Crystal Chemistry of Thallium in Marine Ferromanganese Deposits

Our understanding of the up to 7
orders of magnitude partitioning
of thallium (Tl) between seawater and ferromanganese (FeMn) deposits
rests upon two foundations: (1) being able to quantify the Tl­(I)/Tl­(III)
ratio that reflects the extent of the oxidative scavenging of Tl by
vernadite (δ-MnO2), the principle manganate mineral
in oxic and suboxic environments, and (2) being able to determine
the sorption sites and bonding environments of the Tl­(I) and Tl­(III)
complexes on vernadite. We investigated these foundations by determining
the oxidation state and chemical form of Tl in FeMn crusts and nodules
from the global oceans at a Tl concentration ranging from several
hundred ppm (mg/kg) down to the low ppm level. Seventeen hydrogenetic
crusts and eleven nodules from the Pacific, Atlantic, Arctic, and
Indian Oceans and Baltic Sea were characterized by chemical analysis,
X-ray diffraction, Raman spectroscopy, Mn K-edge X-ray absorption
near-edge structure (XANES) spectroscopy, Tl L3-edge high
energy-resolution XANES (HR-XANES) spectroscopy, and extended X-ray
absorption fine structure (EXAFS) spectroscopy. The Tl concentration
increases linearly from 1.5 to 319 ppm with the Mn/Fe ratio in Fe-vernadite
from hydrogenetic crusts, whereas the percentage of Tl­(III) to total
Tl varies between 62 and 100% independent of both the Mn/Fe and Mn­(III)/Mn­(IV)
ratios. The data, complemented by molecular modeling of the Tl­(III)
coordination and by XANES calculations, suggest that the enrichment
of Tl in Fe-vernadite is driven by (1) the oxidative uptake of octahedrally
coordinated Tl­(III) above the vacant Mn­(IV) sites and on the layer
edges of the vernadite layers, and (2) the sorption of Tl­(I) on the
crystallographic site of Ba at the surface of the vernadite layers,
which is an analogue to the surface site of K. Thus, Tl has a high
affinity for vernadite regardless of its oxidation state, and the
lack of correlation between Tl­(III) and the Mn/Fe ratio in FeMn crusts
is explained by the affinity of Tl­(I) for the Ba site. The Tl concentration
varies between 2 and 112 ppm in surface and buried nodules independent
of the Mn/Fe ratio, and the percentage of Tl­(III) varies between 0
and 100%. Nodules subjected to sediment diagenesis with replacement
of layered vernadite by tunneled todorokite are depleted in Tl and
have more reduced thallium. Knowledge of the complex interplay of
mineralogy, surface chemical processes, and crystallographic siting
is required to understand the variability of Tl concentrations, redox
state, and acquisition processes by marine FeMn deposits.

我们对海水与铁锰（FeMn）沉积物之间铊（Tl）高达7个数量级的分配分异的认知，基于两大核心基础：(1) 可定量表征铊(I)/铊(III)比值，该比值可反映vernadite（δ-MnO2）——有氧与次有氧环境中的主要锰氧化物矿物——对铊的氧化性清除程度；(2) 可确定铊(I)与铊(III)配合物在vernadite表面的吸附位点与键合环境。本研究通过测定全球海洋铁锰结壳与结核中铊的氧化态与化学形态，对上述两大基础展开了探究，样品的铊浓度范围从数百ppm（mg/kg）至低ppm级别。我们共对来自太平洋、大西洋、北冰洋、印度洋及波罗的海的17件水成结壳与11件铁锰结核开展了系统表征，分析手段涵盖化学分析、X射线衍射、拉曼光谱、Mn K边X射线吸收近边结构（XANES）光谱、Tl L3边高能分辨X射线吸收近边结构（HR-XANES）光谱，以及扩展X射线吸收精细结构（EXAFS）光谱。水成结壳中铁vernadite（Fe-vernadite）的锰铁比与铊浓度呈线性正相关，铊浓度从1.5 ppm升至319 ppm；而铊(III)占总铊的百分比介于62%至100%之间，与锰铁比及Mn(III)/Mn(IV)比值均无显著关联。结合铊(III)配位的分子模拟与XANES计算得到的实验数据表明，铁vernadite（Fe-vernadite）对铊的富集受两大机制驱动：(1) 八配位的铊(III)在空位Mn(IV)位点上方及vernadite层的层边缘处发生氧化吸附；(2) 铊(I)吸附于vernadite层表面的钡类晶位，该位点与钾的表面位点具有结构相似性。由此可见，无论氧化态如何，铊对vernadite均具有高亲和性；而铁锰结壳中铊(III)占比与锰铁比无相关性的现象，可通过铊(I)对钡位点的亲和性得到合理解释。表层与埋藏铁锰结核的铊浓度介于2 ppm至112 ppm之间，与锰铁比无关；而铊(III)占比则介于0%至100%之间。经历沉积物成岩作用、以隧道状钙锰矿（todorokite）交代层状vernadite的铁锰结核，其铊含量显著亏损且铊的还原态占比更高。若要理解海洋铁锰沉积物中铊浓度、氧化还原状态与获取过程的变异性，需厘清矿物学、表面化学过程与晶体学位点之间的复杂相互作用。