Glauconite has been synthesized at low temperature by precipitation of Fe-hydroxides from Si-, Fe-, Al-, and K-containing solutions under reducing conditions. The compositions favorable for the synthesis at 20°C and pH 8.5 are 1 ppm Fe, 0.15 ppm Al, 13 ppm Si02, 1000 ppm KC1, and 1000 ppm dithionite. The K-content of the solutions must be sufficiently high to fix K in the precipitate.

Under special early diagenetic conditions glauconite is formed in marine sediments, probably at the interface between reducing and oxidizing zones in the muddy sediments. The silica content of pore waters seems to control the formation of glauconite or chamosite rather than depth or temperatures of the bottom waters.

Fe57 Mössbauer spectroscopy has been used to determine the nature of iron-containing minerals in Lower Greensand samples from an experimental lysimeter at Uffington, Oxfordshire, both before and after a three-year irrigation with a synthetic heavy metal leachate. Analysis of the spectra measured at 300°, 77°, and 4.2°K and at 4.2°K in an applied field of 45 kOe shows that iron is present in the uncontaminated sandstone in fine-grained goethite (α-FeOOH) and glauconite. In the irrigated samples iron is precipitated as a fine-grained ferric hydroxide gel having values for the hyperfine field at 4.2°K of 435 and 470 kOe. The stability of the gel over the three-year period of irrigation may be explained by surface energy considerations.

Mössbauer spectra of 9 glauconite samples from Upper Cretaceous and Lower Tertiary strata in the South Island of New Zealand contain a broad shoulder due to low intensity absorption continuous between 1.0 and 2.5 mm/sec when the absorber is at room temperature; the shoulder is absent, and sharp peaks are apparent in spectra taken with the absorber at 80°K. The data suggest that electron transfer occurs between adjacent Fe3+ and Fe2+ ions at room temperature. The low temperature spectra indicate that all Fe in the glauconites is in octahedral coordination. Fe3+ and Fe2+ ions occur in both eis and trans sites; Fe3+ shows a strong preference for eis sites whereas Fe2+ shows an even stronger preference for trans sites.

The partially variable oxidation state of Fe in glauconite is interpreted in terms of a geochemical model for glauconitization of a degraded or incomplete progenitor phyllosilicate. The model involves exchange of Fe2+ for other cations which temporarily stabilize the progenitor, followed by Fe2+-Fe3+ charge transfer reactions. Each reaction results from the system's tendency towards equilibrium. The model is supported by the observation that artificially leached glauconite increases both its Fe3+ and its Fe2+ content when placed in a solution containing Fe2+ as the only Fe ion present.

Glauconite from the oxidized and reduced zones of soil-geologic columns at two Coastal Plain sites, one in Maryland and one in New Jersey, was examined by Mössbauer spectroscopy. The data indicate that glauconite in the reduced zones had a higher proportion of its structural iron in the ferrous, as opposed to the ferric state. The Fe2+/Fe3+ ratio ranged from 0 to 0.2 for the glauconite from the oxidized zone and was about 0.35 for the glauconite in the reduced zones. Despite the presence of pyrite in the reduced zone, which might be expected to make ferric iron unstable because of the presence of sulfide S, about 75% of the Fe in the glauconite in the reduced zone was in the ferric state. Thin section analysis showed some glauconite in the reduced zones to be intimately associated with pyrite and some aggregates of fine pyrite crystals were locally present in cracks in glauconite pellets. In the oxidized zones, pyrite was absent and the glauconite was more yellow under plane-polarized light, as opposed to more green for the glauconite in the reduced zones. These data indicate that reports of studies of glauconite should stipulate whether samples are from the oxidized or reduced zone of soil-geologic columns.

On the basis of progressive acid dissolution the Thompson-Hower model states that mixed-layer glauconite/smectite and illite/smectite contain potassium in two structural sites: a mica-type K position (site 1) and a position of uncertain structural status more prone to dissolution (site 2). Site 2 was thought not to retain radiogenic argon (40Ar*). Using extensive progressive acid dissolution and K/Ar studies on a sized illite/smectite (I/S), determining the amount of K in site 2 is shown to be somewhat more complicated than previously thought because the dissolution pattern depends on acid normality. More important, site 2 fully retains 40Ar*, and no age correction is thus necessary as is required by the Thompson-Hower model, further affirming the geochronologic value of illite in mixed-layer clay. These data are also relevant to understanding the crystal and particle structure of I/S. Site 2 is probably a partly filled K interlayer that develops as an intermediate kinematic step on the way to being fully filled during the transformation of smectite to illite.

Glauconites in early ankerite concretions, ferroan calcite-cemented sandstones, and uncemented sandstones in the first Wilcox sandstone of the Lockhart Crossing field, Livingston Parish, Louisiana, show a progressive substitution of Fe for octahedral Al with increasing diagenesis. An octahedral Fe content of 0.50 atoms was calculated from glauconite located in early ankeritic concretions. Octahedral Fe averaged 0.60 and 0.90 atoms in later ferroan calcite-cemented sandstone and uncemented sandstone, respectively. Corresponding octahedral Al averages were 1.16, 1.03, and 0.67, respectively. A systematic increase in average interlayer K from 0.49 to 0.54 to 0.61 was also observed, with apparent increases in diagenesis. All element determinations were made with an electron microprobe and recast on an anion equivalent basis to structural formulae based on the O10(OH)2 unit. The clay preserved in the early ankerite concretions was found to be an illite/smectite containing about 20% expandable layers, and the mineral in the glauconite pellets from uncemented areas of the sandstone, an ordered glauconite. “Minus cement” porosities of the sandstone indicate that glauconitization may have taken place at burial depths greater than 0.6 to 1.8 km, but the mechanism for the incorporation of Fe3+ in the glauconite at that depth is not apparent.

The orientation of the OH-vectors in hydroxyl groups of micas of different compositions, polytype modifications, and symmetry were calculated by the method of minimization of electrostatic energy. The orientations are strongly affected by the peculiarities of structures, and even slight deviations from the ideal dioctahedral periodicity can introduce a correction of as much as 10° to the value of the polar angle. Ordering of cations in octahedra lead to twisting of the OH-bond towards the octahedron with lower charge and larger dimension. The latter is less true for cation ordering in tetrahedra. Because of these factors and the dependence of the hydrogen position on the coordinates of heavy atoms, such calculations can give reliable results only if the structure has been refined with high precision. Different polytypes of micas displayed the same orientation of the OH-vectors with respect to the axes of a separate layer, if the stacking sequence did not introduce specific distortions of the 2:1 layer.

Radium sorption efficiencies as a function of temperature, Ra concentration, and secondary mineral sorbate were determined in a 0.01 M NaCl solution. Radium sorption on a characterized clinoptilolite, montmorillonite, nontronite, opal, silica gel, illite, kaolinite, and glauconite under comparable experimental conditions allowed determination of Ra sorption efficiency curves for each, through use of Freundlich constants, over the same temperature and initial Ra solution concentration range. Similar sorption data for U on the same secondary minerals over the same temperatures allowed comparison of sorption efficiencies for Ra and U. Clinoptilolite, illite, and nontronite were the most efficient Ra sorbents, while opal and silica gel were the poorest Ra sorbents. Generally, Ra sorption on secondary minerals was much greater than U sorption under the same experimental conditions.

X-ray powder diffraction (XRD) and energy-dispersive X-ray analyses (EDX) of individual clay particles from hydrothermal mounds in the Galapagos spreading center (GSC) (Deep Sea Drilling Project, hole 509B) and high-resolution transmission electron microscopy (HRTEM) of the <2-µm size fraction of these sediments were carried out to document the mineralogy, geochemistry, and evolution of their clay horizons. The hydrothermal clay minerals of the GSC mounds were found to be intercalated with pelagic sediments and occurred as irregular interstratified illite/smectite according to X-ray powder diffraction analyses. On the basis of TEM, HRTEM, and EDX data, two types of clays appeared to coexist; these types differed in morphology, potassium content, and mode of stacking sequence. Lath-shaped particles having regular 10-Å, spacings were identified as glauconite, and filmy or veil-like particles, having curly edges and variable 10-13-Å spacings were identified as Fe-smectite (nontronite and Fe-montmorillonite). The absence of lattice fringes between Fe-smectite and glauconite crystallites was observed by HRTEM in clay aggregates. This structural discontinuity between Fe-smectite and glauconite layers suggests that a dissolution-recrystallization mechanism was responsible for the textural and chemical transition from the filmy Fe-smectite to the lath-like glauconite.

The 57Fe Mössbauer spectra of an iron-rich montmorillonite, an illite, and two glauconites were measured and computer-fitted with appropriate Fe3+ and Fe2+ doublet resonances. The broad experimental Fe3+ resonance of montmorillonite probably arises from Fe3+ in the octahedral sites and a trans-arrangement of OH groups; however, a large variation in the neighboring environment of these sites exists. In illite this Fe3+ resonance is similar but shows less broadening; it arises from Fe3+ located predominantly in trans-OH octahedral sites, with some Fe3+ being located in cis-OH octahedral sites. Because of the increased iron content less variation exists, compared with montmorillonite, in the neighboring octahedral sites. The Fe3+ resonance is narrower still for the glauconites and represents Fe3+ substituting primarily into cis-OH octahedral sites, similar to that previously reported for nontronite.

The tetrahedral Fe3+ content is very low for montmorillonite and increases progressively for illite and glauconite, suggesting that a higher tetrahedral Fe3+ content directs Fe3+ in the octahedral layer into cis-OH sites. In montmorillonite, the Fe2+ is located only in trans-OH sites; in illite Fe2+ is largely in trans-OH sites and only slightly in cis-OH sites; and in glauconite, Fe2+ is located largely in cis-OH sites and only slightly in trans-OH sites. These assignments suggest that for Fe2-, the doublet with the larger quadrupole interaction arises from Fe2+ in trans-OH sites and the doublet with the smaller quadrupole interaction, from Fe2+ in cis-OH sites.

A new approach to the interpretation of Mössbauer spectra of Fe3+-phyllosilicates having vacant trans-octahedra is based on (1) crystal structure simulation methods that allow for the size and the shape of a Fe3+-octahedron as a function of the nearest surrounding cations; and (2) calculations of electric field gradients (EFG) on Fe3+ in terms of the ionic point-charge model. Calculations were performed by direct summation within the region of radius ≤50 Å. Coordinates for the anions in the coordination octahedra have been assigned to take into account the nearest cationic environment. Atomic coordinates for the rest of the summation volume are those for the average unit cell. EFG calculations for cation combinations responsible for the visible quadrupole splitting Δvis in the spectra of nontronite, “red” muscovite, and celadonite have led to good agreement between Δvis and Δcalc. Computer fitting of the nontronite and celadonite spectra based on EFG calculations for the rest of the possible cation combinations suggests that the distribution of tetrahedral cations in nontronite obeys the Loewenstein rule, and in celadonite, the distribution of R3+ and R2+ over cis-octahedra is predominantly ordered, in agreement with electron diffraction and infrared spectroscopy data. The Mössbauer spectrum of one of the glauconites suggested the presence of celadonite-like and muscovite-like domains in its 2:1 layers.

Well-characterized American Petroleum Institute clay standards, source clays from The Clay Minerals Society, and other secondary minerals were used to determine the effects of U concentration, temperature, and solution composition on U-sorption properties. Uranium concentrations ranged from about 1.00 × 10−4 M to 4.00 × 10−7 M, temperatures from 5° to 65°C and solution compositions containing 0.01 M NaCl and 0.01 M NaHCO3. Silica gel efficiently sorbed uranyl carbonate anion complexes. The higher cation-exchange capacity materials most readily sorbed uranyl ions from the 0.01 M NaCl solution. Temperature increases tended to affect uranyl ion sorption adversely except when the U was present as carbonate complexes. Noncrystalline ferric oxyhydroxides sorbed uranyl ions much more efficiently than any of the secondary crystalline minerals studied. A method for accurately extrapolating U-sorption efficiencies between experimental points based on the Freundlich equation is presented.

The internal fabric of glauconite pellets has been studied by high-resolution transmission electron microscopy (HRTEM) for a better understanding of the glauconitization process. Typical “lamellae” which make up the glauconite pellets showed a spindle-like arrangement of layered crystallite packets. Three main mineral phases were detected: (1) well-ordered glauconite sensu stricto (d(001) = 10 Å) generally in the middle of the spindles; (2) a poorly ordered and undetermined layered-phase “X” with d(001) ~ 12.5 Å, usually sandwiching glauconite such that the interface between the two materials is very sharp; and (3) a noncrystalline or gel-like phase located between the lamellae. A 14-Å smectite-like phase was rarely observed at the periphery of some grains. The glauconite crystallites clearly showed characteristic growth features (e.g., growth steps), whereas the unknown phase X exhibited destabilization characteristics. A structural analysis of the pure glauconite indicates that this dioctahedral mica was present in the IMd (disordered), 1M, and, to a much lesser extent, 2M1 polytypic forms. HRTEM revealed no interlayering of glauconite with the other layered phases. Rather, it appeared to have formed by a layer-growth mechanism at the expense of the unknown phase X which apparently converted into non-crystalline matter before converting to glauconite. The precursor function of the interlamellae “gel” phase during the evolutive process of glauconitization is not understood.

A new approach is described to computer simulate cation distribution in octahedral sheets of dioctahedral 2:1 layer silicates with vacant trans-octahedra. This approach makes use of the information on cation distribution at the one-dimensional level provided by integrated IR optical densities for the region of OH-stretching frequencies. By using this program it is possible to show that (1) the Mössbauer spectrum of glauconite B. Patom conforms to the structural model composed of celandonite-like and illite-like domains whose dimensions are limited by approximately 2 or 4 unit cells; (2) non-equivalency of “left” and “right” cis-positions (with fixed b-direction) with respect to R2+ and R3+ occupancy is a characteristic feature of a celadonite-like domain.

The structural transformation of dioctahedral 2:1 layer silicates (illite, montmorillonite, glauconite, and celadonite) during a dehydoxylation-rehydroxylation process has been studied by X-ray diffraction. thermal analysis, and infrared spectroscopy. The layers of the samples differ in the distribution of the octahedral cations over the cis- and trans-sites as determined by the analysis of the positions and intensities of the 11l, 02l reflections, and that of the relative displacements of adjacent layers along the a axis (c cos ß/a), as well as by dehydroxylation-temperature values. One illite, glauconite, and celadonite consist of trans-vacant (tv) layers; Wyoming montmorillonite is composed of cis-vacant (cv) layers, whereas in the other illite sample tv and cv layers are interstratified. The results obtained show that the rehydroxylated Al-rich minerals (montmorillonite, illites) consist of tv layers whatever the distribution of octahedral cations over cis- and trans-sites in the original structure. The reason for this is that in the dehydroxylated state, both tv and cv layers are transformed into the same layer structure where the former trans-sites are vacant.

The dehydroxylation of glauconite and celadonite is accompanied by a migration of the octahedral cations from former cis-octahedra to empty trans-sites. The structural transformation of these minerals during rehydroxylation depends probably on their cation composition. The rehydroxylation of celadonite preserves the octahedral-cation distribution formed after dehydroxylation. Therefore, most 2:1 layers of celadonite that rehydroxylate (~75%) have cis-vacant octahedra and, only in a minor part of the layers, a reverse cation migration from former trans-sites to empty octahedra occurred. In contrast, for a glauconite sample with a high content in IVA1 and VIAl the rehydroxylation is accompanied by the reverse cation migration and most of the 2:1 layers are transformed into tv layers.

Various size fractions of several samples of glauconitic clay and illite were heated stepwise under vacuum for extended periods to: 1) evaluate radiogenic-argon (Ar) yields in relation to composition and size range; and 2) investigate the kinetics of radiogenic-Ar release. Each sample was heated at 250, 375, 500 and 1000 °C.

The radiogenic-Ar release patterns are nearly the same for the various size fractions of each sample. The Ar yields are not functions of particle size, at least not in these size ranges. This observation suggests that the kinetics of radiogenic-Ar release from these materials under these experimental conditions may be controlled by some mechanism other than diffusion.

The experiments show distinct differences in yield of radiogenic Ar from glauconitic clay and illite, which were most evident in the 375 and 500 °C steps. The yield (relative to the total amount of radiogenic Ar in the sample) at a particular temperature is inversely related to potassium (K) content, and there appears to be a direct dependence of yield on the iron (Fe) content. Because the literature provides evidence that Ar release during heating of phyllosilicates under vacuum is controlled by dehydroxylation and also provides evidence that dehydroxylation of clay may follow first-order kinetics, the kinetic data on Ar release from these samples were compared to simulated first-order reactions. To match the obser- vational data requires more than 2 activation energies in each simulation, which is consistent with the known mineralogical heterogeneity of such samples.

A large and representative collection of clay-size dioctahedral mica minerals differing in their chemical compositions has been studied by infrared (IR) spectroscopy in the OH stretching vibration region. Decomposition of the IR spectra in the individual OH bands has provided unambiguous identification of the band positions for each defined pair of octahedral cations bonded to OH groups. The presence of pyrophyllite-like local structural environments in samples having a deficiency of K in interlayers has been established. A set of the relationships between the OH frequencies corresponding to pairs of cations having different valency and mass has been found.

Boiling glauconite from the El-Gideda area of Egypt in different concentrations of HCl and H2SO4 for different periods led to a modified structure. Treatment resulted in progressive destruction of the structure, leaving X-ray amorphous silica and only relics of the original mineral. The glauconitic material was modified structurally in order to increase its adsorption activity. The glauconite was evaluated in terms of mineralogy, chemistry, morphology, structural modification, octahedral cation leaching rate, surface area and cation exchange capacity using X-ray diffraction, infrared spectroscopy, X-ray fluorescence, scanning electron microscopy and surface area analysis. The ratio of extracted octahedral cations to the total octahedral cations in the untreated glauconite was taken as a measure of octahedral sheet decomposition. A progressive decrease in crystallinity and the formation of X-ray amorphous silica Si-O vibration bands at 1100, 800 and 494 cm−1 accompanied octahedral cation depletion. Acid activation using 2 M and 4 M HCl for 6 h destroyed 30% and 61% of the octahedral sheet, respectively. In contrast, similar treatment using 2.9 and 5.5 M H2SO4 destroyed 48% and 93% of the octahedral sheet, respectively. Depending on the extent of cation depletion, the 4 M HCl product surface areas were as high as 259 m2/g, whereas the surface area of the 5.5 M H2SO4 product was only 63 m2/g. The progressive increase in surface area was due to glauconite morphology alteration. Acid-induced dissolution of Al, Fe, Mg cations from octahedral sheet edges led to a wedge-like splitting of the glauconite crystals, mesopore creation, and greater access to interlayer galleries.

The environmental conditions and reaction paths of shallow-water glauconitization (<500 m water depth, ~15°C) close to the sediment-seawater interface are generally considered to be well understood. In contrast, the key factors controlling deep-sea glauconite formation are still poorly constrained. In the present study, green grains formed in the recent deep-sea environment of the ODP Site 959, Ivory Coast-Ghana Marginal Ridge, (~2100 m water depth, 3-6°C) were investigated by X-ray diffraction and electron microscopic methods in order to determine the rate and mechanism of glauconitization.

Green clay authigenesis at Hole 959C occurred mainly in the tests of calcareous foraminifera which provided post-depositional conditions ideal for glauconitization. Within this organic-rich microenvironment, Fe-smectite developed <10 ky after deposition of the sediments by precipitation from precursor gels containing Fe, Mg, Al, and silica. This gel formation was supported by microbial activity and cation supply from the interstitial solution by diffusion. At a later stage of early marine diagenesis (900 ky), the Fe-smectites reacted to form mixed-layer glauconite-smectite. Further down (~2500 ky), almost pure glauconite with no compositional gaps between the Fe-smectite and glauconite end members formed. This burial-related Fe-smectite-to-glauconite reaction indicates that the glauconitization process was controlled mainly by the chemistry of the interstitial solutions. The composition of the interstitial solution depends heavily on micro-environmental changes related to early diagenetic oxidation of biodegradable (marine) organic matter, microbial sulfate reduction, silicate mineral alteration, carbonate dissolution, and Fe redox reactions. The availability of Fe is suggested as the probable limiting factor for glauconitization, explaining the various states of green-grain maturity within the samples, and this cation may be the most important rate-determining element.

The rate of glauconite formation at ODP Site 959 is given by %GlSed = 22.6·log(ageSed) + 1.6 (R2 = 0.97) where %GlSed is the state of glauconitization in the sediment and ageSed is the sediment age (in ky). This glauconitization rate depends mainly on continuous cation supply (in particular Fe) and is about five times less than that in shallow-shelf regions, suggesting significantly slower reaction at the lower temperature of deep-sea environments.