Enthalpies of drop solution in molten lead borate have been measured for a series of four zoisite and 12 clinozoisite samples, spanning both epidote solid solutions. Mixing within the zoisite series between 0.0 and 0.2 XAl2Fe appears to be ideal. The enthalpy of drop solution varies between 493.0 ± 3.2 and 496.0 ± 3.6 kJ/mol with a slight increase with increasing Fe content, based on a linear fit to the data. The clinozoisite solid solution shows more complex behavior. The enthalpy of drop-solution for samples between 0.28 and 0.65 XAl2Fe varies between 491.7 ± 4.2 and 500.7 ± 3.0 kJ/mol and shows significant scatter. For samples between 0.65 and 0.95 XAl2Fe, the drop solution enthalpies increase abruptly, going from 494.3 ± 3.4 to 521.1 ± 5.4 kJ/mol. End-member drop solution values for Fe-free zoisite were estimated by the linear fit and for clinozoisite by assuming a fictive Fe-free end-member, taking into account the estimated energy of the monoclinic-orthorhombic transition, and by extrapolation of the Fe-rich data to XAl2Fe = 1. Standard enthalpies of formation from the elements for Fe-free zoisite, Al-clinozoisite, and Fe-clinozoisite have been calculated based on the present data and data from the literature. The values are ΔHf,298.150,zoi = −6878.5 ± 6.8 kJ/mol; ΔHf,298.150, Al-czoi = −6882.5 ± 6.9 kJ/mol; ΔHf,298.150,Fe-czo = −6461.9 ± 6.8 kJ/mol. The overall trend in the drop solution data reflects an apparent positive enthalpy of mixing for the clinozoisite series, consistent with the presence of a miscibility gap in the series between about 0.5 and 0.8 XAl2Fe. An attempt to fit the highly asymmetric enthalpy of mixing data with a two-parameter asymmetric Margules mixing model, however, failed to yield realistic values for the interaction parameters and the solvus closure temperature. The significant scatter in the data, between XAl2Fe = 0.0 and 0.65, may be due to short-range ordering, suggested by Fehr and Heuss-Assbichler (1997), or could be due to different states of metastable disorder in the samples. The steep increase in enthalpy going from XAl2Fe = 0.65 to 1.0 the may be due to the effect of possible increasing Fe3+-Al3+ disorder among the M1 and M3 octahedral sites. The break in slope of the enthalpy data occurs at or near the composition where the onset of Fe3+ disorder had been observed previously in clinozoisite.

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