A calorimetric study of synthetic amphiboles along the tremolite- tschermakite join and the heats of formation of magnesiohornblende and tschermakite

A suite of synthetic calcic amphiboles in the tremolite-tschermakite series has been studied by high-temperature drop-solution calorimetry in molten 2PbO.B₂O/₃O. The pseudobinary join, which is shifted toward magnesiocummingtonite by 10 mol%, extends from tremolite to magnesiohornblende, and represents the operation of one full Mg-Tschermak substitution into the tremolite formula. 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 and tschermakite end-members. -from Authors