Barite (BaSO 4 ) is found throughout the ocean, yet seawater is undersaturated with respect to barite, and organisms that could account for the abundance of barite have not yet been identifi ed. The mechanism for barite formation in seawater is not fully understood. Here we show that marine bacteria have the ability to precipitate barite through a metabolically mediated biomineralization process. We precipitated barite in laboratory experiments in the presence of several strains of marine bacteria grown on yeast media enriched with barium (Ba); barite did not precipitate under identical conditions in killed-bacteria controls. The crystals develop from amorphous, phosphorous-rich spherical precursors with fi brous internal textures, common in bacterial mineral precipitation. Bacterial mediation of barite precipitation can explain the distribution of barite in the water column and the occurrence of barite crystals in organic-rich sinking aggregates where bacteria are concentrated. This fi nding has implications for the use of barite and Ba proxies in paleoceanographic research.

Although diverse microbial metabolisms are known to induce the precipitation of carbonate minerals, the mechanisms involved in the bacterial mediation, in particular nucleation, are still debated. The study of aragonite precipitation by Chromohalobacter marismortui during the early stages (3-7 days) of culture experiments, and its relation to bacterial metabolic pathways, shows that: (1) carbonate nucleation occurs after precipitation of an amorphous Ca phosphate precursor phase on bacterial cell surfaces and/or embedded in bacterial films; (2) precipitation of this precursor phase results from local high concentrations of PO(4)(3-) and Ca(2+) binding around bacterial cell envelopes; and (3) crystalline nanoparticles, a few hundred nanometres in diametre, form after dissolution of precursor phosphate globules, and later aggregate, allowing the accretion of aragonite bioliths.

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Spherulites composed of aragonite, magnesian calcite, and calcian-magnesian (-manganoan) kutnahorite-type carbonates were precipitated by two halophilic bacterial strains in porous solid as well as liquid media at high salinity. Although Mg and Ca are geochemically similar elements, Ca is preferentially incorporated into aragonite structures in liquid media whereas Mg remains in the solution and/or precipitates to form struvite crystals. In solid media, crystal growth features clearly correlate with reticular parameters and the Mg content of the Ca-Mg and Ca-Mg(Mn) carbonates. The increased salinity in these media leads to the incorporation of Mg (and Mn) into the carbonate structure under growth conditions farther and farther from equilibrium. Although calcite is the stable phase in the Earth surface environments, carbonates denser than pure calcite, like aragonite and Mg-rich calcite, are kinetically favored in the studied bacterial precipitates.

The thermal dehydration of epsomite (MgSO4*7H2O) crystals grown in the presence and absence of organic additives (phosphonates, carboxylic acids, and polyacrylic acid derivatives) was studied by means of thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray thermodiffraction (XRTD), and environmental scanning electron microscopy (ESEM). In situ XRTD analyses (in air, 30% relative humidity) show an -->epsomite hexahydrite (MgSO4*6H2O) transition at 25-38 degrees C, followed by formation of amorphous phase(s) at T > 43-48 degrees C, and MgSO4 crystallization at approximately 300 degrees C. Kinetic parameters (E(alpha) and A) were determined for the main dehydration step (25-160 degrees C), which corresponds to a MgSO4*7H2O-->MgSO4*H2O transition, by applying two isoconversional methods to nonisothermal TG data obtained at different heating rates (beta= 1, 3, and 5 K*min-1). In situ, hot-stage ESEM observations of the thermal dehydration of epsomite crystals are consistent with the nonisothermal kinetic study and, along with XRTD results, allow us to propose a dehydration mechanism which includes an early nucleation and growth event, followed by the advancement of the reaction interface (3D phase boundary reaction). Both E(alpha) and A values increase in the presence of the most effective crystallization inhibitors tested. H-bonding between additives and epsomite crystal surfaces is consistent with Fourier transform infrared spectroscopy (FTIR) and may account for this effect. The increase of E(alpha) values can be related to the excess energy required to break additive-water bonds in the reactant. These results are likely to further our understanding of the interaction mechanisms between salt hydrates and organic additives which act as growth inhibitors/modifiers.