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首页> 外文期刊>Contributions to Mineralogy and Petrology >HT P2_1/c-C2/c phase transition and kinetics of Fe~(2+)-Mg order-disorder of an Fe-poor pigeonite: implications for the cooling history of ureilites
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HT P2_1/c-C2/c phase transition and kinetics of Fe~(2+)-Mg order-disorder of an Fe-poor pigeonite: implications for the cooling history of ureilites

机译：贫铁方铁矿的HT P2_1 / c-C2 / c相变和Fe〜（2 +）-Mg有序无序动力学：对尿素冷却历史的影响

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摘要

A natural Ca-poor pigeonite (Wo_6En_(76)Fs_(18)) 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 P2_1/c, was annealed up to 1,093°C to induce a phase transition from P2_1/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 P2lc phase) showed a displacive phase transition P2_1/c to Cllc at a transition temperature T_(Tr) = 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 In k_D versus /T yielded the following equation: In k_D = -3717(±416)/r(K) + 1.290(±0.378): (R~2 = 0.988). The closure temperature (T_c) 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 C_0K_(dis)~+ for all three temperatures yielding the following Arrhenius relation: (C_0K_(dis)~+) = In K_0 - Q/(RT) = 20.99(±3.74) - 26406 (±4165)/T(K); (R~2 = 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 X_(Fe) (in the range of X_(Fe) = 0.20-0.50): ln(C_0K_(dis)~+) = (21.185-1 .47X_(Fe)) -(27267-4170X_(Fe)/T(K). The cooling lime constant, η = 6 x 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.
机译：通过原位高温单晶X射线衍射研究了来自陨石陨石样品PCA82506-3中的天然钙贫乏的钙铝榴石（Wo_6En _（（76）Fs_（18））。将单斜P2_1 / c样品退火至1,093°C，以诱导从P2_1 / c到C2 / c对称性的相变。晶格参数和b型反射强度（仅在P2 lc相中出现的h + k = 2n + 1）随温度的升高而变化，在相变处显示出从相变P2_1 / c到Cllc的位移。温度T_（Tr）= 944°C，一阶字符。通过异位单晶X射线衍射在Fe-Mg交换反应的封闭温度和转变温度之间的温度范围内，通过异位单晶X射线衍射研究了Fe-Mg交换动力学。使用IW缓冲液在790、840和865°C的三个晶体上进行了等温无序退火实验。 In k_D对 / T的线性回归得出以下方程式：In k_D = -3717（±416）/ r（K）+ 1.290（±0.378）：（R〜2 = 0.988）。使用该公式计算的关闭温度（T_c）为〜740（±30）°C。根据Mueller模型进行的动力学数据分析考虑了用于定义Fe-Mg有序度的原子分数的esd，从而使我们能够检索以下元素的无序速率常数C_0K_（dis）〜+所有这三个温度都产生以下Arrhenius关系： n（C_0K_（dis）〜+）= In K_0-Q /（RT）= 20.99（±3.74）-26406（±4165）/ T（K）; （R〜2 = 0.988）。 Fe-Mg交换过程的活化能为52.5（±4）kcal / mol。上面的关系用于计算以下根据X_（Fe）修改的Arrhenius关系（在X_（Fe）= 0.20-0.50的范围内）：ln（C_0K_（dis）〜+）=（21.185-1）。 47X_（Fe））-（27267-4170X_（Fe）/ T（K）。冷却石灰常数η= 6 x 10〜（-1）K〜（-1）年〜（-1）（在PCA82506上计算） -3样品提供的冷却速度为1°C / min左右，与冲击开挖后尿素岩母体的极快的后期冷却历史相一致。

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