Kinetics of intracrystalline order-disorder reactions in tremolite

The temperature dependent Fe-Mg distribution in tremolite from Zillertal, Austria was investigated using Mössbauer spectroscopy. The standard free energy change for the exchange reaction Fe²⁺(M4)+Mg(M2)=Mg(M4)+Fe²⁺(M2) decreases with increasing temperature, corresponding to an enthalpy term of 9.2±1.5 kcal/mole and an entropy term of 1.9±1.7 cal/mole K. Kinetic experiments performed as hydrothermal runs, yield an activation energy of 70±7 kcal/mole for the disordering reaction. Difficulties in analysing the very small amount of Mg(M4) in the natural sample introduce large errors in the calculation of cooling rates for the natural rock. The estimated error in the Mg(M4) site occupancy results in a shift of the obtained cooling rate of about three orders of magnitude, making tremolite less useful for estimations of rock cooling rates.