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At least 145 records · Page 8Linked to original sources
USGS1974

An experimental study of the partitioning of a rare earth element (Gd) in the system diopside—Aqueous vapour

The partitioning of Gd in the experimental system diopside-aqueous vapor as a function of temperature, pressure, composition of the phases, time, grain size, solid-liquid ratio and Gd concentration has been investigated. A radioactive tracer measurement was used to determine Gd concentration in the separated phases. Diposides were reacted with aqueous vapor containing tracer Gd and reversibility was tested by reacting Gd-doped diopsides with pure aqueous vapor. Equilibration of Gd between the bulk of the diopside and the liquid was found to be limited by the slow rate of Gd diffusion in diopside, maximum value of D = 2 × 10 −15 cm 2 sec −1 at 800°C and 1 kb. Depending on whether the diopside was previously synthesized or synthesized from an oxide mix during the experiment, Gd concentrations were zoned in the crystal such that higher concentrations existed at the edges or center, respectively. Equilibrium is difficult to achieve in these experiments, but at the optimum experimental conditions for equilibration, the Gd diopside-aqueous vapor distribution coefficient is 20 ± 6 (800°C, 1 kb) in approximate agreement with previous results of 55 ± 23. Changing the composition of the aqueous vapor indicated that possible mechanisms for Gd substitution included coupling of Gd 3+ with H + or Na + replacing 2Ca 2+ , or substitution of 2Gd 3+ for 3Ca 2+ with formation of a cation vacancy.

Geochimica et Cosmochimica Acta↗
USGS1974

40Ar/39Ar age spectra of some undisturbed terrestrial samples

40 Ar/ 39 Ar age spectra and 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochrons were determined by incremental heating for 11 terrestrial rocks and minerals whose geology indicates that they represent essentially undisturbed systems. The samples include muscovite, biotite, hornblende, sanidine, plagioclase, dacite, diabase and basalt and range in age from 40 to 1700 m.y. For each sample, the 40 Ar/ 39 Ar ratios, corrected for atmospheric and neutron-generated argon isotopes, are the same for most of the gas fractions released and the age spectra, which show pronounced plateaus, thus are consistent with models previously proposed for undisturbed samples. Plateau ages and isochron ages calculated using plateau age fractions are concordant and appear to be meaningful estimates of the crystallization and cooling ages of these samples. Seemingly anomalous age spectrum points can be attributed entirely to small amounts of previously unrecognized argon loss and to gas fractions that contain too small (less than 2 per cent) a proportion of the 39 Ar released to be geologically significant. The use of a quantitative abscissa for age spectrum diagrams is recommended so that the size of each gas fraction is readily apparent. Increments containing less than about 4–5 per cent of the total 39 Ar released should be interpreted cautiously. Both the age spectrum and isochron methods of data reduction for incremental heating experiments are worthwhile, as each gives slightly different but complementary information about the sample from the same basic data. Use of a least-squares fit that allows for correlated errors is recommended for 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochrons. The results indicate that the 40 Ar/ 39 Ar incremental heating technique can be used to distinguish disturbed from undisturbed rock and mineral systems and will be a valuable geochronological tool in geologically complex terranes.

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USGS1974

Reference samples for the earth sciences

A revised list of reference samples of interest to geoscientists has been extended to include samples for the agronomist, the archaeologist and the environmentalist. In addition to the source from which standard samples may be obtained, references or pertinent notes for some samples are included. The number of rock reference samples is now almost adequate, and the variety of ore samples will soon be sufficient. There are very few samples for microprobe work. Oil shales will become more important because of the outlook for world petroleum resources. The dryland equivalent of a submarine basalt might be useful in studies of sea-floor spreading and of the geochemistry of basalts. The Na- and K-feldspars of BCS (British Chemical Standards—Bureau of Analysed Samples), NBS (National Bureau of Standards), and ANRT (Association Kationale de la Recherche Technique) could serve as trace-element standards if such data were available. Similarly, the present NBS flint and plastic clays, as well as their predecessors, might be useful for archaeological pottery studies. The International Decade for Ocean Exploration may stimulate the preparation of ocean-water standards for trace elements or pollutants and a standard for manganese nodules.

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USGS1975

Trace element evaluation of a suite of rocks from Reunion Island, Indian Ocean

Reunion Island consists of an olivine-basalt shield capped by a series of flows and intrusives ranging from hawaiite through trachyte. Eleven rocks representing the total compositional sequence have been analyzed for U, Th and REE. Eight of the rocks (group 1) have positive-slope, parallel, chondrite-normalized REE fractionation patterns. Using a computer model, the major element compositions of group 1 whole rocks and observed phenocrysts were used to predict the crystallization histories of increasingly residual liquids, and allowed semi-quantitative verification of origin by fractional crystallization of the olivine-basalt parent magma. Results were combined with mineral-liquid distribution coefficient data to predict trace element abundances, and existing data on Cr, Ni, Sr and Ba were also successfully incorporated in the model. The remaining three rocks (group 2) have nonuniform positive-slope REE fractionation patterns not parallel to group 1 patterns. Rare earth fractionation in a syenite is explained by partial melting of a source rich in clinopyroxene and/or hornblende. The other two rocks of group 2 are explained as hybrids resulting from mixing of syenite and magmas of group 1.

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USGS1975

A high 87Sr 86Sr mantle source for low alkali tholeiite, northern Great Basin

Olivine tholeiites, the youngest Tertiary units (about 8–11 m.y. old) at five widely spaced localities in northeastern Nevada, are geologically related to the basalts of the Snake River Plain, Idaho, to the north and are similar in major element and alkali chemistry to mid-ocean ridge basalts (MORB) and island arc tholeiites. The measured K (1250–3350 ppm), Rb (1·9–6·2 ppm) and Sr (140–240 ppm) concentrations overlap the range reported for MORB. Three of the five samples have low, unfractionated rare earth element (REE) patterns, the other two show moderate light-REE enrichment. Barium concentration is high and variable (100–780 ppm) and does not correlate with the other LIL elements. The rocks have 87 Sr / 86 Sr = 0·7052–0·7076, considerably higher than MORB (~0·702–0·703). These samples are chemically distinct (i.e. less alkalic) from the olivine tholeiites from the adjacent Snake River Plain, but their Sr isotopic compositions are similar. They contain Sr that is distinctly more radiogenic than the basalts from the adjacent Great Basin. About 10 b.y. would be required for the mean measured Rb/Sr (~ 0·02) of these samples to generate, in a closed system, the radiogenic Sr they contain. The low alkali content of these basalts makes crustal contamination an unlikely mechanism. If the magma is uncontaminated, the time-averaged Rb/Sr of the source material must have been ~0·04. A significant decrease in Rb/Sr of the source material (a factor 2̆) thus most probably occurred in the relatively recent (1̌0 9 yr) past. Such a decrease of Rb/Sr in the mantle could accompany alkali depletion produced by an episode of partial melting and magma extraction. In contrast, low 87 Sr 86 Sr "> 87Sr86Sr ratios indicate that the source material of the mid-ocean ridge basalts may have been depleted early in the Earth's history.

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USGS1975

Short chain aliphatic acid anions in oil field waters and their contribution to the measured alkalinity

High alkalinity values found in some formation waters from Kettleman North Dome oil field are due chiefly to acetate and propionate ions, with some contribution from higher molecular weight organic acid ions. Some of these waters contain no detectable bicarbonate alkalinity. For waters such as these, high supersaturation with respect to calcite will be incorrectly indicated by thermodynamic calculations based upon carbonate concentrations inferred from traditional alkalinity measurements

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USGS1975

Interstitial solutions and diagenesis in deeply buried marine sediments: Results from the Deep Sea Drilling Project

Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments. Pelagic clays and slowly deposited (<1 cm/10 3 yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters. In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/10 3 yr the changes chiefly involve gains of Ca 2+ and Sr 2+ and losses of Mg 2+ which balance the Ca 2+ enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr 2+ may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg 2+ for Ca 2+ during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca 2+ and Sr 2+ enrichment is again characteristic, but Mg 2+ losses exceed Ca 2+ gains with the excess being balanced by SO 4 post staggered2- losses. The data indicate that the reactions are similar to those noted above, except that the Ca 2+ released is not kept in solution but is precipitated by the HCO 3 post staggered- produced in SO 4 post staggered2- reduction. In both these types of pore waters Na + is usually conservative, but K + depletions are frequent. In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na + , Si and CO 2 , and slight enhancement in Cl - . The changes are attributed to exchange of Na + for Ca 2+ in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity. Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO 4 post staggered2- , strong HCO 3 post staggered- enrichment, formation of NH 4 post staggered+ , and methane synthesis from H 2 and CO 2 once SO 4 post staggered2- is eliminated. K + and often Na + (slightly) are depleted in the interstitial waters. Ca 2+ depletion may occur owing to precipitation of CaCO 3 . In most cases interstitial Cl - remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl - are observed in some sediments adjacent to continents.

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USGS1976

Mixing of carbonate waters

When mineral solutions of different compositions are mixed, the molalities and activities of individual ions in the mixture are often non-linear functions of their end-member values. This non-linearity is particularly significant in determining mineral saturation levels. Mixtures of saturated solutions may be either undersaturated or supersaturated depending on the end-member compositions and the physical conditions in which end-members and their mixtures exist. In carbonate solutions important non-linear effects occur due to redistribution of carbonate species. In extreme cases this causes mixture pH to be below both the end-member pH values. A simple but precise computer program (WATMIX) has been developed for calculating mixture composition for closed and open system mixing of arbitrary end-members. A number of mixing examples are considered which allow one to isolate three important processes leading to non-linear behaviour: the algebraic effect, the δP CO 2 effect, and the ionic strength effect.

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USGS1976

Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ 18 O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ 18 O H2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite. The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD H2O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. Theδ 18 O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of δ 18 O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ 18 O gradients at the margins and large δ 18 O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ 18 O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates. Almost all the δ 13 C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ 13 C values of most marble units show a general correlation with δ 18 O values. The CO 2 H 2 O "> CO 2 H 2 O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ 13 C values of the CO 2 range from −8.0 to 3.6‰ and indicate that at some localities CO 2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.

Geochimica et Cosmochimica Acta↗
USGS1976

Geochemical controls on lead concentrations in stream water and sediments

The equilibrium distribution of lead in solution and adsorbed on cation exchange sites in sediment theoretically may be calculated from equations representing selectivities of substrate for lead over H + , Ca 2+ and Na + , and the stabilities of lead solute species. Such calculations include consideration of total concentrations of major ions, cation exchange capacity (CEC) of substrate, and pH, at values expected in various natural systems. Measurements of CEC and selectivity coefficients were made for synthetic halloysite, a finely divided amorphous 1:1 clay prepared by precipitation from a mixture of solutions of aluminum and silica. Where suspended sediment having the same properties is present in concentrations of 10-1,000 mg/1 at pH 6–8, more than 90% of the lead present can be adsorbed on sediment surfaces. The cation exchange behavior of lead and other minor cationic species in natural systems could be predicted by this type of model if enough other supporting information were available. Information of the type needed describing natural stream sediments, however, is presently inadequate for accurate predictions.

Geochimica et Cosmochimica Acta↗
USGS1976

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206 Pb/ 204 Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206 Pb/ 204 Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207 Pb/ 204 Pb vs 204 Pb/ 204 Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87 Sr/ 86 Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970). The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.

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USGS1976

The dissolution of calcite in CO 2 -saturated solutions at 25°C and 1 atmosphere total pressure

The dissolution of Iceland spar in CO 2 -saturated solutions at 25&deg;C and 1 atm total pressure has been followed by measurement of pH as a function of time. Surface concentrations of reactant and product species have been calculated from bulk fluid data using mass transport theory and a model that accounts for homogeneous reactions in the bulk fluid. The surface concentrations are found to be close to bulk solution values. This indicates that calcite dissolution under the experimental conditions is controlled by the kinetics of surface reaction. The rate of calcite dissolution follows an empirical second order relation with respect to calcium and hydrogen ion from near the initial condition (pH 3.91) to approximately pH 5.9. Beyond pH 5.9 the rate of surface reaction is greatly reduced and higher reaction orders are observed. Calculations show that the rate of calcite dissolution in natural environments may be influenced by both transport and surface-reaction processes. In the absence of inhibition, relatively short times should be sufficient to establish equilibrium.

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USGS1976

Hydrogen and oxygen isotope exchange reactions between clay minerals and water

The extent of hydrogen and oxygen isotope exchange between clay minerals and water has been measured in the temperature range 100–350° for bomb runs of up to almost 2 years. Hydrogen isotope exchange between water and the clays was demonstrable at 100°. Exchange rates were 3–5 times greater for montmorillonite than for kaolinite or illite and this is attributed to the presence of interlayer water in the montmorillonite structure. Negligible oxygen isotope exchange occurred at these low temperatures. The great disparity in D and O 18 exchange rates observed in every experiment demonstrates that hydrogen isotope exchange occurred by a mechanism of proton exchange independent of the slower process of O 18 exchange. At 350° kaolinite reacted to form pyrophyllite and diaspore. This was accompanied by essentially complete D exchange but minor O 18 exchange and implies that intact structural units in the pyrophyllite were inherited from the kaolinite precursor.

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USGS1976

Studies of quaternary saline lakes-II. Isotopic and compositional changes during desiccation of the brines in Owens Lake, California, 1969-1971

Owens Lake is an alkaline salt lake in a closed basin in southeast California. It is normally nearly dry, but in early 1969, an abnormal runoff from the Sierra Nevada flooded it to a maximum depth of 2·4 m. By late summer of 1971, the lake was again nearly dry and the dissolved salts recrystallized. Changes in the chemistry, pH, and deuterium content were monitored during desiccation. During flooding, salts (mostly trona, halite, and burkeite) dissolved slowly from the lake floor. Their concentration in the lake waters increased as evaporation removed water and salts again crystallized, but winter temperatures caused precipitation of some salts and the following summer warming caused their solution, resulting in seasonal variations in the concentration patterns of some ions. The pH values (9·4–10·4) changed with time but showed no detectable diurnal pattern. The deuterium concentration increased during evaporation and appeared to be in equilibrium with vapor leaving the lake according to the Rayleigh equation. The effective α(D/H in liquid/D/H in vapor) decreased as salinity increased; the earliest measured value was 1·069 [as total dissolved solids (TDS) of lake waters changed from 136,200 to 250,400 mg/1]and the last value (calc.) was 1·025 (as TDS changed from 450,000 to 470,300 mg/1). Deuterium exchange with the atmosphere was apparently small except during late desiccation stages when the isotopic contrast became great. Eventually, atmospheric exchange, combined with decreasing α and lake size and increasing salinity, stopped further deuterium concentration in the lake. The maximum contrast between atmospheric vapor and lake deuterium contents was about 110%.

Geochimica et Cosmochimica Acta↗
USGS1977

Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from Antarctica

Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic 40 Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion. 40 Ar/ 19 Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic 40 Ar with respect to 39 Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic 40 Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the 40 Ar/ 39 Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic 40 Ar during progressive devitrification, although 39 Ar-recoil effects suggested by Turner and Cadogan (1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt, 40 Ar/ 39 Ar age-plateaux may be formed that have no geologic significance.

Geochimica et Cosmochimica Acta↗
USGS1977

Fluorite solubility equilibria in selected geothermal waters

Calculation of chemical equilibria in 351 hot springs and surface waters from selected geothermal areas in the western United States indicate that the solubility of the mineral fluorite, CaF 2 , provides an equilibrium control on dissolved fluoride activity. Waters that are undersaturated have undergone dilution by non-thermal waters as shown by decreased conductivity and temperature values, and only 2% of the samples are supersaturated by more than the expected error. Calculations also demonstrate that simultaneous chemical equilibria between the thermal waters and calcite as well as fluorite minerals exist under a variety of conditions. Testing for fluorite solubility required a critical review of the thermodynamic data for fluorite. By applying multiple regression of a mathematical model to selected published data we have obtained revised estimates of the p K (10,96), ΔG o f (−280.08 kcal/mole), ΔH o f (−292.59 kcal/mole), S° (16.39 cal/deg/mole) and C o P (16.16 cal/deg/mole) for CaF 2 at 25°C and 1 atm. Association constants and reaction enthalpies for fluoride complexes with boron, calcium and iron are included in this review. The excellent agreement between the computer-based activity products and the revised p K suggests that the chemistry of geothermal waters may also be a guide to evaluating mineral solubility data where major discrepancies are evident.

Geochimica et Cosmochimica Acta↗
USGS1977

Ore transport and deposition in the Red Sea geothermal system: a geochemical model

Thermodynamic calculation of distribution of dissolved aqueous species in the Red Sea geothermal brine provides a model of ore transport and deposition in good agreement with observed accumulations of base metal sulfides, anhydrite, and barite. The Red Sea brine is recirculated seawater that acquires high salinity by low-temperature interaction with Miocene evaporites and is subsequently heated to temperatures in excess of 200°C by interaction with recent rift zone intrusive rocks. At temperatures up to 250°C, NaSO − 4 and MgSO 0 4 are the dominant sulfur-bearing species. H 2 S forms by inorganic sulfate reduction at the higher temperatures but is maintained at a uniform concentration of about 2 ppm by the strength of the sulfate complexes. Chloride complexes solubilize metals at the higher temperatures, and thus sulfide and metals are carried together into the Atlantis II Deep. Below 150°C, the brine becomes supersaturated with respect to chalcopyrite, sphalerite, galena, and iron monosulfide due to chloride-complex dissociation. Sulfide precipitation rates, based on the rate of brine influx, are in good agreement with measured sedimentation rates. Anhydrite precipitates as crystalline fissure infillings from high-temperature inflowing brine. Barite forms from partial oxidation of sulfides at the interface between the lower hot brine and the transitional brine layer.

Geochimica et Cosmochimica Acta↗
USGS1977

Reactions of metal ions at surfaces of hydrous iron oxide

Cu, Ag and Cr concentrations in natural water may be lowered by mild chemical reduction involving ferric hydroxide-ferrous ion redox processes. V and Mo solubilities may be controlled by precipitation of ferrous vanadate or molybdate. Concentrations as low as 10 −8.00 or 10 −9.00 M are readily attainable for all these metals in oxygen-depleted systems that are relatively rich in Fe. Deposition of manganese oxides such as Mn 3 O 4 can be catalyzed in oxygenated water by coupling to ferrous-ferric redox reactions. Once formed, these oxides may disproportionate, giving Mn 4+ oxides. This reaction produces strongly oxidizing conditions at manganese oxide surfaces. The solubility of As is significantly influenced by ferric iron only at low pH. Spinel structures such as chromite or ferrites of Cu, Ni, and Zn, are very stable and if locally developed on ferric hydroxide surfaces could bring about solubilities much below 10 −9.00 M for divalent metals near neutral pH. Solubilities calculated from thermodynamic data are shown graphically and compared with observed concentrations in some natural systems.

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