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首页> 外文期刊>The American mineralogist >Hydrogen-bond and cation partitioning in muscovite: A single-crystal neutron-diffraction study at 295 and 20 K
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Hydrogen-bond and cation partitioning in muscovite: A single-crystal neutron-diffraction study at 295 and 20 K

机译：白云母中的氢键和阳离子分配：在295 K和20 K的单晶中子衍射研究

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摘要

The crystal chemistry of a pegmatitic Fe-bearing muscovite (with FeO ?5.1 wt%) from Val di Crana, Arvogno (Verbania, Nothern Italy) has been investigated by means of wavelength-dispersive X-ray spectroscopy and constant-wavelength [λ = 0.9462(2) A°] single-crystal neutron diffraction at 295 and 20 K (2?max = 90°). The structure refinement at 295 K shows no significant deviation from the full occupancy of the K site (using the scattering length of potassium alone), and a disordered Si/Al-distribution in the two independent tetrahedral sites (i.e., T1 and T2) and Fe/Al-distribution in the M2 octahedral site. The difference Fourier map of the nuclear density shows that only one independent H site occurs in the muscovite structure, at both room and low temperature. No evidence of disorder in two sub-sites, as observed in a previous study, was found. The thermal displacement of the H site is here described anisotropically. A trifurcated hydrogen bonding scheme was found: the O6-H bond distance corrected for "riding motion" is ?0.984 A° at 295 K, and three significantly weak hydrogen bonds to the O atoms O2, O4, and O6 occur, with H?O2 = 2.635(5), H?O4 = 2.657(4), and H?O5 = 2.647(4) A° and O6-H?O angles all similar in value (?138°). The structure refinements show that the structure configuration of muscovite is maintained at least down to 20 K: the cation disordering in the tetrahedral and octahedral site and the H-bonding scheme are preserved. The structural evolution induced by decreasing temperature is governed by the contraction along a direction perpendicular to (001), mainly due to the compression of the " inner" K-O bonds, which leads to an increase of the tetrahedral rotation angle (?) of the six-membered ring. A further effect in response to lowering the temperature is a significant reduction of the magnitude of the thermal displacement parameters.
机译：已经通过波长色散X射线光谱法和恒定波长研究了来自Arvogno（意大利北部的韦尔巴尼亚）的瓦尔迪克拉纳（Val di Crana）的含辉长岩的含铁白云母（FeO含量为5.1重量％）的晶体化学。 0.9295（2）A°]单晶中子衍射在295和20 K（2？max = 90°）下进行。在295 K处的结构细化显示，与K位的全部占据（仅使用钾的散射长度）没有显着偏差，并且在两个独立的四面体位（即T1和T2）中Si / Al分布无序。 Fe / Al分布在M2八面体位置。核密度的差异傅立叶图表明，在室温和低温下，白云母结构中仅出现一个独立的H位点。如先前的研究中所观察到的，没有发现两个亚位点疾病的证据。在此各向异性地描述了H位点的热位移。发现了三叉的氢键方案：在“ 295 K”下，经“骑行运动”校正的O6-H键距离为〜0.984 A°，并且与O原子O2，O4和O6发生了3个明显弱的氢键，其中H原子与氢原子键合。 O 2 ＝ 2.635（5），H 2 O 4 ＝ 2.657（4），H 2 O 5 ＝ 2.647（4）A°和O 6 -H 2 O角的值都相似（138°）。结构改进表明，白云母的结构构型至少保持低至20 K：保留了四面体和八面体位点的阳离子无序化和H键合方案。温度降低引起的结构演变受沿垂直于（001）的方向的收缩支配，主要是由于“内部” KO键的压缩，这导致六个键的四面体旋转角（θ）增大环。响应于降低温度的另一效果是显着减小了热位移参数的大小。

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