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Stability of titanian clinohumite: Experiments and thermodynamic analysis

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Abstract

Reversed hydrothermal experiments on a natural titanoclinohumite [Ti-Cl; approximate formula Mg7.5FeTi0.5O16(OH)] show that it breaks down at 475°±11° C (3.5 kbar), 620°±11° C (14 kbar) and 675°±8° C (21 kbar) to the assemblage olivine +ilmenite+vapor. An internal-consistency analysis of the data yields Δ r G /0 s (298 K, 1 bar)=36,760±3,326 cal (mole Ti-Cl)−1. Δ r S /0 s (298 K, 1 bar)=34.14±5.91 cal deg−1(mole Ti-Cl)−1. Linear correlation coefficient r G−S ≈1.0. A solution model that accounts for TiO2-M(OH)2 and F-OH substitution shows that the results for our nearly F-free Ti-Cl are in reasonable agreement with the unreversed breakdown experiments of Mer-rill et al. (1972) on a F-bearing Ti-Cl.

Because fluorine is necessary to stabilize Ti-Cl under mantle conditions, we suggest that Ti-Cl is much more likely to be a “storage device” for fluorine than for water in the mantle.

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  1. Martin Engi

    Present address: Department of Geological Sciences, The University of British Columbia, V6T 1W5, Vancouver, B.C., Canada

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  1. Department of Earth and Space Sciences, State University of New York, 11794, Stony Brook, New York, USA

    Martin Engi & Donald H. Lindsley

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Engi, M., Lindsley, D.H. Stability of titanian clinohumite: Experiments and thermodynamic analysis. Contr. Mineral. and Petrol. 72, 415–424 (1980). https://doi.org/10.1007/BF00371348

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