Lacustrine non‐skeletal carbonates exhibit a diversity of petrographies due to interactions between physico‐chemical and biologically influenced mechanisms. Despite the suggestion that evaporative concentration was involved in the formation of spherulite and shrubby‐bearing carbonate successions in the Pre‐Salt Cretaceous alkaline lakes of the South Atlantic, no consensus exists about the water chemistries promoting these exotic mineral textures. In this work, an experimental approach was developed to evaluate how changes in salinity (NaCl) and biopolymer concentrations (alginic acid) impact calcite growth dynamics from saline and alkaline synthetic solutions. Hydrochemical and petrographical data from selected modern saline/alkaline environments were compared with experimental datasets to further estimate how the underlying (bio)chemical conditions and lake locations probably converge to allow the formation of calcite spherulite grains in evaporitic settings. Spherulitic calcite from Recent saline lakes and experiments arise from waters with moderate to high [Calcium]/[Alkalinity] ratios ([Ca]/[Alk]) rather than in calcium‐depleted and alkaline‐rich environments which tend to produce single‐crystal calcites during abiotic water mixing or lake evaporation. This observation is consistent with the assembly of polycrystalline textures being a kinetically controlled feature, forced by remarkably high rates of nucleation. Also, the data analysed do not support a causative relationship between evaporite‐driven salinity fluctuations and the preferential formation of spherulites, shrubs or their intermediate textures. Ubiquitous in saline lakes, organic substances can lower the kinetic thresholds for spherulitic calcite aggregation while microbial photosynthesis can also raise pH, altogether enhancing calcite supersaturation and promoting spherulite formation in waters with moderate‐high [Ca]/[Alk] ratios and high salinities. Localised observations of abiotic spherulites in Recent soda lakes can occur in restricted mixing zones where [Ca]/[Alk] ratios are enhanced. This work highlights the roles of concentration regimes associated with biopolymers and microbial metabolism against the background salinity fluctuations in determining the morphological and textural transitions in lacustrine carbonate minerals.

Nature produces hierarchical, functional materials by shaping amorphous mineral precursors under physiological conditions. Although biominerals inspire the architectures of synthetic counterparts, the biogenic phase transformations yielding precise crystalline forms, polymorphs and structures are unclear. Elucidating the transformation and structuration of amorphous minerals, herein we show distinct crystallization and structuration schemes synergistically controlled by environmental water contents and the Mg/Ca atomic ratio within amorphous mixed metal carbonates. Control of phase transformation, as well as resultant crystalline micro- and nano-structures, reflects the significance of the amorphous precursors of biominerals as disordered by design. Thereby, we complement the literature-known, suggested (bio)polymer-mediated ‘divide and protect’ mechanism of amorphous mineral stabilization by a Mg²⁺-based ‘unite and protect’ strategy. Altogether, this allows delineating a novel mechanism for mesocrystal formation based on the interface-coupled dissolution–re-precipitation of mesoscale amorphous precursors, which appear important in biomineralization. In the latter case, the recruitment of environmentally abundant Mg²⁺ species can also supplement the functions of biomolecules.

While it is generally known that aging protocols have an important impact on the interaction between crude oil (CRO), brines, and mineral surfaces, the microscopic consequences of the various steps of aging have hardly been described. In this study, we characterize the properties of fluids and carbonate mineral surfaces throughout a series of equilibration steps at 95°C and correlate these microscopic observations with macroscopic contact angle measurements. Chemical equilibration (eq) of CRO and FW leads to transfer of organic molecules from the former to the latter, causing also a pH change in the eqFW. Confocal Raman Microscopy, Atomic Force Microscopy and infrared spectroscopy are used to reveal how consecutive aging of calcite in eqFW and eqCRO induces first (in eqFW) considerable surface reconstruction and precipitation of mineral particles with co-localized organic species and second upon exposure to eqCRO produces a second adlayer primarily composed of polyaromatic hydrocarbon-rich particles. Our results show how these interconnected microscopic chemical and topographical surface modifications give rise to more ‘oil wetting’ contact angles after the two-step aging procedure.