site, Fe2+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Fe–O bond distances ranging from 1.97–2.71 Å. In the third Fe2+ site, Fe2+ is bonded to six O2- atoms to form distorted FeO6 octahedra that share corners with six SiO4 tetrahedra, edges with two FeO6 octahedra, and an edgeedge with one SiO4 tetrahedra. There are a spread of Fe–O bond distances ranging from 2.02–2.69 Å. In the fourth Fe2+ site, Fe2+ is bonded in a 5-coordinate geometry to five O2- atoms. There are a spread of Fe–O bond distances ranging from 2.03–2.56 Å. In the fifth Fe2+ site, Fe2+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with six SiO4 tetrahedra and edges with two FeO6 octahedra. There are a spread of Fe–O bond distances ranging from 2.11–2.31 Å. In the sixth Fe2+ site, Fe2+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with six SiO4 tetrahedra and edges with three FeO6 octahedra. There are a spread of Fe–O bond distances ranging from 2.10–2.34 Å. In the seventh Fe2+ site, Fe2+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with six SiO4 tetrahedra and edges with four FeO6 octahedra. There are a spread of Fe–O bond distances ranging from 2.11–2.33 Å. In the eighth Fe2+ site, Fe2+ is bonded in a distorted rectangular see-saw-like geometry to four O2- atoms. There are a spread of Fe–O bond distances ranging from 2.04–2.13 Å. In the ninth Fe2+ site, Fe2+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with six SiO4 tetrahedra and edges with four FeO6 octahedra. There are a spread of Fe–O bond distances ranging from 2.10–2.34 Å. There are nine inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 34–63°. There are a spread of Si–O bond distances ranging from 1.62–1.66 Å. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with three FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 42–62°. There are a spread of Si–O bond distances ranging from 1.63–1.65 Å. In the third Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four FeO6 octahedra, corners with two SiO4 tetrahedra, and an edgeedge with one FeO6 octahedra. The corner-sharing octahedra tilt angles range from 45–69°. There are a spread of Si–O bond distances ranging from 1.62–1.66 Å. In the fourth Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with three FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 36–62°. There are a spread of Si–O bond distances ranging from 1.63–1.66 Å. In the fifth Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 52–66°. There are a spread of Si–O bond distances ranging from 1.62–1.67 Å. In the sixth Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 46–74°. There are a spread of Si–O bond distances ranging from 1.62–1.68 Å. In the seventh Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with three equivalent FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 39–62°. There are a spread of Si–O bond distances ranging from 1.63–1.66 Å. In the eighth Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with three FeO6 octahedra, corners with two SiO4 tetrahedra, and an edgeedge with one FeO6 octahedra. The corner-sharing octahedra tilt angles range from 9–63°. There are a spread of Si–O bond distances ranging from 1.61–1.69 Å. In the ninth Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with five FeO6 octahedra and corners with two SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 43–63°. There are a spread of Si–O bond distances ranging from 1.63–1.67 Å. There are twenty-seven inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to two Fe2+ and one Si4+ atom. In the second O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three Fe2+ and one Si4+ atom. In the third O2- site, O2- is bonded to three Fe2+ and one Si4+ atom to form a mixture of distorted corner and edge-sharing OFe3Si tetrahedra. In the fourth O2- site, O2- is bonded in a bent 150 degrees geometry to two Si4+ atoms. In the fifth O2- site, O2- is bonded in a 2-coordinate geometry to one Fe2+ and two Si4+ atoms. In the sixth O2- site, O2- is bonded in a distorted bent 120 degrees geometry to two Si4+ atoms. In the seventh O2- site, O2- is bonded in a bent 150 degrees geometry to two Si4+ atoms. In the eighth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to two Fe2+ and one Si4+ atom. In the ninth O2- site, O2- is bonded in a 4-coordinate geometry to three Fe2+ and one Si4+ atom. In the tenth O2- site, O2- is bonded in a 3-coordinate geometry to three Fe2+ and one Si4+ atom. In the eleventh O2- site, O2- is bonded in a 3-coordinate geometry to two Fe2+ and one Si4+ atom. In the twelfth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three Fe2+ and one Si4+ atom. In the thirteenth O2- site, O2- is bonded to three Fe2+ and one Si4+ atom to form distorted corner-sharing OFe3Si tetrahedra. In the fourteenth O2- site, O2- is bonded in a 3-coordinate geometry to one Fe2+ and two Si4+ atoms. In the fifteenth O2- site, O2- is bonded in a distorted trigonal planar geometry to two Fe2+ and one Si4+ atom. In the sixteenth O2- site, O2- is bonded in a 4-coordinate geometry to three Fe2+ and one Si4+ atom. In the seventeenth O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Fe2+ and two Si4+ atoms. In the eighteenth O2- site, O2- is bonded in a 4-coordinate geometry to three Fe2+ and one Si4+ atom. In the nineteenth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three Fe2+ and one Si4+ atom. In the twentieth O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Fe2+ and two Si4+ atoms. In the twenty-first O2- site, O2- is bonded in a distorted T-shaped geometry to two Fe2+ and one Si4+ atom. In the twenty-second O2- site, O2- is bonded in a distorted bent 150 degrees geometry to two Si4+ atoms. In the twenty-third O2- site, O2- is bonded in a 2-coordinate geometry to one Fe2+ and two Si4+ atoms. In the twenty-fourth O2- site, O2- is bonded in a 3-coordinate geometry to two Fe2+ and one Si4+ atom. In the twenty-fifth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to three Fe2+ and one Si4+ atom. In the twenty-sixth O2- site, O2- is bonded in a 3-coordinate geometry to two Fe2+ and one Si4+ atom. In the twenty-seventh O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to two Fe2+ and one Si4+ atom.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Fe(Si2O3)2 by Materials Project

FeSiO3(SiO)3 crystallizes in the triclinic P1 space group. The structure is one-dimensional and consists of one FeSiO3 ribbon oriented in the (1, 0, 0) direction and one SiO ribbon oriented in the (1, 0, 0) direction. In the FeSiO3 ribbon, Fe is bonded in a rectangular see-saw-like geometry to four O atoms. There are a spread of Fe–O bond distances ranging from 1.83–2.09 Å. Si is bonded in a trigonal non-coplanar geometry to three O atoms. There is one shorter (1.65 Å) and two longer (1.68 Å) Si–O bond length. There are three inequivalent O sites. In the first O site, O is bonded in a 3-coordinate geometry to two equivalent Fe and one Si atom. In the second O site, O is bonded in a bent 120 degrees geometry to two equivalent Si atoms. In the third O site, O is bonded in a water-like geometry to two equivalent Fe atoms. In the SiO ribbon, there are three inequivalent Si sites. In the first Si site, Si is bonded in a single-bond geometry to one O atom. The Si–O bond length is 1.64 Å. In the second Si site, Si is bonded in a single-bond geometry to one O atom. The Si–O bond length is 1.69 Å. In the third Si site, Si is bonded to four O atoms to form corner-sharing SiO4 tetrahedra. There are a spread of Si–O bond distances ranging from 1.64–1.66 Å. There are three inequivalent O sites. In the first O site, O is bonded in a bent 150 degrees geometry to two Si atoms. In the second O site, O is bonded in a bent 120 degrees geometry to two Si atoms. In the third O site, O is bonded in a bent 120 degrees geometry to two equivalent Si atoms.

36 MATERIALS SCIENCE↗

NASA NTRS1978

Effects of compositional variation on absorption spectra of lunar pyroxenes

Polarized absorption spectra of lunar pyroxenes with a range of iron, calcium, magnesium, titanium and chromium contents were measured on polished, oriented single crystals; spectral data on pure synthetic FeSiO3 were also recorded. The bands at 1 and 2 microns were found to vary significantly in position with composition within the pyroxene quadrilateral; wavelengths increased with increasing calcium and iron. In the visible region, a weak band at 640 nm correlates in intensity with Cr2O3, but not with titanium as had been previously suggested. The 505-nm ferrous iron peak is a sharp doublet in most low-calcium pyroxenes but a singlet in augites. A peak at 475 nm and an intense absorption edge below 700 nm correlated with titanium content.

Hazen, R. M.↗

NASA NTRS1986

The solar nebula redox state as recorded by the most reduced chondrules of five primitive chondrites

Mafic minerals in the most reduced chondrules of five primitive meteorites were analyzed with an electron microprobe to determine the lower limit on their FeO contents. The accuracy obtained was + or - 0.01 weight percent FeO. The thermodynamic relationship between mole fraction FeSiO3 and pO2 of the ambient nebular gas at the time of mineral equilibration was established, and was used to infer the local O/H ratio of the nebular gas during chondrule formation. The lowest ferrosilite compositions reflected equilibration at 1500 K with a gas 2-18 times more oxidizing than a gas of solar composition. Olivines in low-FeO unequilibrated ordinary chondrites (UOC) chondrules are uniformly more FeO-rich than coexisting pyroxenes. This discrepancy suggests that a significant change in the O/H ratio of the nebular gas occurred between the time of olivine and pyroxene crystallization in the region of the nebula where UOCs formed. Mineral compositions in the chondrules of two C2 chondrites studied suggest they formed in a more homogeneous region of the nebula than the UOCs.

Johnson, M. C.↗

NASA NTRS2016

Examining Metasomatism in Low fO2 Environments: Exploring Sulfidation Reactions in Various Planetary Bodies

Hydrothermal systems are common on Earth in a variety of tectonic environments and at different temperature and pressure conditions. These systems are commonly dominated by H2O, and they are responsible for element transport and the production of ore deposits. Unlike the Earth (fO2~FMQ), many other planetary bodies (e.g., Moon and asteroids) have fO2 environments that are more reduced (IW+/-2), and H2O is not the important solvent responsible for element transport. One example of a texture that could result from element transport and metasomatism, which appears to occur on numerous planetary bodies, is sulfide-silicate intergrowths. These subsolidus assemblages are interpreted to form as a result of sulfidation reactions from a S-rich fluid phase. The composition of fluids may vary within and among parent bodies and could be sourced from magmatic (e.g. Moon) or impact processes (e.g. HED meteorites and Moon). For example, it has been previously demonstrated on the Moon that the interaction of olivine with a hydrogen- and sulfur-bearing vapor phase altered primary mineral assemblages, producing sulfides (e.g. troilite) and orthopyroxene. Formation of these types of "sulfidation" assemblages can be illustrated with the following reaction: Fe2SiO4(ol) + 1/2 S(2 system) = FeS(troi)+ FeSiO3(opx) + 1/2 O2 system. The products of this reaction, as seen in lunar rocks, is a vermicular or "worm-like" texture of intergrown orthopyroxene and troilite. Regardless of the provenance of the S-bearing fluid, the minerals in these various planetary environments reacted in the same manner to produce orthopyroxene and troilite. Although similar textures have been identified in a variety of parent bodies, a comparative study on the compositions and the origins of these sulfide-silicate assemblages has yet to be undertaken. The intent of this study is to examine and compare sulfide-silicate intergrowths from various planetary bodies to explore their petrogenesis and examine the nature of low fO2 (IW+/-2) element migration and sulfidation reactions.

Srinivasan, P.↗