Mossbauer spectroscopy of phyllosilicates: effects of fitting models on recoil-free fractions and redox ratios
Mossbauer spectroscopy of phyllosilicates: effects of fitting models on recoil-free fractions and redox ratios
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DOI：
10.1180/claymin.2008.043.1.02
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发表时间：
2008-03-01
期刊：
影响因子：
1.5
通讯作者：
Bishop, J. L.
中科院分区：
文献类型：
--
作者：
Dyar, M. D.;Schaefer, M. W.;Bishop, J. L.
粘土矿物是地球土壤中普遍存在的成分，偶尔在陨石中发现，也可能在有水的行星表面出现。然而，人们对基本的穆斯堡尔参数知之甚少（固有的同质异能素位移，61，特征穆斯堡尔温度，θ（M），和无反冲分数，f），这是粘土矿物的特征，对正确解释Fe 3 +/Sigma Fe比率以及矿物模式至关重要。具有良好特征的单一矿物样品在多个温度下的光谱可用于f的测定。因此，这里介绍了具有一系列层类型的五层硅酸盐的测量结果：绿脱石、铁蒙皂石、铁蒙脱石、铁云母和黑云母。使用三种不同的软件包拟合光谱：明尼苏达州科学、工程和教育公司的WMOSS;加拿大渥太华大学的Recoil;以及比利时根特大学使用的两个程序。使用了四种不同的方法来建模线形：（1）洛伦兹曲线;（2）伪Voigt（洛伦兹曲线和高斯曲线的卷积）;（3）四极分裂分布（QSD）;以及（4）不假设特定线形的技术（随后称为“模型无关”）。用De Grave和货车Alboom（1991）的方法确定δ（1）、θ（M）和f的值。结果表明，所有模型通常都需要多个双峰来表示Fe位占据，即使当所有同价态的Fe原子都在同一位置时，如双八面体蒙皂石、绿脱石、云母和蒙脱石的情况。对蒙脱石中Fe-M2（3+）的两种分布，得到了一致的中心位移（δ）和四极分裂（δ）值。在钙钛矿，一个单一的Fe 2+双峰清楚地解决，并给出了系统的值为8，A和面积，但两个Fe 3+双峰定义较少。在铁云母中，两个Fe 2+和两个Fe 3+双峰被建模，而三个Fe 2+和一个Fe 3+双峰被用于黑云母。三个不同的程序，使用洛伦兹线的形状给了非常相似的结果，三角洲，三角洲和面积。QSD线形的两种不同实现给出了相似但有时略有不同的结果，并且伪Voigt和模型无关拟合通常落在洛伦兹和QSD结果的范围之间。在所有模型和线形中，delta（1）的值对于Fe 3+类似于0.58 mm/s，对于Fe 2+类似于1.31 mm/s，这是预期的，因为Fe 3+具有额外的屏蔽3d电子。对于绿脱石和铁蒙皂石中的Fe 3+，θ（M）数据的值几乎相同（类似于450 K），在镁橄榄石和黑云母中的Fe 3+略有变化（θ（M）=分别类似于730 K和类似于615 K），并且对于Fe 2+相对不同（类似于350 K）。由于θ作为温度的函数的拟合值的非单调行为，无法确定θ（M）和f的一些值。Fe 3+的f（295）值为0.821-0.917，Fe 2+的f（295）值为0.662-0.743，与云母和其他硅酸盐中无反冲部分的先前研究一致。在较低温度下，作为不同线形状和计算机软件的函数的δ、δ、面积和f值的计算分散度显著降低。在每个计算参数的误差来源进行了讨论。
Clay minerals are ubiquitous constituents in soils on Earth, are occasionally found in meteorites, and may also occur on planetary surfaces in the presence of water. However, little is known about the fundamental Mossbauer parameters (the intrinsic isomer shift, 61, the characteristic Mossbauer temperature, theta(M), and the recoil-free fraction, f) that are characteristic of clay minerals and critical to the correct interpretation of the Fe3+/Sigma Fe ratios as well as the mineral modes. Spectra of well characterized single mineral samples at multiple temperatures may be used for the determinations of f. Hence, measurements of five-layer silicates with a range of layer types are presented here: nontronite, Fe-smectite, glauconite, annite and biotite. The spectra were fitted using three different software packages: WMOSS from Science, Engineering & Education Co. in Minnesota; Recoil, from the University of Ottawa in Canada; and two programs used at the University of Ghent in Belgium. Four different approaches to modelling line shapes were used: (1) Lorentzian; (2) pseudo-Voigt (convolution of Lorentzian and Gaussian curves); (3) quadrupole-splitting distributions (QSD); and (4) a technique that does not assume a particular line shape (subsequently referred to as 'model-independent'). Values of delta(1), theta(M) and f were determined using the method of De Grave & Van Alboom (1991).Results show that multiple doublets are routinely required by all models to represent Fe-site occupancy, even when all the Fe atoms of the same valence are in the same site, as is the case for dioctahedral smectite, nontronite, mica and glauconite. Consistent values of centre shift (delta) and quadrupole splitting (Delta) were obtained for the two distributions of Fe-M2(3+) in the smectites. In glauconite, a single Fe2+ doublet was clearly resolved and gave systematic values for 8, A and area, but the two Fe3+ doublets were less defined. In annite, two Fe2+ and two Fe3+ doublets were modelled, while three Fe2+ and one Fe3+ doublet were used for biotite. Three different programs that use Lorentzian line shapes gave very similar results for delta, Delta and area. The two different implementations of QSD line shapes gave similar but sometimes slightly different results, and the pseudo-Voigt and model-independent fits usually fell between the ranges for Lorentzian and QSD results.The value of delta(1) is similar to 0.58 mm/s for Fe3+ and similar to 1.31 mm/s for Fe2+ across all models and line shapes, which is expected because the Fe3+ has an additional shielding 3d electron. Values for theta(M) data are nearly identical for Fe3+ in nontronite and Fe-smectite (similar to 450 K), somewhat varied for Fe3+ in glauconite and biotite (theta(M) = similar to 730 K and similar to 615 K, respectively), and relatively distinct for Fe2+ (similar to 350 K). Some values for theta(M) and f could not be determined due to the non-monotonic behaviour of the fitted values for 6 as a function of temperature. Values of f(295) were 0.821-0.917 for Fe3+ and 0.662-0.743 for Fe2+, consistent with previous studies of the recoil-free fraction in micas and other silicates. Calculated scatter in delta, Delta, area and f values as a function of different line shapes and computer software was significantly reduced at lower temperatures. Sources of error in each of the calculated parameters are discussed.