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Ordering kinetics of Mg-Fe²⁺ exchange in a Wo₄₃En₄₆Fs₁₁ augite

¹ Dipartimento di Scienze della Terra, Università di Perugia, Perugia, Italy
² Dipartimento di Mineralogia e Petrologia, Università di Padova, Padova, Italy
³ Dipartimento di Scienze della Terra, Università di Pavia, Pavia, Italy
⁴ Centro di Studio per la Cristallochimica e Cristallografia, CNR, Pavia, Italy

Correspondence: ^* E-mail: zanazzi{at}unipg.it

The kinetics of Fe²⁺-Mg exchange between the M1 and M2 sites (Fe_M2^{2 +} Mg_M1 = Fe_M1²⁺ + Mg_M2) were followed by ordering experiments in an augitic clinopyroxene (Ca_0.841Na_0.019Mg_0.888Fe_0.151²⁺Ti_0.011Al_0.028Cr_0.003Fe_0.054³⁺Mn_0.006)(Si_1.914Al_0.086) from a dike on Alicudi (Aeolian Islands, Italy). The same single crystal was annealed in silica tubes in an iron-wüstite buffered Ar atmosphere, at 700, 750, 800, 850, and 950 °C, followed by quenching after each thermal treatment, and was used for the collection of X-ray diffraction data and for microprobe analysis.

Kinetic constants (K) for each isotherm were calculated from the distribution coefficient K_D [=(Fe²⁺/Mg)_M1/(Fe²⁺/Mg)_M2] measured during a series of experiments carried out by increasing annealing times until exchange equilibrium was achieved. Calculations of K were performed using the Ginzburg-Landau equation, the kinetic model of Mueller, and the mathematical formulation proposed by Sha and Chappell (1996) to check the responses of various kinetic models for a Ca-rich pyroxene on the same data set. The activation energy of the Fe²⁺-Mg exchange spans a narrow range, yielding values of 200 (±28) kJ/mol, independent of the method used to calculate K.

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