Structural behavior of C2/m tremolite to 40 GPa: A high-pressure single-crystal X-ray diffraction study
Structural behavior of C2/m tremolite to 40 GPa: A high-pressure single-crystal X-ray diffraction study
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DOI：
10.2138/am-2022-8278
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发表时间：
2022-06
影响因子：
3.1
通讯作者：
Jason N. Ott;B. Kalkan;M. Kunz;G. Berlanga;Ali F. Yuvali;Q. Williams
中科院分区：
文献类型：
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作者：
Jason N. Ott;B. Kalkan;M. Kunz;G. Berlanga;Ali F. Yuvali;Q. Williams
用同步辐射单晶X射线衍射法研究了钙闪石透闪石[Ca2Mg5Si8O22(OH)2]在300K~40 GPa时的高压结构和稳定性。C2/m对称性透闪石比以前研究的斜闪石具有更宽的亚稳范围，在40 Gpa以下没有一级相变。轴向参数比a/b和a/c，结合有限应变对归一化压力的趋势，表明在~5和~20 Gpa的压力下，压缩行为发生变化。利用三阶Birch-Murnaghan状态方程对有限应变趋势进行了分析，得到在0-5 GPA(区域I)、5-20 GPA(II)和20 GPA(III)以上的压缩区域，体积弹性模量(K0T)分别为72(7)、77(2)和61(1)GPA，相应的体积弹性模量(K‘0T)的压力导数为8.6(42)、6.0(3)和10.0(2)。计算结果与透闪石弹性的第一性原理理论计算相一致。透闪石的轴向压缩比为βa：βb：βc=2.22：1.0：0.78(区域I)、2.12：1.0：0.96(II)和1.03：1.0：0.75(III)，表明高压下透闪石的压缩各向异性显著降低，这与斜角闪石的高压各向异性形成了显著的对比。在5 Gpa(I-II)转变时压缩区域的转变归因于结构中空位A位关闭所对应的晶体a轴上的窒息，以及结构I梁的a取向表面的拓扑从凹面到凸面的移位。在20 GPA下II-III区的转变对应于钙多面体压实速率的增加和镁八面体位置的变形增加，这决定了在两个高压压缩区中的压实过程。压力下透闪石结构的键价分析表明，钙离子显著超键(30 GPa时为75%)，镁离子也显著超键(40%)。这意味着透闪石显着的亚稳范围取决于钙离子的成键环境。八配位的钙多面体适应了压力下的显著压实，而钙-氧多面体的几何形状变得越来越规则，并阻止了四面体链的重新定向，从而产生了在其他斜闪石中观察到的相变。在40 GPa以上采集的透闪石的二次分馏数据的峰/背景比，以及我们测定的透闪石的体积模数和压缩系数的状态方程，与我们先前的拉曼研究结果相一致，表明C2/m透闪石可能正在接近其在40 Gpa以上的亚稳极限。我们的结果既与撞击事件中透闪石的亚稳压实有关，也与地球内冷俯冲带内透闪石可能的亚稳持久性有关。
Abstract The high-pressure structure and stability of the calcic amphibole tremolite [Ca2Mg5Si8O22(OH)2] was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C2/m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a/b and a/c, in conjunction with finite strain vs. normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli (K0T) of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0–5 GPa (regime I), 5–20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli (K′0T) of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as βa:βb:βc = 2.22:1.0:0.78 (regime I), 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regime at 5 GPa (I–II) transition is ascribed to stifening along the crystallographic a-axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a-oriented surfaces of the structural I-beam from concave to convex. The II–III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite’s notable metastability range hinges on the calcium cation’s bonding environment. The eightfold-coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of difraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C2/m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth.