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Mixing properties of phengitic micas and revised garnet-phengite thermobarometers

Mixing properties of phengitic micas and revised garnet-phengite thermobarometers
Mixing properties of phengitic micas and revised garnet-phengite thermobarometers
Mixing properties for muscovite–celadonite–ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end-members. When a partially ordered solution model is assumed, the enthalpy of mixing among the end-members muscovite–celadonite–ferroceladonite is nearly ideal, although the Gibbs energies of muscovite–celadonite and muscovite–ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing. Thermodynamic consistency is achieved for data on phengite compositions inassemblages with (a) pyrope+kyanite+quartz/coesite (b) almandine+kyanite+quartz/coesite (c)talc+kyanite+quartz/coesite and (d) garnet–phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low-grade schists. The muscovite–paragonite solvus has been reanalysed using the asymmetric van Laar model, and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end-members. Results are applied to a natural pyrope–coesite–phengite–talc rock from the Western Alps, and to investigate the conditions under which biotite-bearing mica schists transform to whiteschist-like biotite-absent assemblages for average pelite bulk compositions.
0263-4929
683-696
Coggon, R.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Holland, T.J.B.
437b4a3a-999b-4fb3-a74f-65e6110dd3ce
Coggon, R.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Holland, T.J.B.
437b4a3a-999b-4fb3-a74f-65e6110dd3ce

Coggon, R. and Holland, T.J.B. (2002) Mixing properties of phengitic micas and revised garnet-phengite thermobarometers. Journal of Metamorphic Geology, 20 (7), 683-696. (doi:10.1046/j.1525-1314.2002.00395.x).

Record type: Article

Abstract

Mixing properties for muscovite–celadonite–ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end-members. When a partially ordered solution model is assumed, the enthalpy of mixing among the end-members muscovite–celadonite–ferroceladonite is nearly ideal, although the Gibbs energies of muscovite–celadonite and muscovite–ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing. Thermodynamic consistency is achieved for data on phengite compositions inassemblages with (a) pyrope+kyanite+quartz/coesite (b) almandine+kyanite+quartz/coesite (c)talc+kyanite+quartz/coesite and (d) garnet–phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low-grade schists. The muscovite–paragonite solvus has been reanalysed using the asymmetric van Laar model, and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end-members. Results are applied to a natural pyrope–coesite–phengite–talc rock from the Western Alps, and to investigate the conditions under which biotite-bearing mica schists transform to whiteschist-like biotite-absent assemblages for average pelite bulk compositions.

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More information

Published date: September 2002
Organisations: Ocean and Earth Science, Geochemistry

Identifiers

Local EPrints ID: 347890
URI: http://eprints.soton.ac.uk/id/eprint/347890
ISSN: 0263-4929
PURE UUID: 42759b2d-d153-4447-b801-d4098238610e
ORCID for R. Coggon: ORCID iD orcid.org/0000-0002-9228-9707

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Date deposited: 01 Feb 2013 15:18
Last modified: 15 Sep 2021 02:00

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Contributors

Author: R. Coggon ORCID iD
Author: T.J.B. Holland

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