Involving Redox Processes Research Articles

Liquid Ammonia: More than an Innocent Solvent for Zintl Anions.

Liquid ammonia as the original solvent for Zintl anions has been replaced by easier to handle or more versatile solvents in most recent Zintl chemistry. However, methodological advances have made it possible to structurally investigate the anions in ammoniate crystals via crystallography or in the solutions themselves via nuclear magnetic resonance. While in some cases liquid ammonia acts as an innocent solvent, it also provides different possibilities of direct involvement in chemical reactions. In addition to simple dissolution without changes to the anions observed in the solid starting materials, protonation of the anion, incongruent dissolution involving redox processes, and further oxidation and reduction products have been observed. The use of the solvent liquid ammonia under ambient pressure is limited to low temperatures, which in turn allows the monitoring of kinetically stabilized species, some of which cannot be accessed at higher temperatures. In this work, the available literature reports are summarized or referenced, and compounds that have been characterized as new ammoniate crystals are presented and contextualized. Innocent dissolution is observed for clusters involved in K2.9Rb5.1[Si4][Si9]·15NH3, Cs4Sn9·12NH3, Cs4Pb9·5NH3, and [Rb@[18]crown-6]2[Rb@[2.2.2]crypt]Rb[Ge9]·4NH3. Formal protonation of [Ge4]4- results in the crystallization of [Na@[2.2.2]crypt]2[H2Ge4]·3NH3. Tt52- (Tt = Sn or Pb) and HSi93- cannot be accessed in a binary solid state material but can be crystallized in co-crystals of PPh3 in [Rb@[2.2.2]crypt]2[Sn5][PPh3]2·NH3, [Rb@[2.2.2]crypt]2[Pb5][PPh3]2·NH3, and [K@[2.2.2]crypt]3[HSi9][PPh3]·5NH3.

Halide Perovskite Nanocrystal Photocatalysts for CO2 Reduction: Successes and Challenges.

In current research, halide perovskite nanocrystals have emerged as one of the potential materials for light-harvesting and photovoltaic applications. However, because of phase sensitivity, their exploration as photocatalysts in polar mediums is limited. It has been recently reported that these nanocrystals are capable of driving solar-to-chemical production through CO2 reduction. Using bare nanocrystals and also coupling in different supports, several reports on CO2 reduction in low polar mediums were reported, and the mechanism of involved redox processes was also proposed. Considering the importance of this upcoming catalytic activity of perovskites, in this Perspective, details of the developments in the field established to date and supported by several established facts are reported. In addition, some unestablished stories or unsolved pathways surrounding the redox process and the importance of using a polar solvent which confused the understanding of the exclusive roles of perovskite nanocrystals in catalysis are also discussed. Further, the future prospects of these materials that face challenges in dispersing in polar solvents, a key process in redox catalysis for CO2 reduction, are also discussed.

On a heavy path - determining cold plasma-derived short-lived species chemistry using isotopic labelling.

Cold atmospheric plasmas (CAPs) are promising medical tools and are currently applied in dermatology and epithelial cancers. While understanding of the biomedical effects is already substantial, knowledge on the contribution of individual ROS and RNS and the mode of activation of biochemical pathways is insufficient. Especially the formation and transport of short-lived reactive species in liquids remain elusive, a situation shared with other approaches involving redox processes such as photodynamic therapy. Here, the contribution of plasma-generated reactive oxygen species (ROS) in plasma liquid chemistry was determined by labeling these via admixing heavy oxygen 18O2 to the feed gas or by using heavy water H218O as a solvent for the bait molecule. The inclusion of heavy or light oxygen atoms by the labeled ROS into the different cysteine products was determined by mass spectrometry. While products like cysteine sulfonic acid incorporated nearly exclusively gas phase-derived oxygen species (atomic oxygen and/or singlet oxygen), a significant contribution of liquid phase-derived species (OH radicals) was observed for cysteine-S-sulfonate. The role, origin, and reaction mechanisms of short-lived species, namely hydroxyl radicals, singlet oxygen, and atomic oxygen, are discussed. Interactions of these species both with the target cysteine molecule as well as the interphase and the liquid bulk are taken into consideration to shed light onto several reaction pathways resulting in observed isotopic oxygen incorporation. These studies give valuable insight into underlying plasma–liquid interaction processes and are a first step to understand these interaction processes between the gas and liquid phase on a molecular level.

Triangular and Prism‐Shaped Gold‐Zinc Oxide Plasmonic Nanostructures: In situ Reduction, Assembly, and Full‐Range Photocatalytic Performance

Gold‐based nanocatalysts have been traditionally selected for multiple homogeneous and heterogeneous reactions of interest involving redox processes. Likewise, greener routes involving more efficient reactors and the use of inexpensive and nature‐mimicking excitation sources have boosted the research on photocatalysts that can drive these chemical reactions upon excitation with multiple wavelength sources beyond the UV range. In the present work we report on a multi‐step synthesis approach that enables the in situ generation of triangular and prism‐shaped gold nanostructures with a localized surface plasmon resonance effect and their direct assembly onto a ZnO nanostructured semiconductor support. Different LED excitation sources in the whole UV/Vis‐NIR range have been systematically selected to activate these hybrid materials in the selective reduction of p‐nitrophenol (4‐NP), a well‐known contaminant by‐product. While ZnO becomes preferentially active in the UV window, the anisotropic shape of these gold plasmonic nanostructures helps to broaden the photocatalytic response of ZnO towards the visible and NIR range, being especially active under 460 nm blue light irradiation and expanding their potential application in multiple solar‐driven catalytic processes of interest for decontamination and upgrading of toxic chemicals.

Computational Study of Molecular Hydrogen Adsorption over Small (MO2) n Nanoclusters (M = Ti, Zr, Hf; n = 1 to 4).

Hydrogen adsorption on small group 4 metal oxide clusters for both the singlet and the first excited triplet states have been investigated by density functional theory and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The reaction starts with hydrogen physisorption on a metal center followed by formation of metal hydride/hydroxides due to splitting H2 into H- and H+. The hydrogen physisorption energies are predicted to be -1 to -8 kcal/mol for the singlet and -1 to -26 kcal/mol for the triplet, respectively. The formation of metal hydride/hydroxides does not involve redox processes. Chemisorption leading to formation of metal hydride/hydroxides is exothermic by -10 to -50 kcal/mol for the singlet, and exothermic by up to -60 kcal/mol for the triplet. The predicted energy barriers are less than 20 kcal/mol. Formation of metal dihydroxides from the metal hydride/hydroxides is generally endothermic for the monomer and dimer and is exothermic for the trimer and tetramer. Formation of the dihydroxide is a proton coupled electron transfer (PCET) process. The singlet energy barriers for the H-→ H+ transfer process are predicted to be 35-60 kcal/mol, in comparison to triplet energy barriers of less than 15 kcal/mol for the H• → H+ transfer process. For trimers and tetramers, there exist two different pathways: the first is a direct pathway with PCET to a terminal oxygen and the second is a two-step pathway with initial formation of a bridge OH group followed by a proton transfer to generate a terminal OH group. For the singlet, the two-step pathway is preferred for M = Ti and the direct pathway is more favorable for M = Zr and Hf. The two-step pathway is always preferred for the triplet as one-electron transfer is always more likely than two-electron transfer in the direct pathway.

Atomically Resolved Short-Range Order at the Nanoscale in the Ca-Mn-O System.

The elucidation of the reaction mechanisms involving redox processes in functional transition-metal oxides, which usually start in areas of very few nanometers in size, is yet a challenge to be satisfactorily achieved. Atomically resolved HAADF and EELS have provided both chemical and structural information at the nanoscale, which reveal the preservation of short-range cationic order in areas of 2-3 nm length as the driving force behind the reversibility of the Ca2Mn3O8-Ca2Mn3O5 redox process. Oxygen evolution is accommodated by cationic diffusion along the Ca and Mn layers of the cation-deficient Ca2Mn3O8 delafossite related structure, whereas Mn remains octahedrally coordinated.

Appropriate methods for evaluating the efficiency and capacitive behavior of different types of supercapacitors

Abstract The development of new brands of supercapacitors (SCs) has led to a variety of energy storage mechanisms and frequently to performance overestimation or erroneous presentation of the capacitive behavior through applying mathematical relations valid only for electrical double-layer capacitors (EDLCs). This paper addresses a realistic evaluation of capacitive performance and efficiency of SCs based on carbon electrodes. The presented examples of imprecise data processing include misleading information, such as seeming discharge capacitance increase during SC aging and coulombic efficiencies of 90% for SCs involving redox processes whilst energy efficiency is only 50%. Even in typical EDLCs, energy efficiency is 5–10% lower than coulombic efficiency.

Resistive switching and impedance spectroscopy in SiOx-based metal-oxide-metal trilayers down to helium temperatures

In this work, Au/SiOx/TiN layered structures, obtained by magnetron sputtering deposition of silicon oxide (SiOx), are studied by means of I–V characteristics and impedance spectroscopy, in order to understand the underlying mechanisms of the resistive switching (RS) in these structures. After electroforming, the observed RS is bipolar with a resistance ratio higher than 102 between the high resistance state (HRS) and the low resistance state (LRS). The samples were subject to temperature dependent studies and the RS was observed down to 9 K, close to the liquid helium temperature. The switching voltage from the HRS to the LRS is dependent on the temperature and on the voltage application time used during the voltage sweep. The impedance spectra in the LRS and in the HRS show distinct temperature dependences, evidencing big differences between the two states, induced by the RS. There is a weak dependence of the resistance in the LRS on the area of the Au top electrodes. The observed RS is hence of filamentary nature and should involve redox processes and breakage and recovery of Si–O bonds, processes inhibited by isolating the Au top electrode from the surrounding atmosphere.

Accelerated Escherichia coli inactivation in the dark on uniform copper flexible surfaces

The bacterial inactivation of Escherichia coli on Cu/CuO-polyester surfaces prepared by direct current magnetron sputtering was investigated in the dark and under actinic light (360 nm≤ λ ≤ 720 nm; 4.1 mW/cm(2)) as used commonly in hospital facilities. In the dark, complete bacterial inactivation (6log10 reduction) was observed within 150 min and under actinic light within 45 min. Sputtered samples led to nanoparticulate uniform Cu/CuO films ~70 nm thick. The deposition rate used was 2.2×10(15) atoms/cm(2) s as determined by profilometry. X-ray fluorescence was used to determine the sample Cu-content and transmission electron microscopy determined Cu-particles ~20 ± 5 nm in size. The film optical absorption was observed to increase with Cu-content of the sample by diffuse reflection spectroscopy. The bacterial inactivation involved redox processes between Cu/CuO-polyester and the bacteria as observed by x-ray photoelectron spectroscopy. During sample recycling, the amount of Cu-release was determined by inductively coupled plasma-mass spectroscopy. The values required for E. coli inactivation were below the cytotoxicity level threshold allowed for mammalian cells. The E. coli inactivation by Cu/CuO-polyester seems to involve an oligodynamic effect since bacterial inactivation was achieved at very low Cu-concentrations.

Nonvolatile Transport States in Ferrite Thin Films Induced by Field‐Effect Involving Redox Processes

The conductivity and magnetoresistance of a practically useful ferrite compound are found to be electrically programmable in a nonvolatile manner. Electric-double-layer gating using an ionic liquid changes the surface oxidation state of spinel zinc ferrite via reversible redox processes. This unique electric-field effect, distinct from conventional electrostatic carrier doping, provides a new route for developing oxide-based field-effect devices.

De Novo-Designed Metallopeptides with Type 2 Copper Centers: Modulation of Reduction Potentials and Nitrite Reductase Activities

Enzymatic reactions involving redox processes are highly sensitive to the local electrostatic environment. Despite considerable effort, the complex interactions among different influential factors in native proteins impede progress toward complete understanding of the structure-function relationship. Of particular interest is the type 2 copper center Cu(His)3, which may act as an electron transfer center in peptidylglycine α-hydroxylating monooxygenase (PHM) or a catalytic center in copper nitrite reductase (CuNiR). A de novo design strategy is used to probe the effect of modifying charged amino acid residues around, but not directly bound to, a Cu(His)3 center embedded in three-stranded coiled coils (TRI-H)3 [TRI-H = Ac-G WKALEEK LKALEEK LKALEEK HKALEEK G-NH2]. Specifically, the peptide TRI-EH (=TRI-HK22E) alters an important lysine to glutamate just above the copper binding center. With a series of TRI-EH peptides mutated below the metal center, we use a variety of spectroscopies (EPR, UV-vis, XAS) to show a direct impact on the protonation equilibria, copper binding affinities, reduction potentials, and nitrite reductase activities of these copper-peptide complexes. The potentials at a specific pH vary by 100 mV, and the nitrite reductase activities range over a factor of 4 in rates. We also observe that the affinities, potentials, and catalytic activities are strongly influenced by the pH conditions (pH 5.8-7.4). In general, Cu(II) affinities for the peptides are diminished at low pH values. The interplay among these factors can lead to a 200 mV shift in reduction potential across these peptides, which is determined by the pH-dependent affinities of copper in both oxidation states. This study illustrates the strength of de novo protein design in elucidating the influence of ionizable residues on a particular redox system, an important step toward understanding the factors that govern the properties of this metalloenzyme with a goal of eventually improving the catalytic activity.

Native Cu from the oceanic crust: Isotopic insights into native metal origin

Ocean drilling has revealed that, although a minor mineral phase, native Cu ubiquitously occurs in the oceanic crust. Cu isotope systematics for native Cu from a set of occurrences from volcanic basement and sediment cover of the oceanic crust drilled at several sites in the Pacific, Atlantic and Indian oceans constrains the sources of Cu and processes that produced Cu0. We propose that both hydrothermally-released Cu and seawater were the sources of Cu at these sites. Phase stability diagrams suggest that Cu0 precipitation is favored only under strictly anoxic, but not sulfidic conditions at circum-neutral pH even at low temperature. In the basaltic basement, dissolution of primary igneous and potentially hydrothermal Cu-sulfides leads to Cu0 precipitation along veins. The restricted Cu-isotope variations (δ65Cu=0.02–0.19‰) similar to host volcanic rocks suggest that Cu0 precipitation occurred under conditions where Cu+-species were dominant, precluding Cu redox fractionation. In contrast, the Cu-isotope variations observed in the Cu0 from sedimentary layers yield larger Cu-isotope fractionation (δ65Cu=0.41–0.95‰) suggesting that Cu0 precipitation involved redox processes during the diagenesis, with potentially seawater as the primary Cu source. We interpret that native Cu precipitation in the basaltic basement is a result of low temperature (20°–65°C) hydrothermal processes under anoxic, but not H2S-rich conditions. Consistent with positive δ65Cu signatures, the sediment cover receives major Cu contribution from hydrogenous (i.e., seawater) sources, although hydrothermal contribution from plume fallout cannot be entirely discarded. In this case, disseminated hydrogenous and/or hydrothermal Cu might be diagenetically remobilized and reprecipitated as Cu0 in reducing microenvironment.

Redox Processes in Neurodegenerative Disease Involving Reactive Oxygen Species

Much attention has been devoted to neurodegenerative diseases involving redox processes. This review comprises an update involving redox processes reported in the considerable literature in recent years. The mechanism involves reactive oxygen species and oxidative stress, usually in the brain. There are many examples including Parkinson’s, Huntington’s, Alzheimer’s, prions, Down’s syndrome, ataxia, multiple sclerosis, Creutzfeldt-Jacob disease, amyotrophic lateral sclerosis, schizophrenia, and Tardive Dyskinesia. Evidence indicates a protective role for antioxidants, which may have clinical implications. A multifaceted approach to mode of action appears reasonable.

A Mechanistic Hypothesis for the Cytochrome P450-Catalyzed Cis–Trans Isomerization of 4-Hydroxytamoxifen: An Unusual Redox Reaction

We provide a detailed description of the cis-trans isomerization of 4-hydroxytamoxifen/endoxifen catalyzed by several isoforms from the cytochrome P450 (CYP) superfamily, including CYP1B1, CYP2B6, and CYP2C19. We show that the reactions mainly involve redox processes catalyzed by CYP. DFT calculation results strongly suggest that the isomerization occurs via a cationic intermediate. The cationic cis-isomer is more than 3 kcal/mol more stable than the trans form, resulting in an easier conversion from trans-to-cis than cis-to-trans. The cis-trans isomerization is a rarely reported CYP reaction and is ascribed to the lack of a second abstractable proton on the ethenyl group of the triarylvinyl class of substrates. The cationic intermediates thus formed instead of the stable dehydrogenation products allow for isomerization to occur. As a comparison, the reactions for the tamoxifen derivatives are compared to those of other substrates, 4-hydroxyacetanilide and raloxifene, for which the stable dehydrogenation products are formed.

Comparison of Isoelectronic 8-HO-G and 8-NH2-G Derivatives in Redox Processes

8-Oxo-7,8-dihydroguanine (8-oxo-G) is the major lesion of oxidatively generated DNA damage. Despite two decades of intense study, several fundamental properties remain to be defined. Its isoelectronic 8-aminoguanine (8-NH(2)-G) has also received considerable attention from a biological point of view, although its chemistry involving redox processes remains to be discovered. We investigated the one-electron oxidation and one-electron reduction reactions of 8-oxo-G and 8-NH(2)-G derivatives. The reactions of hydrated electrons (e(aq)(-)) and azide radicals (N(3)(*)) with both derivatives were studied by pulse radiolysis techniques, and the transient absorption spectra were assigned to specific tautomers computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The protonated electron adducts of 8-NH(2)-G and 8-oxo-G showed a substantial difference in their absorption spectra, the unpaired electron being mainly delocalized in the imidazolyl ring and in the six-membered ring, respectively. On the other hand, the deprotonated forms of one-electron oxidation of 8-NH(2)-G and 8-oxo-G showed quite similar spectral characteristics. In a parallel study, the one-electron reduction of 8-azidoguanine (8-N(3)-G) afforded the same transient of one-electron oxidation of 8-NH(2)-G, which represents another example of generation of one-electron oxidized guanine derivatives under reducing conditions. Moreover, the fate of transient species was investigated by radiolytic methods coupled with product studies and allowed self- and cross-termination rate constants associated with these reactions to be estimated.

Authigenic Ti-Bearing Crystals in a Precambrian Clay from Buenos Aires Province, Argentina

AbstractThe Precambrian Villa Mónica Formation clay was analyzed using petrographic and scanning electron microscopy, X-ray diffraction and chemical analysis, with the aim of characterizing the Ti-bearing phases present and determining their possible origin and/or source. Two Ti-bearing minerals were found to be present, rutile and anatase. The crystals are needles between 5 and 15 µm in size, with no evidence of abrasion or corrosion. They are commonly found in association with the (001) faces of illite flakes or in the pores between the flakes. Their disposition is similar in the three quarries studied. No zonation or differential settling due to differences in specific gravity was observed. In addition, chemical analyses indicated a positive correlation between TiO2 and K2O suggesting that both oxides behave similarly after the deposition of the sediment. The TiO2 content in the bulk fraction ranges from 0.8 to 1.98 wt.%, values that are similar to those exhibited in other clay deposits from different ages and geneses.All observations are consistent with an in situ origin of these Ti-bearing phases during post-depositional processes that included recrystallization of existing minerals and crystallization of new phases. The original detrital minerals such as biotite, ilmenite as well as detrital illite, were the primary sources of the TiO2 and of the Fe oxides that coat the clay. Understanding the origin and the reaction mechanisms involved during the post-depositional alteration of the Villa Mónica Formation suggests that the Ti-bearing phases in different sediment types were formed by similar mechanisms involving redox processes at low to medium temperatures, even in Precambrian sediments where the presence of rutile could lead to an incorrect assumption of high temperatures involved.

Derivate des Hydrazins als Liganden in Vanadium(III)-, -(IV)- und -(V)-Komplexen. Darstellung und Charakterisierung von [V(dipic)(NH2NHCOPh)(H2O)2][VO(dipic)(NHNCOPh)]·2H2O , [VO(dipic) (NHNHCO2Me)], [VO(dipic)(NH2NHCO2Me)], [VO(dipic)(NHNHCSPh)] und [VO('ONO')(NHNHCOPh)]·2H2O / Substituted Hydrazines as Ligands in Vanadium(III), -(IV) and -(V) Complexes. Synthesis and Characterization of [V(dipic)(NH2NHCOPh)(H2O)2][VO(dipic)(NHNCOPh)]·2H2O , [VO(dipic) (NHNHCO2Me)], [VO(dipic)(NH2NHCO2Me)], [VO(dipic)(NHNHCSPh)] und [VO('ONO')(NHNHCOPh)]·2H2O

The reaction of [VO(dipic)(H2O)2]·2H2O with benzoylhydrazine, thiobenzoylhydrazine and O-methylcarbazate yields [V(dipic)(NH2NHCOPh)(H2O)2][VO(dipic)(NHNCOPh)·2H2O (1), [V0(dipic)(NHNHCO2Me)] (2), [VO(dipic)(NH2NHCO2Me)] (3) and [VO(dipic)(NHNHCSPh)] (4). 1 is probably formed in solution by redox disproportionation. The reactions giving 2,3 and 4 also involve redox processes with atmospheric oxygen as oxidant and the respective hydrazine as reductant. [VO('ONO')(NHNHCOPh)]·H2O (5) was formed using aqua-oxo-salicylaldehyde-L-alaninato-vanadium(IV) as precursor. The structures of 1,2,3 and 5 have been determined by single crystal X-ray crystallography. The cation of 1 has a seven-coordinated vanadium(III) center with the two H2O molecules at the apices of a pentagonal bipyramide, the anion contains the doubly deprotonated hydrazido( 2-) ligand NHNCOPh2- as N,O-chelate. 2 and 3 have very similar structures, but differ in the oxidation state of the vanadium atom and in the degree of deprotonation of the hydrazine. In 3 neutral NH2NHCOOMe and in 2 the single deprotonated hydrazido(l -) ligand NHNHCOOMe- are bound as N,O-chelates, with the O-donor in rrans-position to the VO group. The remaining coordination sites are occupied by the ONO-donating dinegative dipic ligand. 5, where dipic is replaced by the three-dentate salicylaldehyde-benzoyl-hydrazone, has a very similar structure to 2. 4 with a VO3N2S coordination sphere is the first vanadiun (V) complex containing the N,S-donating hydrazido(l-) ligand NHNHCSPh-. The following parameters are characteristic for the bonding of the NHNHR- and NHNR2- ligands in 1,2 and 5: V-N 189 ± 1 pm, N-N 134 ± 1 pm and V-N-N 122 ± 2°.

The interaction of group IB metals with van der waals faces of semiconducting metal dichalcogenides

AbstractThe interaction of UHV cleaved van der Waals surfaces of the layered semiconductors SnS2, SnSe2, ZrS2, MoS2 and WS2 with deposited Cu, Ag and Au has been studied with XPS and LEED. It was observed for Cu on SnS2 and for Cu and Ag on SnSe2 that a bulk reaction ocurs leading to the formation of a new phase containing reduced Sn and oxidized Cu or Ag. Even at high coverages a metallic overlayer was not formed. An interfacial reaction at low coverages and subsequently a metallic layer was obtained for Cu on ZrS2 and Ag on SnS2 and Au on SnSe2. Cu, Ag and Au on MoS2 or WS2 form a metallic overlayer with an atomically abrupt semiconductor/metal interface. Epitaxial growth of a Au(111) layer was observed on WS2(0001). The differences in reactivity are related to differences in electrochemical standard potentials of the involved redox processes, structural features and to kinetic factors, e.g. ion transport.

Application of Lacustrine-Humate Model to New Mexico Grants Mineral Belt and Relation Between Ore Types and Hydrologic History of San Juan Basin: ABSTRACT

In the Grants mineral belt, greenish-gray lacustrine claystones and mudstones of the Brushy Basin Member and K End_Page 571------------------------------ shales of the Morrison Formation may have served as rocks for the humate in uranium-bearing sandstones of the Westwater Canyon Member and Poison Canyon sandstone (of economic usage). This hypothesis provides a convenient, local source for the humic substances associated with tabular ore, as the K shales are interspersed with, and the Brushy Basin Member immediately overlies, the ore-bearing sandstones. The reduced nature of these claystones and mudstones suggests that organic matter incorporated during sedimentation would have been initially preserved in the pore waters of lake-bottom sediments. The diagenetic alteration of volcanic ash contained in these sediments would have raised the pH of the pore water, causing solution of humic substances. Subsequent compaction would ha e forced the alkaline, humic-rich pore fluids out of the claystones and mudstones, into the nearby sandstones. The two stages of ore in the Grants mineral belt--the primary tabular or trend ore and the secondary redistributed or stacked ore--can be related to the hydrology of a compacting basin. During early burial, formation water moves generally upward and laterally toward basin margins. Meanwhile, fresh meteoric water flows downdip from recharge areas that flank the basin. A similar early-burial hydrology is envisioned for the formation of tabular ore in the San Juan basin. In this case, humic substances, derived from pore waters expelled from the greenish-gray mudstones and claystones, moved into sandstone beds that served as escape conduits for formation water. When these alkaline formation waters encountered fresh meteoric water, the pH was lowered, facilitating organo-clay reactions tha resulted in precipitation of humate. The meteoric water also delivered uranium to the humate masses; the uranium probably had been leached by ground water moving through the sandstone beds. Sometime after compaction ceased, the basin achieved hydrologic unity, and water flowed from areas of recharge to areas of discharge deeper in the basin. Redistribution of ore into stacked or roll-type orebodies probably occurred during this time, destroying some preexisting tabular humate ores. Redox mechanisms were involved in this redistribution of uranium into rolls but not in the formation of the earlier tabular ores. Location of tabular ores within reduced ground and the association with humate indicates that uranium in these orebodies was fixed by organic materials, which does not necessarily involve redox processes. End_of_Article - Last_Page 572------------