Fe2+-Mg order-disorder in orthopyroxene: equilibrium fractionation between the octahedral sites and thermodynamic analysis
Fe2+-Mg order-disorder in orthopyroxene: equilibrium fractionation between the octahedral sites and thermodynamic analysis
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斜方辉石中的 Fe2 -Mg 有序无序：八面体位点之间的平衡分馏和热力学分析
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
G. Molin
中科院分区：
文献类型：
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作者：
M. Stimpfl;J. Ganguly;G. Molin
通过5 5 0~10 0 0 °C不同温度下的退火实验和X射线单晶结构修正，测定了三种天然的近二元Fe~(2+)-Mg斜方辉石晶体(FS14、FS15和FS49)中Fe~(2+)、Mg在M1和M2位之间的晶内平衡分配系数Kd~*。此外，还给出了Molin等 等人早些时候收集的斜方辉石晶体(Fs23)的X射线数据。(1991年)在700至1000 °C之间，被重新提炼。数据通过两个不同的改进程序(SHELXL-93和RFINE90)处理，既使用单位和个人重量，也使用离子和原子散射因子。计算的场地占有率被发现在其估计的标准误差内一致。然而，与使用原子散射因子相比，使用离子散射因子可以得到更好的拟合优度和一致指数，并且位置占有率的标准偏差也更小。此外，即使在两个过程中使用相同的反射集合，加权精化产生的场地占有率的标准偏差也比未加权精化小得多。这项研究的场地占有率数据与选定的已发表数据相结合，得出了Kd*作为温度和组成的函数的表达式。超额构型熵ΔSXS的计算表明，斜方辉石应视为双参数对称溶液，而不是“简单混合物”。由已有的阳离子交换数据计算的ΔSXS值和过量的吉布斯混合自由能导致斜方辉石固溶体中的混合热略为负。
Abstract The equilibrium intracrystalline distribution coefficient, kD*, of Fe* (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of kD* as a function of temperature and composition. Calculation of the excess configurational entropy, ΔSXS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a “simple mixture”. The calculated ΔSXS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution.