Crystal Chemistry of Thallium in Marine Ferromanganese Deposits br

Our understanding of the up to 7 orders of magnitude partitioningof thallium (Tl) between seawater and ferromanganese (FeMn) deposits restsupon two foundations: (1) being able to quantify the Tl(I)/Tl(III) ratio thatreflects the extent of the oxidative scavenging of Tl by vernadite (delta-MnO2), theprinciple manganate mineral in oxic and suboxic environments, and (2) being ableto determine the sorption sites and bonding environments of the Tl(I) and Tl(III)complexes on vernadite. We investigated these foundations by determining theoxidation state and chemical form of Tl in FeMn crusts and nodules from theglobal oceans at a Tl concentration ranging from several hundred ppm (mg/kg)down to the low ppm level. Seventeen hydrogenetic crusts and eleven nodulesfrom the Pacific, Atlantic, Arctic, and Indian Oceans and Baltic Sea werecharacterized by chemical analysis, X-ray diffraction, Raman spectroscopy, Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopy, Tl L3-edgehigh energy-resolution XANES (HR-XANES) spectroscopy, and extended X-ray absorptionfine 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, whereasthe percentage of Tl(III) to total Tl varies between 62 and 100% independent of both the Mn/Fe and Mn(III)/Mn(IV) ratios. Thedata, complemented by molecular modeling of the Tl(III) coordination and by XANES calculations, suggest that the enrichment ofTl in Fe-vernadite is driven by (1) the oxidative uptake of octahedrally coordinated Tl(III) above the vacant Mn(IV) sites and onthe layer edges of the vernadite layers, and (2) the sorption of Tl(I) on the crystallographic site of Ba at the surface of the vernaditelayers, which is an analogue to the surface site of K. Thus, Tl has a high affinity for vernadite regardless of its oxidation state, and thelack 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 Tlconcentration varies between 2 and 112 ppm in surface and buried nodules independent of the Mn/Fe ratio, and the percentage ofTl(III) varies between 0 and 100%. Nodules subjected to sediment diagenesis with replacement of layered vernadite by tunneledtodorokite are depleted in Tl and have more reduced thallium. Knowledge of the complex interplay of mineralogy, surface chemicalprocesses, and crystallographic siting is required to understand the variability of Tl concentrations, redox state, and acquisitionprocesses by marine FeMn deposits.