How do uncertainties of structure refinements influence the accuracy of the prediction of enthalpy of formation? Examples on muscovite and natrolite

Abstract

Abstract.A method to predict the enthalpy of formation is based on the parameter AHO2-cation (comp) (Vieillard 1982; Vieillard and Tardy 1988a). The calculation of this parameter requires the knowledge of cristallographical and optical properties of minerals (refractive index, molar volume, average distances and shortest bond lengths of cation-oxygens in all sites, existence of hydrogen bonding, presence of a order-disorder in a site of the involved compound). This method has been tested on a great number of minerals and artificial compounds belonging to the system: Li20-Na20-K20-BeO -MgO -CaO -MnO -FeO -Fe203 -A1203 -SiO 2 -H20 (180 minerals) (Vieillard 1994a, b). When several structure refinements of the same mineral are present, several factors such as precision of crystal structure determination, chemical composition, existence of any disorder in a site, presence of hydrogen bonding and estimated standard deviation on bond lengths seem to play an important role in the predication of enthalpy of formation from oxides. Muscovite and natrolite, for which numerous structure refinements are available, are used and discussed in this study.

Further study of the cation ordering in phengite 3T by neutron powder diffraction

“…This is in keeping with the fact that the 3T polytype formed at high pressure conditions, within an environment dominated by baric effects. These condition can promote crystallization of minerals with ordered cation distribution to minimize the Gibbs energy, by exploiting energetically non-equivalent sites (see the discussion on the role of the static energy at high pressure conditions in Wentzcovitch and Stixrude, 1997). The present results and those of Pavese et al (1997) agree that the cation partitioning inferred from the room temperature re®nements is preserved upon heating.…”

“…The force ®elds warranting the stability of structures are a sensitive function of the cation ordering, and the use in calculations of an incorrect atomic distribution causes failure in evaluating both static and vibrational energy (Vieillard, 1995).…”

“…Energy calculations would produce distorted results in predicting the stability ®eld of polytype 3T, if the incorrect partitioning reported by Pavese et al (1997) was used. The force ®elds warranting the stability of structures are a sensitive function of the cation ordering, and the use in calculations of an incorrect atomic distribution causes failure in evaluating both static and vibrational energy (Vieillard, 1995).…”