A suite of synthetic calcic amphiboles in the tremolite-tschermakite (Tr-Ts) series has been studied by high-temperature drop-solution calorimetry in molten 2PbO·B2O3. The pseudobinary join, which is shifted toward magnesiocummingtonite (Mc) by 10 mol%, extends from tremolite (Tr), Ca1.8Mg5.2Si8O22(OH)2, to magnesiohornblende (MgHb), Ca1.8Mg4.2Al2Si7O22(OH)2, and represents the operation of one full Mg-Tschermak substitution, [6]Mg,[4]Si ⇌ [6]Al, [4]Al, into the tremolite formula. Transmission electron microscope (TEM) examination of the amphiboles shows that all the samples contain a low concentration of (010) chain multiplicity faults (CMFs), suggesting that the increase of Al in the amphibole does not significantly increase or decrease the tendency of these defects to form. The calorimetric measurements indicate that the energy change associated with one full Mg-Tschermak substitution is quite small, on the order of −5 to −10 kJ/mol. Enthalpies of formation from the elements at 298.15 K and 1 bar have been calculated using the new calorimetric data for the magnesiohornblende (MgHb) and tschermakite (Ts) end-members. The values of ΔHf0 for MgHb and Ts are −12401.2 ± 10.6 and −12527.7 ± 16.4 kJ/mol, respectively. The entropy of MgHb, at 298.15 K and 1 bar, has also been calculated using four different site-mixing activity models involving the octahedral (M2) and tetrahedral (T1) sites and the experimental phase-equilibrium data of Jenkins (1994). Two of the models, the two-site and four-site coupled models, are essentially indistinguishable and gave the most consistent values for SMgHb0 and 575.6 ± 3.9 J/(mol· K), respectively.

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