W. Crawford Elliott

Phosphorus K-edge XANES spectroscopy of mineral standards.

Phosphorus K-edge XANES spectra are presented for a diverse set of 44 phosphate minerals.

The kinetics of the smectite to illite transformation in Cretaceous bentonites, Cerro Negro, New Mexico

The thermal effects, as well as the survivability and origins of microorganisms in Cretaceous rocks, are evaluated from the timing and extent of the smectite to illite transformation in Cretaceous bentonites collected from cores outside the thermal aureole of the Pliocene Cerro Negro volcanic neck. Overall, randomly ordered mixed-layered illite-smectite (I-S) is the predominant clay mineral in these bentonites, and the K-Ar ages of I-S range from 36 to 48 Ma (21 analyses, two additional analyses were outside this range). Increased temperature from burial is thought to be the primary factor forming I-S in these bentonites. Kinetic model calculations of the smectite to illite transformation are also consistent with I-S formed by burial without any appreciable thermal effects due to the emplacement of Cerro Negro. In a core angled toward Cerro Negro, the percentages of illite layers in I-S from the bentonite closest to Cerro Negro are slightly higher (32-37%) than in most other bentonites in this study. The K-Ar ages of the closest I-S are slightly younger as a group (38-43 Ma; Average = 41 Ma; N = 4) than those of I-S further from Cerro Negro in the same core (41-48 Ma; Average = 44 Ma; N = 6). A small amount of illite in this I-S may have formed by heat from the emplacement of Cerro Negro, but most illite formed from burial. Vitrinite reflectance, however, appears to record the effects of heating from Cerro Negro better than I-S. Tentatively, the temperature of this heat pulse, based on vitrinite data alone, ranged from 100 to 125°C and this is most evident in the CNAR core. The upper temperature, 125°C, approximates the sterilization temperatures for most microorganisms, and these temperatures probably reduced a significant portion of the microbial population. Thermophiles may have survived the increased temperatures from the combined effects of burial and the intrusion of Cerro Negro.

EVALUATION OF KINETIC MODELS FOR THE SMECTITE TO ILLITE TRANSFORMATION

Three different models have been reported previously to describe the kinetics of the transformation of smectite to illite (Pytte 1982; Velde and Vasseur 1992; Huang et al. 1993). In order to evaluate the general utility of these models to calculate the timing and extent of this transformation, each model was applied to four different geologic settings (Denver Basin, Gulf Coast, the Salton Sea Geothermal System, and Paris Basin) in which the ages, geothermal gradients and potassium ion activities vary markedly. The model results are compared to the measured percentages of illite in illite/smectite (I/S) and the K/Ar ages of I/S (if available) to test the utility of a given model to a particular basin.Although individual models can be applied to study this transformation within a specific setting, none of these models was successful in simulating the transformation for all four basins. The Salton Sea was simulated best using the model by Huang et al. (1993), which incorporated an increased geothermal gradient during the last 20,000 years. These results indicate that a large fraction of illite formed due to this increased geothermal gradient, and underscores that temperature is a dominant kinetic factor in forming illite. The Denver Basin was simulated well by the models of Velde and Vasseur (1992) and Pytte (1982). The Gulf Coast was simulated very well by the model of Huang et al. (1993) using a term that terminates the transformation at 75% illite. For the Paris Basin, the results are mixed. The models can be refined by comparing the calculated and measured ages of illite such as the K/Ar ages of I/S to understand the thermal history of a particular basin. The calculated ages of illitization derived from these refined models can be used to indicate the time at which source rocks became thermally mature to form oil and gas.

Growth of thermophilic and hyperthermophilic Fe(III)-reducing microorganisms on a ferruginous smectite as the sole electron acceptor.

ABSTRACT Recent studies have suggested that the structural Fe(III) within phyllosilicate minerals, including smectite and illite, is an important electron acceptor for Fe(III)-reducing microorganisms in sedimentary environments at moderate temperatures. The reduction of structural Fe(III) by thermophiles, however, has not previously been described. A wide range of thermophilic and hyperthermophilic Archaea and Bacteria from marine and freshwater environments that are known to reduce poorly crystalline Fe(III) oxides were tested for their ability to reduce structural (octahedrally coordinated) Fe(III) in smectite (SWa-1) as the sole electron acceptor. Two out of the 10 organisms tested, Geoglobus ahangari and Geothermobacterium ferrireducens, were not able to conserve energy to support growth by reduction of Fe(III) in SWa-1 despite the fact that both organisms were originally isolated with solid-phase Fe(III) as the electron acceptor. The other organisms tested were able to grow on SWa-1 and reduced 6.3 to 15.1% of the Fe(III). This is 20 to 50% less than the reported amounts of Fe(III) reduced in the same smectite (SWa-1) by mesophilic Fe(III) reducers. Two organisms, Geothermobacter ehrlichii and archaeal strain 140, produced copious amounts of an exopolysaccharide material, which may have played an active role in the dissolution of the structural iron in SWa-1 smectite. The reduction of structural Fe(III) in SWa-1 by archaeal strain 140 was studied in detail. Microbial Fe(III) reduction was accompanied by an increase in interlayer and octahedral charges and some incorporation of potassium and magnesium into the smectite structure. However, these changes in the major element chemistry of SWa-1 smectite did not result in the formation of an illite-like structure, as reported for a mesophilic Fe(III) reducer. These results suggest that thermophilic Fe(III)-reducing organisms differ in their ability to reduce and solubilize structural Fe(III) in SWa-1 smectite and that SWa-1 is not easily transformed to illite by these organisms.

Coexisting altered glass and Fe–Ni oxides at the Cretaceous–Tertiary boundary, Stevns Klint (Denmark): direct evidence of meteorite impact

Abstract The Cretaceous–Tertiary (K–T) boundary at Stevns Klint, Denmark, is noteworthy for its large Ir anomaly that is taken as evidence of extraterrestrial components, but the origin of the smectite in this marl has been variously interpreted to have a detrital, meteorite impact, or volcanic origin. We have carried out scanning electron microscopy and transmission electron microscopy (TEM)/analytical electron microscopy observations of the impact and contiguous layers within the K–T marl at Stevns Klint. TEM images show abundant smectite, much of which occurs with layers curving around and grading into cores of nanometer-scale glass shards. The smectite composition is unusual in having both significant octahedral Al and Mg. The glass and smectite major element compositions are similar and unique relative to glasses of terrestrial and extraterrestrial origin with the exception for one kind of glass at the K–T boundary in Haiti. Abundant 10–20-nm diameter iron oxides having as much as 10% Ni and minor Zn are intergrown with smectite. We interpret these domains to be altered meteorite fragments, which formed when impact glass was transformed to smectite. The direct association of unique glass and meteorite fragments is unambiguous evidence for meteorite impact. These data may imply fall-out of globally distributed impact-derived particles over an extended time period. The relations imply that TEM observations may be a powerful tool in detecting other impact events in the geological record.

Long-Term Selective Retention of Natural Cs and Rb by Highly Weathered Coastal Plain Soils

Naturally occurring Cs and Rb are distinctly more abundant relative to K in the highly weathered upland soils of the Savannah River Site, South Carolina, than in average rock of Earths upper continental crust (UCC), by factors of 10 and 4, respectively. Naturally occurring Cs has been selectively retained during soil evolution, and Rb to a lesser extent, while K has been leached away. In acid extracts of the soils, the Cs/K ratio is about 50 times and the Rb/K ratio about 15 times the corresponding ratios for the UCC, indicating that relatively large amounts of natural Cs and Rb have been sequestered in soil microenvironments that are highly selective for these elements relative to K. Cation exchange favoring Cs and Rb ions, and subsequent fixation of the ions, at sites in interlayer wedge zones within hydroxy-interlayered vermiculite particles may account for the observations. The amounts of stable Cs retained and the inferred duration of the soil evolution, many thousands of years, provide new insights regarding long-term stewardship of radiocesium in waste repositories and contaminated environments. Study of natural Cs in soil adds a long-term perspective on Cs transport in soils not available from studies of radiocesium.

Molecular Models of Cesium and Rubidium Adsorption on Weathered Micaceous Minerals

Understanding the adsorption mechanisms of metal cations onto soils and sediments is of critical importance in the protection of the environment, especially for the case of radioactive materials including the fission product (137)Cs. Mechanism-based adsorption models for the long-term interaction of chemical and radionuclide species with clay minerals are needed to improve the accuracy of groundwater reaction and flow models, as well as related simulations for performance assessment of waste sites and repositories. Toward this goal, molecular simulation using geometry optimization and molecular dynamics methods have been used to investigate the adsorption behavior of Cs(+) and Rb(+) cations at frayed edge wedges (a proxy for frayed edge sites, FES) and in the interlayer region formed as a result of the transformation of muscovite to Al-hydroxy interlayered vermiculite (HIV) during weathering and pedogenesis. Frayed edge wedges, formed both on individual smectite and illite phases and on the mica-HIV intergrade, have previously been recognized as significant sinks for the strong adsorption of Cs(+) and Rb(+). Atomic density profiles, interlayer adsorption site maps, radial distribution functions, and adsorption enthalpies derived from the equilibrated structural models are used to evaluate the optimal adsorption configurations and thermodynamics for Cs- and Rb-endmembers, a 50:50 Cs-Rb composition for the aqueous interlayer of vermiculite, and for the interlayer wedge zone as mica is transformed to HIV (i.e., HIV-mica wedge). Adsorption enthalpies for both cations are significantly larger for the frayed edge wedges (as represented by the HIV-mica wedge model) compared to values for the vermiculite and mica interlayers. Cesium cation binds more strongly than Rb(+) in the vermiculite interlayer, while Rb(+) binds more strongly than Cs(+) in the HIV-mica wedge. In all cases, the derived adsorption enthalpies for both cations indicate a preference for the wedge environment where electrostatic interaction is enhanced due to the presence of layer charge and the increased size of interlayer at the wedge accommodating cations larger than K(+).

Freundlich and dual Langmuir isotherm models for predicting 137Cs binding on Savannah River Site soils.

Distribution of 137Cs and stable cesium between aqueous solution and near-surface soil samples from five locations at the Savannah River Site was measured in order to develop a predictive model for 137Cs uptake by the soils. Sorption of 137Cs in these soils appears to be mostly by hydroxy-interlayered vermiculite. Batch sorption studies with 4 d for equilibration were conducted at three cesium concentrations and at two backing electrolyte (NaNO3) concentrations. The soil-solution mixtures were pH-adjusted to evaluate the effects of pH on cesium sorption. Sorbed cesium was related to the equilibrium aqueous cesium concentrations by a Freundlich isotherm model. Model fits on logarithmic scales have a common slope of 0.60 ± 0.03 for acidic mixtures and 0.69 ± 0.04 for neutralized mixtures but have unique intercepts that are influenced by backing electrolyte concentration and pH. An ion-exchange model is proposed that pertains to all five soils and relates the Freundlich isotherms to the cation exchange capacity of soil and the aqueous concentrations of cesium, sodium, and a third ionic species that was hydrogen in the acidic mixtures and potassium in the neutralized mixtures. Model fits are consistent with Kd values in the entire range of 5–2,300 L kg−1 determined for the five soil types. As an alternate model, dual Langmuir isotherms were fitted to the data. The results suggest cesium sorption by (1) relatively few interlayer-wedge sites, highly selective for cesium, and (2) much more abundant but less selective sites on internal and external planar surfaces.

The chemical character of fluids forming diagenetic illite in the Southern Appalachian Basin

Abstract Diagenetic illite (mixed layer illite-smectite, I-S) from Ordovician and Devonian potassium bentonite (K-bentonite) and shales was studied in the proximal southern Appalachian Basin (Alabama through Virginia) to understand in more detail the timing of illite formation as well as attempt to describe the chemistry of the fluids that formed illite during the Alleghanian Orogeny through combined K/Rb and oxygen isotope analyses. The clay fraction of the K-bentonites is composed predominantly of illite-smectite (I-S) and chlorite. Most I-S exhibited Kalkberg order (IISI) with >85% illite layers. The K-Ar dates of I-S from Ordovician K-bentonites and Devonian K-bentonites mostly range from 260-310 Ma and indicate I-S formed during the Alleghanian Orogeny. Internally concordant dates and similar percentages of illite layers of I-S are measured from several samples collected within a thick (1 m) Ordovician K-bentonite at Fort Payne, Alabama. The oxygen isotopic values (δ18O) of I-S range from 18-23‰ SMOW. At lowest levels of thermal maturity and burial, as seen by conodont alteration index values (CAI) of 1.5-2.0, the range in δ18O values of water in equilibrium with I-S is 2.5-3.5‰. At higher thermal maturity (CAI = 2.5-4.5), the δ18O of water in equilibrium with I-S is from 4.5 to 12‰. The δ18O analyses are mostly consistent with I-S having formed in the presence of orogenic (i.e., saline) waters. The mean K/Rb of the Appalachian illites from this study is 325 (N = 20). This mean K/Rb is also consistent with the formation of diagenetic I-S in the presence of a saline fluids having higher K activities than meteoric waters. The combined data indicate these diagenetic illites in K-bentonites formed from saline/orogenic waters during the Alleghanian Orogeny along the Valley and Ridge Province in the southern Appalachian Basin. Although the data rule out formation of diagenetic illite in the presence of meteoric water or long-term reaction with connate formation waters, the combined data cannot distinguish the source and the mechanism (tectonic push or flush from tectonic highlands) for the movement of saline fluids in the Appalachian Basin.

Enrichment of Cesium and Rubidium in Weathered Micaceous Materials at the Savannah River Site, South Carolina

The enrichment of Cs and Rb relative to Ba, Sr, and K in three soils representing a range of soil maturities was determined to investigate the long-term sorption behavior of these elements in upland soils of the Savannah River Site (SRS). Elemental mass fractions normalized to upper continental crust (UCC) decreased in the order Cs > Rb > Ba > K > Sr in the soil fine fractions. Only the UCC-normalized amount of Cs was greater than unity. The UCC-normalized amounts in strong-acid extracts decreased as Cs > Rb > Ba > K ≈ Sr. In all three soil cores, the trends of the UCC-normalized amounts of acid-extractable metals were similar to trends of cation-exchange capacity (CEC) calculated from synchrotron-X-ray diffractometry measurements of soil mineralogy. Consequently, the relative enrichment of Cs and Rb is largely controlled by selective sorption to micaceous minerals, including hydroxy-interlayered vermiculite, that dominate the CEC. Where high clay content had caused retention of soil solution, amounts of acid extractable K, Sr, and Ba were enhanced. The retention of natural Cs by these three soils, which developed over many thousands of years, is a strong indicator that radiocesium will likewise be retained in SRS soils.