Kinetics of the Fe 2+ -Mg, Order-Disorder Reaction in Anthophyllites: Quantitative Cooling Rates

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Abstract
The kinetics of the Fe2+-Mg, order-disorder phenomenon in a highly ordered natural anthophyllite have been determined over the temperature range from 400° to 720°C at a pressure of 2 kilobars. At temperatures of 600°C and above, equilibrium is attained by disordering as well as ordering reactions. The intracrystalline exchange is defined by a standard Gibbs free energy of 4247 ± 54 calories per formula unit. Rate studies at 550° and 500°C show that equilibrium is attained by ordering but not by disordering within the same time scale and that the exchange reaction is characterized by an activation energy of∼55 kilocalories per formula unit. An equilibration temperature for the natural anthophyllite of 270°C is determined from the termination of the ordering process owing to excessively slow reaction kinetics after∼107 years. From the rate constants of the exchange process, for different crystallization temperatures, the apparent equilibration temperature of 270°C defines a maximum linear cooling rate for the rock of 1 x 104°C per year.