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 ( Tr-Ts) series has been studied by high-temperature drop-solution calor imetry in molten 2PbO.B2O3. The pseudobinary join, which is shifted to ward magnesiocummingtonite (Mc) by 10 mol%, extends from tremolite (Tr ), Ca1.8Mg5.2Si8O22(OH)2, to magnesiohornblende (MgHb), Ca1.8Mg4.2Al2S i7O22(OH)2, and represents the operation of one full Mg-Tschermak subs titution, [6]Mg,[4]Si reversible [6]Al,[4]Al, into the tremolite formu la. Transmission electron microscope (TEM) examination of the amphibol es shows that all the samples contain a low concentration of (010) cha in multiplicity faults (CMFs), suggesting that the increase of Al in t he 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 i s quite small, on the order of -5 to -10 kJ/mol. Enthalpies of formati on from the elements at 298.15 K and 1 bar have been calculated using the new calorimetric data for the magnesiohornblende (MgHb) and tscher makite (Ts) end-members. The values of DELTAH(f)0 for MgHb and Ts are -12401.2 +/- 10.6 and -12527.7 +/- 16.4 kJ/mol, respectively. The entr opy of MgHb, at 298.15 K and 1 bar, has also been calculated using fou r 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 coup led models, are essentially indistinguishable and gave the most consis tent values for S(MgHb,)0 575.1 +/- 3.9 and 575.6 +/- 3.9 J/(mol.K), r espectively.