Mixing properties and activity-composition relationships of chlorites in the system MgO-FeO-Al2O3-SiO2-H2O
Holland, Tim; Baker, Judy; Powell, Roger
European Journal of Mineralogy Volume 10 Number 3 (1998), p. 395 - 406
published: Jun 22, 1998
manuscript accepted: Nov 21, 1997
manuscript received: May 14, 1997
ArtNo. ESP147051003001, Price: 29.00 €
Abstract The thermodynamic mixing properties of chlorites in the system MgO-FeO-Al2O3-SiO2-H2O are derived from the experimental study of Baker & Holland (1996) together with observations on the crystal chemistry and natural partitioning behaviour of chlorite and garnet. Ordering of octahedral Al strongly into the M4 site is apparently required by the MgO-AhO3-SiO2-H2O experimental data, and so a simple Bragg-Williams type of model has been constructed and calibrated. Disordered (mixing-on-sites) models for chlorites not only cannot fit the experimental data, but also yield calculated chlorite compositions which are excessively low in Al for certain assemblages. The ordering model developed here rectifies this situation, and is readily extended into iron-bearing systems, where it appears to be in good agreement with garnet-chlorite Fe:Mg partitioning in nature. A simplification for compositions more aluminous than clinochlore should provide a basis for constructing new thermobarometers involving chlorite.