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首页> 外文期刊>Contributions to Mineralogy and Petrology >HTP21/c–C2/c phase transition and kinetics of Fe2+–Mg order–disorder of an Fe-poor pigeonite: implications for the cooling history of ureilites
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HTP21/c–C2/c phase transition and kinetics of Fe2+–Mg order–disorder of an Fe-poor pigeonite: implications for the cooling history of ureilites

机译：贫铁褐铁矿的HTP2 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 (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 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 ln k D versus 1/T yielded the following equation: ln ktextD = - 3717( ±416)/T(K) + 1.290( ±0.378); (R2 = 0.988) ln ,k_{text{D}} = - 3717( pm 416)/T(K) + 1.290( pm 0.378);quad (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 0 K dis+ for all three temperatures yielding the following Arrhenius relation: ln( C0 Ktextdis + ) = ln K0 - Q/(RT) = 20.99( ±3.74) - 26406( ±4165)/T(K); (R2 = 0.988) ln left( {C_{0} K_{text{dis}}^{ + } } right) = ln ,K_{0} - Q/(RT) = 20.99( pm 3.74) - 26406( pm 4165)/T(K);quad (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( C0 Ktextdis + ) = (21.185 - 1.47XtextFe ) - frac(27267 - 4170XtextFe )T(K) ln left( {C_{0} K_{text{dis}}^{ + } } right) = (21.185 - 1.47X_{text{Fe}} ) - {frac{{(27267 - 4170X_{text{Fe}} )}}{T(K)}} . 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.
机译：从不含陨石的溶解的陨石样品PCA82506-3中获得了天然的贫Ca皂石（Wo 6 En 76 Fs 18 ）。通过原位高温单晶X射线衍射研究。将单斜P2 1 / c样品退火至1,093°C，以诱导从P2 1 / c到C2 / c对称的相变。晶格参数和b型反射强度（h + k = 2n + 1，仅在P2 1 / c相中存在）的强度随温度的升高而变化。在转变温度T Tr = 944°C时，P2 1 / c转换为C2 / c，其特征是一阶。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。 ln k D 与1 / T的线性回归得出以下方程式：ln k textD =-3717（±416）/ T（K）+ 1.290（±0.378） ; （R 2 = 0.988）ln，k_ {text {D}} =-3717（pm 416）/ T（K）+ 1.290（pm 0.378）;四（R ^ {2} = 0.988 ）。用该方程式计算的关闭温度（T c ）为〜740（±30）℃。根据穆勒模型对动力学数据进行了分析，并考虑了用于定义Fe–Mg有序度的原子分数的esd，这使我们能够检索无序速率常数C 0 K dis + 对于所有三个温度均产生以下阿伦尼乌斯关系：ln（C 0 K textdis < sup> + ）= ln K 0 -Q /（RT）= 20.99（±3.74）-26406（±4165）/ T（K）; （R 2 = 0.988）ln左（{C_ {0} K_ {text {dis}} ^ {+}}右）= ln，K_ {0}-Q /（RT）= 20.99 （pm 3.74）-26406（pm 4165）/ T（K）; quad（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 （在X Fe = 0.20–0.50范围内）而变的Arrhenius关系：ln（C 0 K textdis + ）=（21.185-1.47X textFe ）-frac（27267-4170X textFe < / sub>）T（K）ln左（{C_ {0} K_ {text {dis}} ^ {+}}右）=（21.185-1.47X_ {text {Fe}}）-{frac {{（（27267 -4170X_ {text {Fe}}）}} {T（K）}}。在PCA82506-3样品上计算得出的冷却时间常数η= 6×10 -1 K -1 年 -1 速率为1°C / min的速率与冲击开挖后尿素体母体的极快的后期冷却历史保持一致。

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