Organic–mineral interfacial chemistry drives heterogeneous nucleation of Sr-rich (Ba _x , Sr _{1− x} )SO ₄ from undersaturated solution

Abstract

Sr-bearing marine barite [(Ba x , Sr 1−x )SO 4 ] cycling has been widely used to reconstruct geochemical evolutions of paleoenvironments. However, an understanding of barite precipitation in the ocean, which is globally undersaturated with respect to barite, is missing. Moreover, the reason for the occurrence of higher Sr content in marine barites than expected for classical crystal growth processes remains unknown. Field data analyses suggested that organic molecules may regulate the formation and composition of marine barites; however, the specific organic-mineral interactions are unclear. Using in situ grazing incidence small-angle X-ray scattering (GISAXS), size and total volume evolutions of barite precipitates on organic films were characterized. The results show that barite forms on organic films from undersaturated solutions. Moreover, from a single supersaturated solution with respect to barite, Sr-rich barite nanoparticles formed on organics, while micrometer-size Sr-poor barites formed in bulk solutions. Ion adsorption experiments showed that organic films can enrich cation concentrations in the adjacent solution, thus increasing the local supersaturation and promoting barite nucleation on organic films, even when the bulk solution was undersaturated. The Sr enrichment in barites formed on organic films was found to be controlled by solid-solution nucleation rates; instead, the Sr-poor barite formation in bulk solution was found to be controlled by solid-solution growth rates. This study provides a mechanistic explanation for Sr-rich marine barite formation and offers insights for understanding and controlling the compositions of solid solutions by separately tuning their nucleation and growth rates via the unique chemistry of solution-organic interfaces.Sr-rich marine barite | organic-mineral interactions | solid solution | nucleation and growth | paleoenvironments S r-bearing barite [(Ba x , Sr 1−x )SO 4 ] is an important marine authigenic mineral present in the seawater column and marine sediments (1-5). Marine barite formation, essential to the global cycles of organic carbon, sulfate, and oxygen, can be used to reconstruct the changes in geochemical cycles and paleoproductivity, and Sr incorporation may reflect the oceanographic conditions (2, 5-7). Despite the geochemical importance of marine (Ba x , Sr 1−x )SO 4 formation, two paradoxes still exist. First, the global oceans are undersaturated with respect to barite, while barite is extensively present in seawater column and marine sediments (5,7,8). Second, Sr-rich marine barite (>50 mol% strontium) formation (7, 9) is not thermodynamically favorable (7, 10), as the solubility of SrSO 4 (K sp,SrSO4 = 10 −6.63 ) is three orders of magnitude higher than BaSO 4 (K sp,BaSO4 = 10 −9.98 ) (3, 10). Field and laboratory studies provide evidence for the association of organics with both paradoxes (7, 11); however, the specific roles of organics in these thermodynamically unfavorable mineralization processes are not clear (7,11,12).Organic-mineral interac...

Organic interfaces enhance strontium content of marine barite

“…Unfortunately, experimental difficulties (2) and contradictory theoretical results (14,15) have thus far preempted an accurate assessment of miscibility in the barite-celestite series. Consistent with expectations, Deng et al (6) observed almost no strontium substitution in their homogeneous crystallization experiments. However, the results of their heterogeneous crystallization experiments were quite unexpected: They found that the compositions of the barite grains that formed in association with the organic film were substantially altered, with 20 to 99% of their barium content replaced by strontium.…”

“…However, the results of their heterogeneous crystallization experiments were quite unexpected: They found that the compositions of the barite grains that formed in association with the organic film were substantially altered, with 20 to 99% of their barium content replaced by strontium. Through a particularly apt usage of a kinetic model of solidsolution nucleation previously developed by Pina and Putnis (16), Deng et al (6) show that the strontium enrichment they observed in the heterogeneous precipitates is consistent with a transition from a growth-dominated regime in the bulk fluid to a nucleation-dominated regime at the organic interface.…”

“…This local environment may also be necessary to explain the formation of strontium-enriched marine barites, which are expected to be less stable than more barium-rich compositions in seawater (2). In PNAS, Deng et al (6) advance the understanding of marine barite formation by investigating the competitive adsorption of strontium and barium on organic interfaces and the influence of surface-directed nucleation on the strontium content of synthetic barite.…”

“…(10), which indicates that strontium isotope ratios in marine barites are not at all influenced by the crystallization process that occurs in nature. Deng et al (6) found that strontium and barium preferentially adsorb to organic substrates (terminated by sulfhydryl and carboxyl moieties) at levels that locally exceed the solubility limit of (Ba x , Sr 1−x )SO 4 , even when the bulk solution is undersaturated with respect to both barite and celestite (SrSO 4 ) (Fig. 1).…”

Reduction of structural Fe(III) in nontronite by humic substances in the absence and presence of Shewanella putrefaciens and accompanying secondary mineralization

“…This observation is not surprising considering that our system is heterogeneous due to the presence of minerals and HS. In fact, multiple studies have shown mineral nucleation and growth in under-saturated bulk solutions if organic materials and solid interfaces serve as nucleation templates to locally enrich ions (Deng et al, 2019;Hövelmann et al, 2018;Roberts et al, 2004). For example, Deng et al (2019) showed barite formation on organic films, despite their bulk solution was under-saturated with respect to barite, likely because of local super-saturation near the organic molecules.…”