Abstract--The formation and structure of hematite aggregates were examined by dynamic and static light scattering techniques. A large range in coagulation kinetics was studied by varying either indifferent electrolyte (KC1) concentration or surface complexing anion (H2PO4) concentration, PT, at pH 6.0 ~ 0.1. Diffusion limited aggregation (DLA) was induced by counterion screening at [KC1] > 80 mM or by surface charge neutralization at Pr = 31 IxM (and ionic strength = 1.0 raM). In DLA, the fractal dimension, de, of aggregates formed by either surface charge neutralization or counterion screening was 1.7 2 0.1. A reduction in the rate of coagulation in KC1 for [KC1] < critical coagulation concentration (CCC) produced an increase in df to 2.1 _+ 0.1. For aggregation induced by phosphate adsorption at constant ionic strength, there was no apparent trend in df with coagulation rate. The value of df was consistently less than 1.8 when reaction limited aggregation (RLA) resulted from surface charge neutralization rather than counterion screening. TEM observations of aggregates formed in the presence or absence of phosphate confirm that, when RLA is induced by phosphate adsorption, resulting aggregates are much looser in structure than those formed by counterion screening. The results suggest that the high-affinity binding of phosphate to hematite may result in a nonrandom distribution of surface charge that facilitates the coalescence of positive and negative charge crystal faces.

We used a model rock-biota system to demonstrate that ecosystem composition (microbial and plant) regulates the fate of REE during early biota-rock interactions.Graphical abstract: word cloud with article keywords, Buffalograss, and X-ray diffractogram. Main Manuscript 2The rare earth elements (REE) are of increasing importance in a variety of science and economic fields, including (bio)geosciences, paleoecology, astrobiology, and mining. Despite their great promise, REE fractionation in early plant-microbe-rock systems has largely remained elusive. We tested the hypothesis that REE mass-partitioning during the incipient weathering of basalt, rhyolite, granite and schist depends on the activity of microbes, plant, and arbuscular mycorrhiza.Pore-water element abundances reflected a rapid transition from abiotic to biotic weathering, the latter associated with lower aqueous loss and higher uptake. Abiotic dissolution contributed 38.6±19% to total denudation. Microbes incremented denudation, particularly in rhyolite, this effect associating with decreased bioavailable solid fractions in this rock. Total mobilization (aqueous+uptake) was ten times greater in planted treatments compared to abiotic control, REE masses in plant generally exceeding those in water. Plants of larger biomass further increased solid fractions, consistent with soil genesis. Mycorrhiza had a generally positive effect on total mobilization. The incipient REE weathering was dominated by inorganic dissolution enhanced by biotic respiration, the patterns of denudation largely dictated by mineralogy. A consistent biotic signature was observed in La:phosphate, mobilization:solid fraction in all rocks, as well as in the general pattern of denudation and uptake.The transformation of rock to soil supports Earth's terrestrial life. Soil genesis and evolution have been under scientific scrutiny for over a century. With recent advances in analytical chemistry, molecular/evolutionary biology, hydrology, ecology and remote sensing we are only now beginning to understand how different components of geosphere, hydrosphere, atmosphere, and biosphere work together at different scales to shape the surface of Earth and transform parent rock into soil that sustains the ecosystem 1,22,3,1,4,5 . Among the most reactive phases in the transformation of crustal rock are the earliest stages of mineral weathering when first microbial and plant communities interact with bedrock and spark the flow of energy and nutrients that feed into major biogeochemical cycles. The igneous rock-Earth's nutrient store, exhibits its largest thermodynamic disequilibrium at surface pressure and temperatures, where the oxic aqueous environment makes it highly susceptible to weathering. Rapid mineral transformations of crustal rock often accompany the earliest stages of weathering, and they are intensified under mineral colonization by microbiota and plant roots. Studying these early interactions in the natural soil-forming system has classically been focused on nutrients that Main Manuscript...