Excess CFSE of mixing

Therefore, according to this calculation, there is an excess CFSE (enthalpy) of mixing of about -1.6 kJ/(mole of olivine). Similarly, the formation of all intermediate olivines by mixing of Mg2Si04 and Fe2Si04 components results in an excess CFSE of mixing. This is illustrated in fig. 7.3. These results imply that there is a heat of mixing term and that the olivine series is not an ideal solid-solution of forsterite and fayalite. 

Figure 7.4 Excess CFSE of mixing for Mg2+-Ni2+ olivines of the forsterite-liebenber-gite series (see table 7.2).

This calculation shows that the formation of intermediate liebenbergite by mixing of Mg2Si04 and Ni2Si04 components is accompanied by an excess CFSE of mixing of -5.65 kJ/mole. These results suggest that Mg2+-Ni2+ olivines depart considerably from ideal solution behaviour (Bish, 1981). This is further demonstrated in fig. 7.4 by the compositional variation of excess CFSE of mixing for the suite of synthetic Mg2+-Ni2+ olivines for which site occupancy and CFSE data are available (table 7.2). 

It should be noted, however, that in a multisite substitution where cation ordering occurs, a heat of mixing term could arise even in the absence of a compositional variation of the CFSE. For example, the excess CFSE of mixing would increase to about -8.20 kJ/mole in solid-solution formation of the liebenbergite (Mg0 49Ni0 51)2SiO4, eq. (7.14), if the CFSE of Ni2+ ions in the Ml and M2 sites were to remain constant at -143.1 and -113.2 kJ/mole, respectively. 

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