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Discovery of Panguite, a New Ultra-Refractory Titania Mineral in Allende Chi Ma1*, Oliver Tschauner1,2, John R

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Discovery of Panguite, a New Ultra-Refractory Titania Mineral in Allende Chi Ma1*, Oliver Tschauner1,2, John R 42nd Lunar and Planetary Science Conference (2011) 1276.pdf Discovery of Panguite, a New Ultra-Refractory Titania Mineral in Allende Chi Ma1*, Oliver Tschauner1,2, John R. Beckett1, Boris Kiefer3, George R. Rossman1, Wenjun Liu4; 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125; 2High Pressure Science and Engineering Center and Department of Geoscience, University of Nevada, Las Vegas, NV 89154; 3Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131; 4Advanced Photon Source, Argonne National Laboratory, Chicago, IL 60439; *[email protected]. Introduction: During our nano-mineralogy inves- tigation of the Allende meteorite, we identified a new titania mineral named “panguite” in an ultra-refractory inclusion within an amoeboid olivine aggregate (AOA). It has an orthorhombic Pbca structure related to the Ia3 bixbyite type and a formula unit 4+ (Ti ,Al,Sc,Mg,Zr,Ca,□)2O3. We used electron probe microanalysis (EPMA), high-resolution scanning elec- tron microscope (SEM), electron backscatter diffrac- tion (EBSD), synchrotron micro-Laue diffraction with subsequent energy scans, and micro-Raman analyses to characterize the composition and structure. Panguite is not only a new mineral but also a new phase as syn- Fig. 1. SEM backscattered electron (BSE) image of 4+ thetic Ia3 (Ti ,Al,Sc,Mg,Zr,Ca,□)2O3 is not known. the ultra-refractory inclusion in an Allende AOA. We report here the type occurrence of this new titania in nature and discuss implications of this phase for processes very early in the history of our solar system. The mineral and the mineral name (panguite) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2010-057). Occurrence, Chemistry and Crystallography: Panguite occurs as small irregular to subhedral crystals (~0.5-2 m) with Ti-rich davisite in an irregular ultra- refractory inclusion in Allende thick section Caltech MC2Q (Fig. 1). The refractory inclusion is about 30 μm × 20 μm in the section plane and resides within an AOA, surrounded by a matrix of mostly fine-grained Fig. 2. Enlarged BSE showing the area where pan- olivine and troilite. Two compositionally distinct guite crystals (type material) occur with smaller Zr- groups of panguite are observed in Allende MC2Q. rich panguite in davisite (Fig. 1). The type panguite is Ti-, Sc-rich and Zr-, Y-poor and [(Ti Zr Si )4+ (Al Sc Y V Cr )3+ generally larger than Zr-rich panguite (smaller white 0.75 0.15 0.07 Σ0.97 0.21 0.20 0.06 0.02 0.01 Σ0.5 (Mg Ca Fe )2+ ] O for type panguite with grains in Fig. 2). We also observed panguite with pe- 0.17 0.10 0.04 Σ0.31 Σ1.78 3 a general formula of (Ti4+,Al,Sc,Mg,Zr,Ca) O , or rovskite, spinel and davisite in two other ultra- 1.8 3 (Ti4+,Al,Sc,Mg,Zr,Ca,□) O Associated Zr-rich panguite refractory inclusions in Allende. 2 3. (Fig. 2) has an empirical formula The mean chemical composition of type panguite [(Ti Zr Si )4+ (Al Sc Y Cr V )3+ by EPMA is (wt%) TiO 45.94, ZrO 13.94, Sc O 0.55 0.28 0.15 Σ0.98 0.14 0.12 0.12 0.01 0.01 Σ0.40 2 2 2 3 (Ca Mg Fe )2+ ] O and host davisite 10.64, Al O 8.28, MgO 5.38, Y O 4.91, CaO 4.20, 0.20 0.15 0.09 Σ0.44 Σ1.82 3 2 3 2 3 (Ca Mg ) [(Sc Ti Y V Al )3+ (Mg Fe SiO 3.37, FeO 2.18, V O 0.91, Cr O 0.48, HfO 0.96 0.04 Σ1.00 0.34 0.23 0.01 0.01 0.01 Σ0.60 0.23 2 2 3 2 3 2 )2+ (Ti Zr )4+ ] (Si Al Ti4+ ) O , 0.09, sum 100.33. Repeated electron probe analysis of 0.05 Σ0.28 0.08 0.04 Σ0.12 Σ1.00 1.12 0.83 0.05 Σ2.00 6 where Ti3+ and Ti4+ are partitioned based on stoichi- panguite with O measured gave similar results, imply- ometry. Note that most of the Ti in davisite coexisting ing that Ti in the structure is dominately 4+. The Ra- with the panguite, is trivalent. Small crystals of Ru-Ir- man spectrum is also consistent with minimal Ti3+ be- Mo-Fe-Os alloys are included in small amounts in dav- ing present based on comparisons with tistarite (Ti O ) 2 3 isite, or in contact with panguite. The surrounding oli- and davisite. Assuming all Ti as Ti4+ leads to an em- vine is Fa Fo pirical formula, based on 3 oxygens, of 21 79. 42nd Lunar and Planetary Science Conference (2011) 1276.pdf EBSD patterns of panguite can not be matched tive constraints on origin. Panguite is particularly in- against the structures of tistarite, perovskite, Ti3O5, teresting as a potential sensor of environment because Sc2TiO5, armalcolite, pseudobrookite, anatase, rutile, it is a titanate that contains significant concentrations or brookite. They do imply a CaF2-type structure, of Al (ionic radius in octahedral coordination of which assisted us in identifying its bixbyite-related 0.48Å), Sc (0.73Å), and Y (0.89Å), so it should also 3+ structure (a defect form of the CaF2 structure) by syn- be able to readily accept large concentrations of Ti chrotron analysis. (0.67Å). The absence of measureable Ti3+ in our Al- Synchrotron micro-Laue-diffraction on one pan- lende panguite therefore strongly suggests that these guite grain was carried out at the 34ID-E undulator crystals equilibrated in a highly oxidizing environ- beamline of the Advanced Photon Source in the Ar- ment. Yet they coexist with davisite that has gonne National Laboratory using a 0.50.5 m2 poly- Ti3+/Ti4+~2, strongly suggesting very reducing condi- chromatic X-ray beam. An energy scan by synchrotron tions. This intimate juxtaposition of highly oxidized mono-beam yielded a set of 17 reflections whose Q- and highly reduced phases (Fig. 2) is thematically values indicate an orthorhombic distortion of the cubic similar to the presence in type B inclusions of Ti4+- cell proposed by the EBSD patterns. Moreover the Q- enriched spinels hosted by Ti3+-enriched clinopyrox- values of these reflections imply a unit cell larger than ene [8]. Zr-Y-Sc enriched phases are, however, likely that of fluorite. Related structure types with larger unit to provide more direct clues to the source of extrava- cells, which are common among anion-deficient fluo- gant dichotomies in redox conditions because they rite-type structures, are the pyrochlore and the bixbyite contain many more independent constraints on their type. We modelled orthorhombic distorted fluorite-, environment. pyrochlore- and bixbyite cells of panguite using sub- Panguite is more than 20% higher in density than groups of the cubic aristotype cells and found a match tistarite, the other Ti2O3-polymorph found in Allende of all reflections only for the bixbyite type in space [9]. This suggests formation of panguite may have group Pbca. Trial indexing with Jade 6.5 also indi- formed at high pressure and opens the interesting pos- cated cell shapes compatible with a primitive ortho- sibility that panguite formed during a very early shock rhombic distorted bixbyite-type cell. event potentially prior to encapsulation in the CAI. We Based on the indices from trial indexing and sub- examined this possible high pressure origin by ab ini- sequent cell refinement using the program UnitCell, tio calculations. Our results indicate that the stable we evaluated the observed and calculated structure high pressure form of Ti2O3 is not the panguite- but the factor moduli and found agreement with a Rint of 6.1%. Th2S3-structure [10]. However, among all plausible The observed structure factors show no evidence for A2O3-structures, panguite-type Ti2O3 has the lowest ordering of cations among the four available sites. The H compared to tistarite at ambient and to Th2S3-type orthorhombic distorted bixbyite cell of panguite was Ti2O3 at elevated pressure. Therefore, if panguite is a used to calculate an EBSD pattern and compared to the high pressure phase, it either formed directly from observed patterns. The EBSD patterns can be indexed condensation at elevated pressure because of a low nicely by the Pbca structure. seed-formation energy consistent with the Oswaldt- Panguite has a cation-deficient Pbca structure re- rules, or upon high-temperature release out of the lated to the Ia3 bixbyite group, showing a unit cell: a = Th2S3-type structure. Finally, it is possible that pan- 9.781(1) Å, b = 9.778(2) Å, c = 9.815(1) Å, V = 938.7 guite formed at low pressures due to stabilization by (1) Å3 and Z = 16. solution of Sc, Al, Ca, Mg, and REE. We will examine Origin and Significance: Zr is a key element in these possibilities by static and dynamic compression deciphering some of the earliest and most extreme experiments. environments before and during formation of the solar References: [1] Lodders K. (2003) App. J. 591, system because its oxide is highly refractory in both 1220. [2] Nicolussi et al. (1997) Science 277, 1281. [3] reducing and oxidizing gases [1] and it holds isotopic Allen J.M. et al. (1980) GCA 44, 685. [4] Hinton R.W. clues to nucleosynthetic contributions to the solar sys- and Davis A.M. (1988) GCA 52, 2573. [5] El Goresy tem and how they were introduced [2]. A variety of Zr, A. et al. (2002) GCA 66, 1459. [6] Ma C.
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