HT P21/c-C2/c phase transition and kinetics of Fe2+-Mg order-disorder of an Fe-poor pigeonite : implications for the cooling history of ureilites

A natural Ca-poor pigeonite (Wo6En76Fs18) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P21/c, was annealed up to 1,093°C to induce a phase transition from P21/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P21/c phase) showed a displacive phase transition P21/c to C2/c at a transition temperature TTr = 944°C, first order in character. The Fe-Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in a range of temperatures between the closure temperature of the Fe-Mg exchange reaction and the transition temperature. Isothermal disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression of ln kD versus 1/T yielded the following equation: In KD = -3717(±416)/T(K)+1.290(±0.378); (R2 = 0.988). The closure temperature (Tc) calculated using this equation was ~740(±30)°C. Analysis of the kinetic data carried out taking into account the e. s. d.'s of the atomic fractions used to define the Fe-Mg degree of order, performed according to Mueller's model, allowed us to retrieve the disordering rate constants C0Kdis+ for all three temperatures yielding the following Arrhenius relation: In(C0K+dis) = In K0 - Q/(RT) = 20.99(±3.74) -26406 (±4165)/T(K); (R2 = 0.988) An activation energy of 52. 5(±4) kcal/mol for the Fe-Mg exchange process was obtained. The above relation was used to calculate the following Arrhenius relation modified as a function of XFe (in the range of The cooling time constant, η = 6 × 10-1 K-1 year-1 calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late cooling history of the ureilite parent body after impact excavation.