Margules parameters

Chapitre I : Stabilité, déshydratation et évolution minéralogique des smectites di- et trioctaédriques en fonction de P, T, et de l’activité d’eau

The (K or Na) – (dried vacancy) exchanges along the pyrophyllite-muscovite or -paragonite solid-solutions are known to be largely non-ideal (large miscibility gap). They are described by large and positive W[mica-pyrophyllite] (> 20kJ/mol, see Vidal and Parra, 2000; Parra et al., 2002). Following our assumption that K-smectite, illite and muscovite can be described by the same solid-solution model (Fig. 1a), the coexistence of K-rich (illite) and K-poor (smectite) layers in illite/smectite is also taken to indicate a positive W[Smect.A – mica]. However, unmixing should not preclude the stability of smectite and vermiculite at low temperature, and the miscibility gap between K-poor and K-rich compositions should narrow with decreasing temperature, as hydration increases. This is consistent with the topology proposed by Loucks (1991), who predicted a decrease of K-content and an increase of mica hydration with a decrease of T, during continuous evolution from muscovite to illite. This is also consistent with the results of the solubility experiments of Rosenberg et al. (1990) and Aja et al. (1991), who conducted their experiments using illite of variable composition mixed with other phases (quartz (Qtz), kaolinite (Kln), microcline (K-Felds)). From the composition of solutions measured after the experiments, these authors found that clay with various interlayer charges (K0.3, 0.5, 0.7, 0.9) were stable at

different conditions. A compilation by Aja (1991) indicates that the range of possible K-contents

increases with increasing temperature, and that most compositions plot on two limbs in a

pyrophyllite-muscovite vs temperature diagram (Fig. 3). The K-rich limb shows an evolution of

clay composition from K-beidellite to high-charge smectite, then illite and then muscovite with increasing temperature, whereas the composition of clay along the K-poor limb is almost constant and corresponds to that of the K-beidellite end-members used in the present study. This topology is consistent with a narrowing of the miscibility gap between K-beidellite and K-rich clay compositions with decreasing temperature. A similar evolution occurs at higher temperature for the Na-smectite – paragonite solid solution. Chatterjee (1973) showed that Na-beidellite is stable in the presence of quartz and albite at a temperature of 315 to 335°C at pressures in the range 2-7 kbar. With increasing temperature, several regular paragonite-beidellite mixed-layer clays were observed up to about 450°C. Eberl and Hower (1977) also showed that mixed layers were stable above 300°C, a result that has been confirmed by Yamada and Nakazawa (1993), Yamada et al. (1999) and Vidal (1997). In order to insure unmixing between R-rich (illite, mica, rectorite, vermiculite, high charge smectite) and R-poor clays (beidellite and saponite), a value of

W[Smect.A – mica]> 15 kJ was imposed for K-, Na-, Ca and Mg-clay. According to Ransom and

Helgeson (1994), the Margules parameters describing non-ideal interaction between Smect.mnH2O

Chapitre I : Stabilité, déshydratation et évolution minéralogique des smectites di- et trioctaédriques en fonction de P, T, et de l’activité d’eau

constraints, we initially assumed that assumed that W[Smect.mnH2O – mica] = 0 for all interlayer

cations.

Finally, the enthalpy of R-clay.0.7H2O, .2H2O, .4H2O and 7H2O and the Margules parameters

were adjusted within the limits of the constraints discussed above. For a given interlayer cation, the

values of W[Smect.m_nH₂O –Smect.A], W[Smect.m_nH₂O – mica] and W[Smect.A – mica] were

assumed to be constant for 0 < n ≤ 3, but possibly different for n = 0. The contribution of interlayer

water δHf°mnH2O was constrained to decrease with hydration with a continuous downward convex

curve. An additional constraint was that the values of δH°fm2H2O derived by Ransom and

Helgeson (1994) for K, Na, Ca and Mg-smectites were reproduced within + 2 kJ/molH2O.

2.5. Results

The calculated standard state properties and the values of δH°fm2H2Oare listed in Table 3, and

the Margules parameters are listed in Table. 4. The formation enthalpy of Na-Beid.A, K-Beid.A

and Ca-Beid.A predicted by oxide summation were adjusted by -0.8 kJ and +1 kJ and +2 kJ,

respectively. These changes are lower than the uncertainties on the estimated H°f (about 10 kJ),

but they were necessary in order to obtain the topology reported in Fig. 10a, b and Fig. 11c. For the

same reason, W[Smect.m0H2O –Smect.A] was decreased to zero and W[Smect.m0H2O – mica]

was decreased to -5 kJ.

Figure 4a shows that for all interlayer cations, δH°fm2H2O decreases as the number of water

layers increases. For a given number of water molecules, δH°fm2H2O increase with the nature of

the interlayer cation (K<Na<Ca<Mg) and converge at low ionic potential of interlayer cation (IPIC = charge/radius) (Fig. 4b). The trends depicted in Figure 4 can be used to estimate the amount of

interlayer water in smectite as a function of the IPIC, a(H2O), pressure and temperature. This is

illustrated in Fig. 5, which shows that the amount of interlayer water calculated using the

δH°fm2H2O vs IPIC trends of Fig. 4 is in fair agreement with the values derived by Bérend et al.

(1995) and Cases et al. (1997) from their experimental measurements at a(H2O) = 0.5. The

reliability of the thermodynamic properties estimated in the present study can be also checked by

comparing the value of Hf°Na-Smectite derived from differential scanning calorimetry and

adiabatic calorimetric measurements by Gailhanou (2005) with the values predicted with the same

Chapitre I : Stabilité, déshydratation et évolution minéralogique des smectites di- et trioctaédriques en fonction de P, T, et de l’activité d’eau

composition of smectite investigated by Gailhanou (2005)

(Si3.7Al0.39)^IV(Al1.59Mg0.23Fe²⁺0.04Fe³⁺0.18)^VI(K0.03Ca0.01Na0.44)O10(OH).zH2O, the difference

between the estimated and measured values is about 10 kJ/mol for the dehydrated end-member,

and less than 5 kJ for all hydrated compositions (zH2O = 4.18, 4.77 and 5.44). These differences

are in the limit of the cumulative experimental uncertainties reported by Gailhanou (2005). The good agreement between the calorimetric measurements and our predicted values for all hydrated states suggests that the thermodynamic data listed in Table 3 can be used to predict the phase

relations, stability limits and dehydration of clays as a function of T, P, a(H2O) and composition.