Banin A, Ben-Shlomo T, Margulies L, Blake D F, Mancinelli R L, Gehring A U
Department of Soil and Water Sciences, The Hebrew University, Rehovot, Israel.
J Geophys Res. 1993 Nov 25;98(E11):20,831-53.
A series of surface-modified clays containing nanophase (np) iron oxide/oxyhydroxides of extremely small particle sizes, with total iron contents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive studies of the iron mineralogy in these "Mars-soil analogs" were conducted using chemical extractions, solubility analyses, pH and redox, x ray and electron diffractometry, electron microscopic imaging, specific surface area and particle size determinations, differential thermal analyses, magnetic properties characterization, spectral reflectance, and Viking biology simulation experiments. The clay matrix and the procedure used for synthesis produced nanophase iron oxides containing a certain proportion of divalent iron, which slowly converts to more stable, fully oxidized iron minerals. The clay acted as an effective matrix, both chemically and sterically, preventing the major part of the synthesized iron oxides from ripening, i.e., growing and developing larger crystals. The precipitated iron oxides appear as isodiametric or slightly elongated particles in the size range 1-10 nm, having large specific surface area. The noncrystalline nature of the iron compounds precipitated on the surface of the clay was verified by their complete extractability in oxalate. Lepidocrocite (gamma-FeOOH) was detected by selected area electron diffraction. It is formed from a double iron Fe(II)/Fe(III) hydroxy mineral such as "green rust," or ferrosic hydroxide. Magnetic measurements suggested that lepidocrocite converted to the more stable maghemite (gamma-Fe2O3) by mild heat treatment and then to nanophase hematite (alpha-Fe2O3) by extensive heat treatment. After mild heating, the iron-enriched clay became slightly magnetic, to the extent that it adheres to a hand-held magnet, as was observed with Mars soil. The chemical reactivity of the iron-enriched clays strongly resembles, and offers a plausible mechanism for, the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the iron oxide/oxyhydroxides and silicate phase surfaces. The reflectance spectrum of the clay-iron preparations in the visible range is generally similar to the reflectance curves of bright regions on Mars. This strengthens the evidence for the predominance of nanophase iron oxides/oxyhydroxides in Mars soil. The mode of formation of these nanophase iron oxides on Mars is still unknown. It is puzzling that despite the long period of time since aqueous weathering took place on Mars, they have not developed from their transitory stage to well-crystallized end-members. The possibility is suggested that these phases represent a continuously on-going, extremely slow weathering process.
通过将氯化亚铁溶液中的铁沉积在粘土表面，制备了一系列含有极细颗粒尺寸的纳米相（np）氧化铁/羟基氧化铁的表面改性粘土，其总铁含量与火星土壤中的含量一样高。利用化学萃取、溶解度分析、pH值和氧化还原分析、X射线和电子衍射、电子显微镜成像、比表面积和粒径测定、差热分析、磁性表征、光谱反射率以及海盗号生物学模拟实验，对这些“火星土壤类似物”中的铁矿物学进行了全面研究。粘土基质和合成过程产生了含有一定比例二价铁的纳米相氧化铁，二价铁会缓慢转化为更稳定的、完全氧化的铁矿物。粘土在化学和空间上都起到了有效的基质作用，防止了大部分合成氧化铁的熟化，即防止其生长并形成更大的晶体。沉淀出的氧化铁呈现为等径或略呈拉长状的颗粒，尺寸范围为1 - 10纳米，具有较大的比表面积。通过草酸对其完全可萃取性，证实了沉淀在粘土表面的铁化合物的非晶态性质。通过选区电子衍射检测到了纤铁矿（γ - FeOOH）。它由双铁Fe（II）/Fe（III）羟基矿物如“绿锈”或氢氧化亚铁形成。磁性测量表明，纤铁矿通过温和热处理转化为更稳定的磁赤铁矿（γ - Fe₂O₃），然后通过长时间热处理转化为纳米相赤铁矿（α - Fe₂O₃）。经过温和加热后，富含铁的粘土变得稍有磁性，以至于能附着在手持磁铁上，这与火星土壤的情况相同。富含铁的粘土的化学反应性与海盗号生物学实验中有些令人困惑的观察结果非常相似，并提供了一个合理的机制。它们独特的化学反应性归因于氧化铁/羟基氧化铁和硅酸盐相表面的联合催化作用。粘土 - 铁制剂在可见光范围内的反射光谱通常与火星上明亮区域的反射曲线相似。这加强了火星土壤中纳米相氧化铁/羟基氧化铁占主导地位的证据。这些纳米相氧化铁在火星上的形成方式仍然未知。令人困惑的是，尽管火星上发生水风化已有很长时间，但它们尚未从过渡阶段发展到结晶良好的最终形态。有人提出，这些相代表了一个持续进行的、极其缓慢的风化过程。
