Ti 3+ – and Ti 4+ – rich fassaites at the birth of the solar system: Thermodynamics and applications

Clicks: 2
ID: 303273
2017
Free PDF
Article Quality & Performance Metrics
Overall Quality
Not rated
Combines reader engagement with the AI quality analysis. This article has not been analysed, so there is no overall score — reader engagement is measured and shown alongside.
AI Quality Assessment
Not analyzed
Readership in this journal
Emerging

Ranked #847 of 8,486 articles by views in american journal of science

Most read Least read

Bar heights use a square-root scale. Only the 120 most-read articles are drawn; the journal has 8,486 in total.

Mint this article as an NFT
Not yet minted

Create a permanent, verifiable on-chain record of this article on the Scimatic Network. The NFT is held in your Journament account, and you can withdraw it to your own wallet at any time.

5 SUSD one-off · no wallet required
Sign in to mint
Abstract
A thermodynamic model is developed for pyroxenes in the chemical (mineralogical) system CaMgSi~2~O~6~ (diopside, Di, 1) -- CaTiAlSiO~6~ (grossmanite, Gr, 2) --CaMg~1/2~ Ti~1/2~AlSiO~6~ (alumino -- buffonite, AlBf, 3) -- CaAl~2~SiO~6~ (Calcium Aluminum Tschermaks, CATS, 4), *aka* fassaites. It is formulated for the assumptions of molecular mixing between AlBf and Di, Gr and CATS components, and that the reduction in configurational entropy resulting from short -- range Al -- Si ordering on tetrahedral sites may be described using, as a proxy, a long--range, convergent ordering parameter for ordering of Al and Si between A and B tetrahedral sites on the AlBf -- free tetrahedral sublattice. Nine out of sixteen independent thermodynamic parameters are calibrated to be consistent with calorimetric measurements of Benisek and others (2007), spectroscopic and statistical mechanical constraints on tetrahedral Al -- Si ordering in pyroxenes on the Di -- CATS binary (Cohen, 1986; Vinograd, 2001; Warren and others, 2001), and phase equilibrium constraints on Di -- AlBf miscibility gaps in the Di -- AlBf -- CATS ternary subsystem (Yang 1975, 1976). The remaining seven independent parameters (for Gr -- bearing fassaites) are constrained to be consistent with the distribution of fassaite compositions in calcium -- aluminum -- rich inclusions in carbonaceous chondrites (CAIs), experimental determinations of fassaite single -- phase regions produced at the extremely low oxygen fugacities prevailing during their primary condensation/crystallization (for example, Beckett, ms, 1986), and the assumption that secondary fassaites in Wark -- Lovering rims enclosing CAIs were in local equilibrium with the outer surface layers of the earlier (canonical) AlBf -- rich fassaites at their layer contacts at temperatures of around 1473 K (Wark and Lovering, 1982; Meeker and others, 1983; Murrell and Burnett, 1987; Ruzicka, 1997; Dyl and others, 2011). In this model there is a miscibility gap between Di--rich and AlBf--rich fassaites on the Di -- AlBf join which displays only very limited extension into the fassaite tetrahedron. On the Di -- AlBf -- CATS face of this tetrahedron there is a second miscibility gap (upper buffonite gap, UBG) with 0.08 ≤ X~4~^*fas*^ \< 0.22 and 0.06 ≤ X~4~^*fas*^ \< 0.25 at 1505 and 1473 K. This gap is modestly ellipsoidal with its long axis roughly radial to AlBf and it is roughly symmetrically disposed between Di and AlBf at 1505 K. At 1473 K this gap extends to the AlBf -- CATS binary, intersecting it at very low and intermediate X~4~^*fas*^, with most of its CATS -- rich binode being metastable with respect to assemblages composed of the minerals melilite, spinel, anorthite, perovskite and CATS -- poor fassaite. A third miscibility gap extends from the CATS--rich portion of the AlBf -- CATS binary, and occupies most of the CATS -- rich portion of this ternary at 1505 and 1473 K. It is metastable with respect to assemblages composed of melilite, spinel, anorthite, perovskite and hibonite, and, has a CATS -- rich binode that is virtually coincident with the Di--CATS binary at high CATS contents, in accordance with compositions of metastable, virtually AlBf--free CATS--rich fassaites (kushiroites) found in hibonite ± grossite + fassaite spherules in CAIs (for example, Krot and others, 1999). Within the fassaite tetrahedron at 1473 K the UBG is rapidly destabilized with addition of the Gr component, diminishing in dimension and collapsing to a critical endpoint at X~2~^*fas*^ \< 0.06. With further increase in Ti(Al)~−1~ exchange potential, the third miscibility gap in Di -- AlBf -- CATS fassaites (where it is metastable) becomes less extensive, and, it separates into an interior gap and one emanating from the Gr -- AlBf -- CATS ternary at a Ti(Al)~−1~ exchange potential corresponding to a reference Gr -- CATS fassaite with X~2~^*fas*^ (≡X~2~^*ref*^) ≈ 0.22. Both of these gaps continue to decrease in size with further increases in Ti(Al)~−1~ exchange potential, with the gap emanating from the Gr -- AlBf -- CATS ternary terminating there in a critical point at X~2~^*ref*^ ≈ 0.30. With further increase in Ti(Al)~−1~ exchange potential the more Di--rich gap continues to move toward the Di -- Gr -- CATS ternary until X~2~^*ref*^ ≈ 0.50. At higher Ti(Al)~−1~ exchange potentials there is complete miscibility in quaternary fassaites, except in the immediate vicinity of the Di -- AlBf binary, and, at very high Ti(Al)~−1~ exchange potentials, in fassaites near the Di -- Gr binary. Evaluating the Ti(Al)~−1~ exchange potentials at which the extensive gap that is metastable in the diopside -- alumino -- buffonite -- CATS ternary subsystem becomes stable in the quaternary system at given temperatures awaits development of a comprehensive database for CAI fassaites and their associated minerals, particularly the solid solutions melilite and hibonite (for example, Waldbaum, 1973; Waldbaum and Woodhead, 1975; Doyle and others, 2014). The model for fassaite thermochemistry is used to develop a Ti^3+^ -- Ti^4+^ fassaite oxygen cosmobarometer based on experimental constraints (for example, Beckett, ms, 1986). Values of oxygen fugacity calculated with this cosmobarometer for fassaites and melilites, judged by Grossman and others (2008) to be coeval in refractory CAIs from the Allende carbonaceous chondrite, are more than three orders of magnitude below that of a gas of solar composition.
Reference Key
openalex_W2756171392 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Richard O. Sack, M. S. Ghiorso
Journal american journal of science
Year 2017
DOI
10.2475/07.2017.02
URL
https://doi.org/10.2475/07.2017.02
Keywords Keywords not found

Citations

No citations found. To add a citation, contact the admin at info@scimatic.org

Comments

No comments yet. Be the first to comment on this article.