Synthetic and natural tremolite in equilibrium with forsterite,enstatite, diopside and fluid

The following equilibrium among tremolite forsterite, diopside, and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials: Ca₂Mg₅Si₈O₂₂(OH)₂+Mg₂SiO₄ =2 CaMgSi₂O₆+5MgSiO₃+H₂O A significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite. For example, in nearly pure H₂O with the H₂ content of the fluid buffered by nickel-bunsenite at one kilobar (10⁸ pascals), the breakdown of the assemblage with synthetic amphibole occurs at 708±20° C. The breakdown of the assemblage with natural amphibole, Ca_2.16Mg_4.94Fe_0.03Si_7.92 Al_0.01O₂₂(OH)₂F_0.03 occurs at 841±47° C. The shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments. The reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the NB, OH (XG, COH) buffer. Analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160.92 joules/degree is used. The resulting enthalpy of reaction is 113.96±1.82 kilojoules with the natural amphibole and 104.83±0.12 kilojoules with synthetic tremolite. Deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability.