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Superhydrous hematite and goethite: A potential water reservoir in the red dust of Mars?
Geology ( IF 5.8 ) Pub Date : 2021-11-01 , DOI: 10.1130/g48929.1
Si Athena Chen 1 , Peter J. Heaney 1 , Jeffrey E. Post 2 , Timothy B. Fischer 3 , Peter J. Eng 4, 5 , Joanne E. Stubbs 4
Affiliation  

Water can be stored in nominally anhydrous minerals as substitutional hydroxyl, generating vast but commonly unrecognized H2O reservoirs in ostensibly dry regimes. Researchers have long known that hematite (α-Fe2O3) can accommodate small concentrations of hydroxyl through the substitution of Fe3+ by 3H+. Our study of natural hematite has demonstrated the occurrence of “hydrohematite” phases that are 10–20 mol% deficient in Fe and accordingly contain 3.6–7.8 mol% structural water. Intergrown with natural hydrohematite samples were superhydrous goethite-like phases exhibiting an Fe deficiency of 10–20 mol% relative to end-member goethite (α-FeOOH). We synthesized hydrohematite in alkaline solutions (pH 9–12) at low temperatures (T < 200 °C) using fresh ferrihydrite as the transient precursor, and we observed a nonclassical crystallization pathway involving vacancy inoculation by Fe as nanocrystals evolved. The high level of incorporation of H2O in iron (hydr)oxides dramatically alters their behaviors as catalysts and pigments, and the presence of hydrohematite in rocks may rule out high-T diagenesis. We propose that hydrohematite is common in low-T occurrences of Fe oxide on Earth, and by extension it may inventory large quantities of water in apparently arid planetary environments, such as the surface of Mars.

中文翻译:

超水赤铁矿和针铁矿:火星红尘中的潜在水库?

水可以作为替代羟基储存在名义上的无水矿物中,在表面上干燥的情况下产生大量但通常未被识别的 H2O 水库。研究人员早就知道赤铁矿 (α-Fe2O3) 可以通过 3H+ 取代 Fe3+ 来容纳小浓度的羟基。我们对天然赤铁矿的研究表明,存在 10-20 mol% 的 Fe 缺乏的“水赤铁矿”相,因此含有 3.6-7.8 mol% 的结构水。与天然水赤铁矿样品共生的是超水合针铁矿样相,相对于端元针铁矿 (α-FeO​​OH),铁缺乏 10-20 mol%。我们在低温 (T < 200 °C) 的碱性溶液 (pH 9-12) 中合成了水赤铁矿,使用新鲜的水铁矿作为瞬态前体,并且我们观察到了一种非经典的结晶途径,包括随着纳米晶体的发展,Fe 的空位接种。铁(氢)氧化物中高水平的 H2O 结合显着改变了它们作为催化剂和颜料的行为,并且岩石中水赤铁矿的存在可能排除高 T 成岩作用。我们提出,水赤铁矿在地球上低 T 发生的 Fe 氧化物中很常见,并且通过扩展,它可能在明显干旱的行星环境中储存大量的水,例如火星表面。
更新日期:2021-11-03
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