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THE THERMODYNAMICS of Sc- and Ti-BEARING REFRACTORY MINERAL PHASES in CALCIUM-ALUMINUM-RICH INCLUSIONS: REVISTING the CONDENSATION SEQUENCE of the EARLY SOLAR NEBULA

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THE THERMODYNAMICS of Sc- and Ti-BEARING REFRACTORY MINERAL PHASES in CALCIUM-ALUMINUM-RICH INCLUSIONS: REVISTING the CONDENSATION SEQUENCE of the EARLY SOLAR NEBULA 51st Lunar and Planetary Science Conference (2020) 2547.pdf THE THERMODYNAMICS OF Sc- AND Ti-BEARING REFRACTORY MINERAL PHASES IN CALCIUM-ALUMINUM-RICH INCLUSIONS: REVISTING THE CONDENSATION SEQUENCE OF THE EARLY SOLAR NEBULA . V.R.Manga1,2, T.J. Zega1,2 . 1Lunar and Planetary Laboratory, 2Dept. of Materials Sci- ence and Engineering, University of Arizona, Tucson, AZ 85721, USA. ([email protected]). Introduction: Calcium-aluminium-rich inclusions With the advent of sophisticated high-performance (CAIs) are radiometrically age dated to 4.567 billion computing, computational thermodynamics and kinet- years [1,2] and contain refractory materials that formed ics paradigms have reached a level of maturity Where at very high temperatures. They are the oldest solids to they can be reliably used for describing the formation, have formed in the solar system and the constituent min- structure, and phase stability of molecules, engineering eral phases of these inclusions offer clues to the thermal materials, and minerals. landscape of the early solar nebula. Theoretical methods and calculations: First-prin- Understanding this landscape requires thermody- ciples quantum-mechanics-based methods have led to namic models in conjunction With experimental charac- the development of a computational framework to pre- terization of the CAI assemblages. Crucial to this ap- dict the thermochemistry of a Wide range of phases, in- proach is thermochemical data pertaining to the com- cluding those relevant to the solar system. The em- plex multicomponent solid solutions of the mineral ployed computational framework predicts thermochem- phases within the inclusions. However, knowledge gaps ical data such as heat capacity (Cp), enthalpies (H), and within thermochemical databases of the myriad solid so- entropies (S) of formation of stoichiometric endmem- lutions Within CAI materials limits the applicability of bers and of their solid solutions. The free-energy (G) thermodynamic calculations of the system of interest, descriptions of these materials can therefore be ob- i.e., the solar nebula. tained. One such eXample of an important phase within We performed first-principles quantum-mechanics CAIs that is limited by the sparsity of thermochemical calculations employing Vienna Ab initio Simulation data is pyroxene. The monoclinic form of pyroXene, cli- Package (VASP) [8,9] to calculate the thermochemical nopyroxene (space group C2/c), exhibits a family of data of several solid solutions (see beloW). Special qua- solid solutions With a large composition space involving sirandom structures (SQS) predict enthalpies of miXing > 8 elements. Experimental determination of the ther- in solid solutions as a function of composition With re- mochemical data of such parameter space and the dis- spect to their end-members. The entropic contributions crete number of phases that compose it is not trivial. As to the free energy are obtained from phonon and Debye a consequence, the thermodynamic modeling efforts of calculations [10]. The eXchange correlation functional pyroxene employs approximations, which in turn leads as described by Purdew-Burke-Ernzerhof (PBE) is used to different predictions of the composition of phases and in the calculations [9]. the sequence in Which they condense from the modeled Thermodynamic modeling is conducted Within the (solar) system. CALculation of PHAse Diagrams (CALPHAD) frame- In the case of the thermochemistry of Al-Ti-rich py- work. To be clear, CALPHAD is not software but rather roXenes relevant to CAI phases, they were initially cal- a framework in Which the condensation and stability of culated With Ti only in the 4+ oxidation state [3,4]. Later minerals can be modeled by combining the computed calculations incorporated Ti3+, but several approXima- and available experimental thermochemical data With tions Were made in the parametrization of the model [5]. crystal structure-based models. Gibbs free energy de- Hence, a critical knowledge gap exists of the thermo- scriptions of solution phases in their entire composition chemical data for pyroXenes. Closing this gap is crucial space are developed to calculate their crystal chemistry for their thermodynamic modeling. Most importantly, and precise thermochemical origins under nebular con- the assement of Whether chemistry in the early solar ditions in addition to predicting their post-formation his- nebula occurred under equilibrium or non-equilibrium tories. conditions requires knowing the thermochemical prop- The modeling includes, but is not limited to, all the ex- erties of the solid solutions in their entire composition. perimentally identified refractory phases. Thus, as part In addition, many neW mineral phases such as (Allen- of this ongoing effort, We calculated the condensation deite, Sc4Zr3O12, Panguite (Ti+4,Sc, Al,Zr,Mg,Ca)1.8O3 sequence of refractory minerals that are found Within etc.) and others have been discovered in CAIs [e.g., 6,7]. primitive meteorites . The modeling includes all the In light of the limitations in the existing thermo- pertinent stoichiometric and solid solution phases such chemical data and the discovery of new mineral phases, as V-alloyed perovskite (CaTiO3), V-alloyed spinel more comprehensive thermodynamic modeling is de- (MgAl2O4), grossite (CaAl4O7), hibonite (CaAl12O19), manded. To this end, we have undertaken first-princi- melilite (CaAl2Si2O7), corundum (Al2O3), and pyroxene ples based computation of thermochemistry of refrac- (Ca(Mg,Ti)(Al,Si)2O6). We are also including the other tory mineral phases pertaining to CAI assemblages. solid-solution phases that are recently found Within 51st Lunar and Planetary Science Conference (2020) 2547.pdf calcium and aluminium-rich inclusion (CAIs) [6,7]. For gap(s), With the additions of Sc, Ti3+ and Fe, Will be dis- the gas phase, We consider all the elements/species per- cussed at the meeting. tinent to the solar nebula. The gas phase is modeled by 1800 incorporating all the species (O2, O, O3, Al, Mg, Ca, Ti, V, Al1O1, Al1O2, Al2, Al2O1, Al2O2, Al2O3, Mg1O1, Ca1O1, Mg2, Ca2, H, H2, H2O1, H1O2, H1O1, H2O2, C, Pv C1O2, C1O1, Si, Ti1O1, Ti1O2, V1O1, V1O2, Si1O1, Si1O2). Vapor Results and discussion: We highlight some of the Px calculated enthalpies of formation (in kJ/mol) of pyrox- ene endmembers relative to the binary oXides (CaO, 1600 Co+Px MgO, TiO2, Ti2O3, Sc2O3, Fe2O3) in Table 1. The pre- dicted CaScAlSiO6 endmember energies (-75.209 kJ/mol) allow the calculation of Davisite, Hb +Px Hb +Px+ Gr 3+ 3+ 4+ 2+ Px+Gr+Ml Ca(Sc ,Ti ,Ti ,Mg )AlSiO6 . The first principles cal- culations have considered the random mixing of Al and Gr+Px Si on the tetrahedral site of the pyroXene, and enable the Temperature [K] Ml+Px+Hb+Sp Hb 1400 Co Hb+Ml+Px determination of excess free energy arising due to the + + Px Ml miXing. Co+Ml+Px+Sp Px endmember DHf (kJ/mol) Ml+Sp+Px CaTiAlSiO6 -38.64 CaTiAl2O6 -62.785 Sp+Px CaScAlSiO6 -75.209 1200 CaAlAlSiO6 -51.271 10-5 10-4 CaFeAlSiO6 -51.896 Pressure [bar] Using these data, we calculated the condensation of Fig. 1. The stability diagram of refractory mineral solids from a gas of solar composition under equilib- phases as a function of T and P, depicting the conditions rium conditions (i.e. complete equilibriation in terms of of their condensation Within solar nebula. chemical potentials of all elements betWeen the gas phase and the condensates at a given T and P). Figure 1 In summary, the revised condensation sequences shoWs this sequence as a function of temperature and that are being proposed from our on-going thermody- pressure. We highlight tWo key points here. First, the namic effort have important implications for the identi- sequence looks remarkably different from those availa- fication of equilibrium versus non-equilibirum nature of ble in the literature, e.g., [3,11]. Specifically, We find condensation based on the reported microstructures and that cubic perovskite is the first phase to condense With phase relationships Within CAI and other planetary-ma- a condensation temperature varying betWeen 1682 K terials mineral assemblages. and 1637 K in the pressure range of 1×10-3 to 3×10-5 bar. Acknowledgements Research funded by NASA un- The eXisting literature [e.g. 3,11] invariably predict co- der the grant NNX15AJ22G and 80NSSC19K0509. First-principles calculations Were carried out on the rundum (Al2O3) as the first phase to condense from a gas of solar composition. The reason for the difference NAS (NASA) cluster and on the high-performance is that our calculations take into account all three phases computational (HPC) cluster at the University of Ari- of perovskite (cubic, tetragonal, and orthorhombic) for zona. which we calculated the thermochemical data employ- References: [1] Amelin Y. et al. (2002) Science ing first-principles. Second, We find high condensation 297:1678-1683. [2] Connelly J. N. et al. (2012) Science temperatures of Al-Ti-rich pyroXenes with an endmem- 338: 651-655. [3] Ebel D.S. and Grossman L. (2000) Geochimica et Cosmochimica Acta 64:339-366. [4] ber composition of CaTiAl2O6, also in sharp contrast to the previous calculations. At 10-4 bar the phase con- Sack R.O. and Ghiorso M.S. (1994) Contributions to the denses at temperatures as high as 1670 K at a total gas Mineralogy and Petrology, 116: 277-286. [5] Sack R.O. pressure of 10-4 bar. and Ghiorso M.S. (2017) American Journal of Science , The incorporation of ultra-refractory components 317: 807-845. [6] Ma C. et al. (2009) LPSC, XL . [7] such as Sc3+ as Well as Ti3+, can further modify the con- Ma C. et al. (2012) American Mineralogist 97: 1219- densation of clinopyroxenes and lead to changes in the 1225. [8] Kresse G. and Joubert J. (1999) Phys. Rev. B condensation sequence. In the current model, the pyroX- 59: 1758-1775. [9] PerdeW J.P. et al. (1996) Phys. Rev. ene (Fig. 1) phase exhibits a miscibility gap with Al-Ti- Lett., 77, 3865-3868 .
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