Experimental Formation of Non Expandable Dehydrated Smectite Layers in Interstratified Clays at 300°C and 100 bar

Michel Cathelineau (michel.cathelineau@g2r.u-nancy.fr) & Régine Mosser-Ruck (regine.ruck@g2r.u-nancy.fr)

G2R (UMR 7566), Faculté des sciences, UHP, BP-239, 54506-Vandoeuvre Cedex, France

The study of the hydrothermal reactivity of a K-smectite under 300°C, 100 bars in (Na,K) chloride fluids has shown the typical transformation in an ordered mixed layer I/S containing less than 30% expandable layers. This transformation involves two processes: (i) a partial dissolution of the smectite layers, (ii) the crystallization of ordered I/S with high percentage of non expanded layer. The chemical changes consist in the substitution of Si⁴⁺ by Al³⁺ in tetrahedral sites which is compensated by the introduction of Na and K (from the solution) in the interlayer space of the clay. Octahedral contents are remarkable stable. Non expandable layer percents in I/S run products are determined by XRD (Newmod Program). If it is assumed that Si content of smectite layers in I/S are constant, at 3.90 per half formula and newly formed illite in I/S run products contain 3.37 Si per half formula (Meunier and Velde, 1989), the mean Si content of I/S run products can be calculated (using XRD results) and compared to microprobe and TEM chemical analysis of the clays. The discrepancy between the two values is explained by the existence of dehydrated K or Na smectite among the non expandable layers of the I/S. Kinetic equations of illitization are thus different when calculated only from XRD data or XRD/crystal-chemical data. This study puts forward the crucial determination of the nature and composition of the non expandable layers forming the I/S.

Phase Relations in the System MgO-P₂O₅-H₂O: Stability Field of the New Mineral Raadeite, Mg₇(PO₄)₂(OH)₈

Christian Chopin (chopin@geologie.ens.fr) & Fabrice Brunet (brunet@geologie.ens.fr)

E.N.S., Laboratoire de Géologie, 24 rue Lhomond, 75005 Paris, France

Raadeite is a new mineral found in nodules of apatite + Mg-phosphates within a serpentinite body of the Modum district, S. Norway, from which althausite and holtedahlite, two polymorphs of Mg₂PO₄OH, have been described (Raade and Mladeck, 1979). Raadeite is isostructural with the Mn-arsenate allactite, i.e. contains face-sharing Mg octahedra, as the high-pressure Mg-phosphates holtedahlite and phospho- ellenbergerite.

Raadeite could be synthesized from 1.5 kbar, 450°C, to 22 kbar, 700°C, from or along with the alternative assemblage brucite + Mg₂PO₄OH (either the ß polymorph [= OH-wagnerite], or (epsilon)). At higher T, a still unidentified polymorph of raadeite (Mg₇(PO₄)₂(OH)₈-II) is obtained. The phase transition raadeite/Mg₇(PO₄)₂(OH)₈-II has been reversed between 1.5 and 3 kbar at 550°C and shows a steep positive Clapeyron slope. Toward lower pressures, raadeite breaks down to brucite + (epsilon)-Mg₂PO₄OH. The large volume change of this water-conserving reaction suggests a flat Clapeyron slope. Ongoing bracketing experiments locate the invariant point involving brucite, raadeite, Mg₇(PO₄)₂(OH)₈-II and (epsilon)-Mg₂PO₄OH near 1 kbar, 530°C, so making Mg₇(PO₄)₂(OH)₈-II the high-T phase, brucite + (epsilon)-Mg₂PO₄OH the low-P assemblage, and raadeite the relatively high-P, low-T phase. This is consistent with the natural occurrence of raadeite with althausite + holtedahlite, and with the phase relations of the latter two phases (Brunet et al., 1998).

We were unsuccessful in our attempts at synthesizing the related compounds that can be derived by a water-conserving reaction of the type

m Mg₂PO₄OH + n Mg(OH)₂ = Mg₂ m+n(PO₄)_m(OH)_m+2n

or, to emphasise the similarity with the humite family,

m Mg₃(PO₄)₂ + n Mg(OH)₂ = Mg₃ m+n(PO₄)₂ m(OH)_2n

where Mg₃(PO₄)₂ can have the olivine structure.

Raade G & Mladeck MH, Lithos, 12, 283-287

Brunet F, Chopin C & Seifert F, Contrib. Mineral. Petrol, 131, 54-70

The Theoretical Equation of State for the Fluids of H-O-C-N-S-F-Cl-Br-I-B-Si-Noble Gases System

Sergey Churakov (sergey@gfz-potsdam.de) & Matthias Gottschalk (mgott@gfz-potsdam.de)

GeoForschungsZentrum, Telegrafenberg, 14473, Potsdam, Germany

Fluid phases are involved in most geological processes. The calculation of metamorphic reactions or the interpretation of experimental mineral equilibria requires accurate thermodynamic properties for the fluid components. Natural fluids are usually complicated mixtures of various species, mainly water, CO₂, CH₄ and N₂. The properties of the principal pure fluids are well known. For most important binary mixtures like H₂O-CO₂ experimental data are available only in a limited P-T range. Only very few experiments exist for ternary systems. In order to extrapolate fluid properties various semi-empirical equations are used. Because of their different functional form, it is impossible to combine such expressions to obtain properties of more complicated fluid mixtures. In addition, the behavior of minor fluid compounds are usually unknown at high pressure and temperatures. These difficulties can be solved by introducing an equation of state (EOS) for complex fluid systems which satisfies the following conditions: a) a general functional form for all pure compounds of interest and accurate prediction of its mixing properties; b) a minimum of adjustable parameters; c) a correct extrapolation behavior to extreme pressure and temperature where experimental investigations are impossible.

The proposed EOS is based on a physically evident model of intermolecular interaction. The fluids behavior is approximated by the model of Stockmayer pair interaction potential. The EOS for this potential was developed using the thermodynamic perturbation theory (Verlet & Weis, 1972; Stell et al., 1974). It includes only three adjustable parameters, which are the molecular dipole moment, and parameters of Lennard-Jones interaction. These parameters are obtained from experimental P-V-T data. If no experimental data are available these values can be calculated from the properties of pure fluids at its critical point.

The developed EOS provides fugacities and volumetric properties for fluid mixtures of more then 30 components. The calculated activities in the H₂O-CO₂ binary showed excellent agreement with recent high temperature-pressure experimental results (Aranovich & Newton, 1999).

Aranovich LY & Newton RC, Amer.Miner., 84, 1319-1332, (1999).

Stell G, Rasaiah JC & Narang H, Mol.Phys., 7, 1393-1414, (1974).

Verlet L & Weis JJ, Mol. Phys, 4, 1013-1024, (1972).