Dehydration of dioctahedral aluminous phyllosilicates: thermodynamic modelling and implications for thermobarometric estimates

  • Benoît Dubacq
  • Olivier Vidal
  • Vincent De Andrade
Original Paper

DOI: 10.1007/s00410-009-0421-6

Cite this article as:
Dubacq, B., Vidal, O. & De Andrade, V. Contrib Mineral Petrol (2010) 159: 159. doi:10.1007/s00410-009-0421-6

Abstract

We propose a solid-solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations, including hydration, observed in natural smectites, interlayered illite/smectite, illites, and phengites from diagenetic to high-grade metamorphic conditions. The suggested formalism involves dehydrated micas and hydrated pyrophyllite-like thermodynamic end-members. With these end-members, the equilibrium conditions of quartz + water + K-bearing mica-like phyllosilicates of fixed 2:1 composition are represented by a line in PT space along which the interlayer water content varies. The relevant thermodynamic properties required for the calculation of equilibrium conditions were derived using a set of 250 natural data of known maximal temperature and pressure conditions, which covers a range between 25°C and few MPa to 800°C and 5 GPa. The temperatures calculated at fixed pressure with our model are in fair agreement with those reported in the literature for the 250 natural data. At low temperature and pressure, the amount of interlayer water in K-deficient phengite and illite is predicted to reach 100% of the apparent vacancies, which is consistent with previous values reported in the literature. Although the amount of interlayer water is predicted to decrease with pressure and temperature, it is calculated to be significant in K-deficient phengite from LT–HP pelites metamorphosed at about 350°C, 10 kbar. The presence of molecular water in the interlayer site of such phengites has been confirmed by FTIR mapping. Its implications for PT estimates are discussed.

Keywords

Thermodynamics Clay minerals Hydration Thermobarometry Phengites Solid solution 

Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Benoît Dubacq
    • 1
    • 3
  • Olivier Vidal
    • 1
  • Vincent De Andrade
    • 2
  1. 1.CNRS, Université Joseph Fourier Grenoble, LGCAGrenoble CedexFrance
  2. 2.European Synchrotron Radiation FacilityGrenobleFrance
  3. 3.Department of Earth SciencesUniversity of CambridgeCambridgeUK

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