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DOE OSTI2020

Materials Data on MgTi2O5 by Materials Project

MgTi2O5 crystallizes in the orthorhombic Cmc2_1 space group. The structure is three-dimensional. Mg2+ is bonded to six O2- atoms to form distorted MgO6 octahedra that share corners with two equivalent MgO6 octahedra, corners with two TiO6 octahedra, and edges with six TiO6 octahedra. The corner-sharing octahedra tilt angles range from 28–63°. There are a spread of Mg–O bond distances ranging from 1.98–2.27 Å. There are two inequivalent Ti4+ sites. In the first Ti4+ site, Ti4+ is bonded to six O2- atoms to form distorted TiO6 octahedra that share a cornercorner with one MgO6 octahedra, corners with three TiO6 octahedra, edges with three equivalent MgO6 octahedra, and edges with three equivalent TiO6 octahedra. The corner-sharing octahedra tilt angles range from 22–63°. There are a spread of Ti–O bond distances ranging from 1.87–2.15 Å. In the second Ti4+ site, Ti4+ is bonded to six O2- atoms to form distorted TiO6 octahedra that share a cornercorner with one MgO6 octahedra, corners with three TiO6 octahedra, edges with three equivalent MgO6 octahedra, and edges with three equivalent TiO6 octahedra. The corner-sharing octahedra tilt angles range from 22–36°. There are a spread of Ti–O bond distances ranging from 1.87–2.16 Å. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded to one Mg2+ and three Ti4+ atoms to form distorted OMgTi3 tetrahedra that share corners with two equivalent OMgTi3 tetrahedra, corners with two OMg2Ti2 trigonal pyramids, and edges with four OMg2Ti2 trigonal pyramids. In the second O2- site, O2- is bonded to two equivalent Mg2+ and two Ti4+ atoms to form distorted OMg2Ti2 trigonal pyramids that share a cornercorner with one OMgTi3 tetrahedra, corners with three OMg2Ti2 trigonal pyramids, edges with two equivalent OMgTi3 tetrahedra, and edges with two equivalent OMgTi3 trigonal pyramids. In the third O2- site, O2- is bonded to one Mg2+ and three Ti4+ atoms to form distorted OMgTi3 trigonal pyramids that share a cornercorner with one OMgTi3 tetrahedra, corners with three OMg2Ti2 trigonal pyramids, edges with two equivalent OMgTi3 tetrahedra, and edges with two equivalent OMg2Ti2 trigonal pyramids. In the fourth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and two Ti4+ atoms. In the fifth O2- site, O2- is bonded in a distorted T-shaped geometry to one Mg2+ and two Ti4+ atoms.

36 MATERIALS SCIENCE↗

NASA NTRS2002

Stability of Pseudobrookite-Type Titanium Oxides

Orthorhombic, (Bbmm), (Al, Fe, Cr, Ti)(sub 2) TiO5-(Mg, Fe)Ti2O5 solid solutions (pseudobrookites, s.l.) are found either as an oxidation product of ilmenite and/or spinel or a primary crystallizing phase in igneous and metamorphic rocks on Earth (e.g., basalt flows, crustal and mantle xenoliths, hornfels), and basaltic rocks on the Moon. Moreover, orthorhombic oxides are often part of the crystalline matrix in glass/ceramics with useful applications, and play a major role in the industrial production of TiO2. To fully exploit the potential of these compounds as petrogenetic indicators and/or useful materials we need to quantitatively understand the factors controlling their properties and stability, and thus, to extrapolate beyond the calibrating experiments. For that purpose, we need to combine thermochemistry, phase equilibrium, and in situ P-V-T-cation disorder experimental data that presently either are incomplete or lacking. Perhaps, the most complete data set is that for MgTi2O5 (karrooite) which allows the calibration of models for the Gibbs free energy of the MgTi2O5 as a function of pressure, temperature, and the Mg2+-Ti4+ distribution between the two nonequivalent octahedral sites. Consequently, the effect of cation disorder on MgTi2O5 stability, and the phase relations among MgTi2O5, other titanium oxides, and silicate minerals can be examined. Calculated phase relations in the Mg-Ti-Si-O system and phase equilibrium experiments in Fe-bearing compositions suggest that pseudobrookite-type oxides may be a more common in rocks than previously realized. However, homogeneous and heterogeneous equilibria, and crystallization paths likely affect their stability. For example, isobaric increases in temperature favor disordering and thus entropy-stabilization, in contrast, isothermal increases in pressure have the opposite effect. Although, currently, the potential effect of composition to cation disorder cannot be fully explored, it appears that enrichment in trivalent cations probably enhances entropy-stabilization and thus may increase the stability of (Al, Fe, Cr, Ti)-rich pseudobrookites relative to that of (Mg, Fe)-rich ones. In addition, high-temperature, nearly isothermal, decompression paths of olivine+orthopyroxene+oxide assemblages may favor pseudobrookites (s.l.) over rutile and/or ilmenite, in contrast, cooling at low pressures seems to favor ilmenite and/or rutile. In the case of crustal and mantle xenoliths, the presence or absence of orthorhombic oxides is probably controlled by reactions with olivine, orthopyroxene, ilmenite, and rutile. In oceanic mantle xenoliths such reactions may also involve a TiO2-enriched but not SiO2-enriched melt/fluid, because pseudobrookites (s.l.) would react with the SiO2-enriched melt/fluid to form orthopyroxene and rutile. Parenthetically, experiments and model calculations in the Mg-Ti-Si-O system suggest that low degree partial melting of low-TiO2 bulk compositions may produce Ti-enriched liquids in equilibrium with olivine, orthopyroxen ad=nd MgTi2O5, rutile or ilmenite.

Xirouchakis, Dimistrios↗

NASA NTRS1975

The effects of Al/3+/, Cr/3+/, and Ti/3+/ on the stability of armalcolite

Experimental studies on the binary system FeTi2O5-MgTi2O5 yield unrealistically high values for the lower thermal stabilities of lunar armalcolites, since these typically contain 10-15 mole% (Al, Cr, Ti/3+/)2TiO5 components. Pure binary Fe0.5Mg0.5Ti2O5 begins to break down at 1010 + or - 20 C whereas Fe0.5Mg0.5Ti2O5)0.85(Ti2/3+/TiO5)0.15 persists to approximately 900 C and (Fe0.5Mg0.5Ti2O5)0.84(Al2TiO5)0.04(Cr2TiO5)0.03(Ti2/3+/TiO5)0.09 (a model Apollo 17 armalcolite) remains stable down at least to 850 C. The lower thermal stability of the model Apollo 17 armalcolite is raised by approximately 35 C per kilobar of load pressure. The breakdown curve intersects the solidus of likely source materials for high-Ti lunar basalts at 9-10 kbar; thus, the maximum depth at which this armalcolite is stable is 180-200 km. Subsolidus isochemical reaction of Ti/3+/ and Fe/2+/ components in armalcolite can yield rutile and metallic Fe. This assemblage could be misidentified as due to an externally imposed reduction.

Kesson, S. E.↗