Sublattice solid solution model and its application to orthopyroxene (Mg, Fe)2Si2O6

Abstract

Abstract. The theory of sublattice solid solution model and optimization methods have been described for modelling the geochemically important multicomponentmultisite silicate solid solution systems. Some new X-ray Mg-Fe z § site occupancy data along with some selection from the existing data on heated orthopyroxene in the temperature range 600 to 1000~ C have been used in thermodynamic modelling of the orthopyroxene (Mg, Fe)2Si206 solid solution using the sublattice solution model. The optimized interaction energy solution parameters are: L~lg, ve = 13 600.7-4.92650 9 T L~2, Ve = 13 308.

Fe-Mg order-disorder in orthopyroxenes

“…These applications all rely on a knowledge of equilibrium distribution and exchange kinetics. Hence, considerable effort has been expended to quantify the equilibrium distribution of Mg and Fe between the M1 and M2 sites (Saxena and Ghose, 1971;Ganguly, 1982;Anovitz et al, 1988;Shi et al, 1992;Yang and Ghose, 1994;Kroll et al, 1997;Stimpfl et al, 1999).…”

Thermodynamic modelling of non-convergent ordering in orthopyroxenes: a comparison of classical and Landau approaches

“…The signs of L~x and L~2 cannot be predicted, although one might expect that they are both positive or negative due to the relative similarity of the sites. Sack and Ghiorso (1989) and Shi et al (1992) report positive Es with L~, >LG2 9 A positive interaction parameter means a tendency to exsolution. "Exsolution" of M1 and M2 is equivalent to ordering (or anti-ordering) of the crystal as a whole.…”

“…The fact that the contribution of the (L~I--LG2) term to the internal field changes sign across the orthopyroxene series causes the isotherms in a Roozeboom diagram (e.g. Shi et al 1992) to be asymmetric: At compositions X<0 they plot above, at X> 0 they plot below their symmetric equivalents (L~ =c?.9.…”

“…The second term in 557 square brackets models the excess free energy due to independent mixing of Fe 2+ and Mg on the M1 and M2 sites. Cross-site interactions are incorporated in the reference term which in general is a non-planar G~ (Shi et al 1992, Fig. 2).…”

“…Correspondence to: H. Kro]l in minerals: the classical thermodynamic approach based on the reciprocal solution model (Thompson 1969;1970;Sack and Ghiorso 1989;Shi et al 1992; Ghiorso and Sack 1991, and citations therein) and the Landau theory for order/disorder processes (Salje and Kroll 1991;Carpenter et al 1994;Carpenter and Salje 1994a).…”