Cr(III) solubility in aqueous fluids at high pressures and temperatures
Cr(III) solubility in aqueous fluids at high pressures and temperatures
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DOI：
10.1016/j.gca.2013.10.054
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发表时间：
2014-02
影响因子：
5
通讯作者：
A. Watenphul;C. Schmidt;S. Jahn
中科院分区：
文献类型：
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作者：
A. Watenphul;C. Schmidt;S. Jahn
三价铬通常被认为相对不溶于水性流体和熔体。然而，自然界中的许多反例表明，在含铬铁矿岩石的变质或热液蚀变过程中，或伟晶岩熔体中，含水流体使Cr（III）活化。到目前为止，对这些流体中的铬浓度和形态知之甚少。在这项研究中，在1.6-4.2 m HCl水溶液中，在高达1 GPa的高压和400 - 700° C的温度范围内，使用同步加速器微X射线荧光光谱法（μ-XRF）原位测定了铁铬云母（Cr 2 O 3）的溶解度。溶解的Cr的测定浓度范围在约900- 18，000 ppm之间，在500° C和861 MPa下发现最高浓度。在所研究的温度范围内，Cr（III）在含水HCl流体中的溶解度会发生倒退，并随着压力的增加而增加。此外，通过拉曼光谱法在400和600° C、0.3-1.6 GPa下在与埃司科莱石平衡的12.3质量% HCl流体上探索这些流体中的Cr（III）络合。所有的光谱都在275和325 cm− 1处显示出两个明显的Cr-Cl伸缩带，这显示出一些精细结构，在一些光谱中，在380和500 cm− 1之间的区域有弱带。两个主要谱带的积分强度之和定性地揭示了与μ-XRF测定的流体中Cr（III）浓度相同的沿等容线随温度沿着变化、随恒定温度下的压力变化以及随平衡所需的时间变化。采用互补从头算分子动力学方法，在两种不同密度（0.8和0.97 g/cm 3）和温度（427和727° C）下对4 m HCl溶液进行了模拟，研究了不同CrCl x（H2 O）y 3-x和CrCl x（H2 O）y（OH）z 3-x-z配合物（3 <$x+ z <$4和0 <$y <$2）的振动性质.准简正模分析表明，CrCl 4（H2 O）0-2-的四面体对称和反对称Cr-Cl伸缩振动的特征频率都在实验观察到的两个最强的拉曼谱带范围内，而羟基氯络合物的Cr-O伸缩振动则发生在400 cm− 1以上的波数处。Cr（III）的溶解度和络合作用强烈依赖于Cl−和H+的活性。在高H+和Cl−活性下，结果与CrCl 3-4（H2 O）0-2-1-0络合物作为主要Cr（III）物种一致，其Cr配位数随着压力的增加而变得更加水合。在低Cl−活性下，即我们在高温低压条件下的研究中，数据表明混合的CrCl x（H2 O）y（OH）z 3-x-z络合物的Cl-Cr比小于3。用铁铬土+（H2O +29mass%Na2CO3）和铁铬土+（H2O +44mass%Na2Si3O7）进行的原位μ-XRF溶解度实验表明，溶解的Cr浓度等于或低于500 ppm的检测限。因此，酸性氯化物流体似乎是更有效的代理Cr（III）的动员和运输在地壳条件下比含水碱金属碳酸盐或硅酸盐溶液。
Trivalent chromium is generally considered relatively insoluble in aqueous fluids and melts. However, numerous counterexamples in nature indicate Cr (III) mobilization by aqueous fluids during metamorphism or hydrothermal alteration of chromite-bearing rocks, or by pegmatite melts. So far, very little is known about the chromium concentrations and speciation in such fluids. In this study, the solubility of eskolaite (Cr 2 O 3) in 1.6–4.2 m aqueous HCl solutions was determined in situ at elevated pressures up to 1 GPa and temperatures ranging between 400 and 700° C using synchrotron micro–X-ray fluorescence spectroscopy (μ-XRF). Determined concentrations of dissolved Cr ranged between about 900–18,000 ppm, with the highest concentrations found at 500° C and 861 MPa. The Cr (III) solubility in aqueous HCl fluids is retrograde in the studied temperature range and increases with pressure. In addition, Cr (III) complexation in these fluids was explored by Raman spectroscopy on a 12.3 mass% HCl fluid in equilibrium with eskolaite at 400 and 600° C, 0.3–1.6 GPa. All spectra show two prominent Cr–Cl stretching bands at about 275 and 325 cm− 1, which display some fine structure, and in some spectra weak bands in the region between 380 and 500 cm− 1. The sum of the integrated intensities of the two dominant bands reveals qualitatively the same changes with temperature along an isochore, with pressure at constant temperature, and with the time required for equilibration as the Cr (III) concentrations in the fluid determined by μ-XRF. Complementary ab initio molecular dynamics simulations of a 4 m HCl solution at two different densities (0.8 and 0.97 g/cm 3) and temperatures (427 and 727° C) were performed to investigate the vibrational properties of various CrCl x (H 2 O) y 3-x and CrCl x (H 2 O) y (OH) z 3-x-z complexes with 3⩽ x+ z⩽ 4 and 0⩽ y⩽ 2. Quasi-normal mode analysis reveals that both the tetrahedral symmetric and antisymmetric Cr–Cl stretching vibrations of CrCl 4 (H 2 O) 0-2-have characteristic frequencies in the range of the two strongest experimentally observed Raman bands, whereas Cr–O stretching vibrations of hydroxy-chloride complexes occur at wavenumbers above 400 cm− 1. Solubility and complexation of Cr (III) depend strongly on the activities of Cl− and H+. At high H+ and Cl− activity, the results are consistent with CrCl 3-4 (H 2 O) 0-2-1-0 complexes as major Cr (III) species, the Cr coordination number of which increases with pressure by becoming more aquated. At low Cl− activity, ie in our study at high-temperature low-pressure conditions, the data indicate mixed CrCl x (H 2 O) y (OH) z 3-x-z complexes with Cl–Cr ratios less than three. In situ μ-XRF solubility experiments conducted with eskolaite+(H 2 O+ 29 mass% Na 2 CO 3) and kosmochlor+(H 2 O+ 44 mass% Na 2 Si 3 O 7) resulted in dissolved Cr concentrations at or below the detection limit of 500 ppm. Thus, acidic chloridic fluids seem to be more efficient agents for Cr (III) mobilization and transport at crustal conditions than aqueous alkali carbonate or silicate solutions.