Volume 87, Issue 11-12

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 (δ 18 O) 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 δ 18 O values of water in equilibrium with I-S is 2.5-3.5‰. At higher thermal maturity (CAI = 2.5-4.5), the δ 18 O of water in equilibrium with I-S is from 4.5 to 12‰. The δ 18 O 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.

K-Ar dates of illitic clays from sedimentary rocks may contain “mixed ages,” i.e., may have ages that are intermediate between the ages of end-member events. Two phenomena that may cause mixed ages are: (1) long-lasting reaction during the burial illitization of smectite; and (2) physical mixing of detrital and diagenetic components. The first phenomenon was investigated by simulation of illitization reactions using a nucleation and growth mechanism. These calculations indicate that values for mixed ages are related to burial history: for an equivalent length of reaction time, fast burial followed by slow burial produces much older mixed ages than slow burial followed by fast. The type of reaction that occurred in a rock can be determined from the distribution of ages with respect to the thickness of illite crystals. Dating of artificial mixtures confirms a non-linear relation between mixed ages and the proportions of the components. Vertical variation of K-Ar age dates from Gulf Coast shales can be modeled by assuming diagenetic illitization that overprints a subtle vertical trend (presumably of sedimentary origin) in detrital mineral content.

PolyQuant, a new method to quantify illite polytypes in physical mixtures, has been coupled with Illite Age Analysis (IAA, Pevear 1992) to determine accurate K-Ar ages of the individual polytypes. K-Ar radioisotope dates from illite in shales and sandstones are typically meaningless because these rocks are mixtures of diagenetic and detrital components in unknown proportions. IAA uses linear extrapolation to unmix measured K-Ar ages and establish the end-member ages. This procedure requires samples to have a diagenetic Illite-Smectite (I-S) component mixed with a detrital discrete illite component because they are separable and quantifiable using X-ray diffraction (XRD) of oriented aggregate samples. We extend IAA for use in mixtures of diagenetic and detrital illite with no I-S component by coupling a genetic algorithm (GA) with the WILDFIRE programs (1993) of Reynolds in a new computer program, PolyQuant, which quantifies illite polytypes in a natural sample. Based on first principles of XRD, WILDFIRE is a forward model that calculates hkl reflections by varying parameters such as rotational disorder, octahedral cation occupancy (i.e., cis- and transvacancies), chemical substitutions (K and Fe), orientation, and crystallite thickness. PolyQuant executes several forward models, using the GA to vary the parameters and weight percents of each polytype to automatically achieve a “best fit” with an experimental XRD pattern. Sensitivity tests reveal that changes in the weight percent of an illite polytype, disorder (P 0 , P cv ), and the orientation function affect the pattern more than other parameters. In addition, PolyQuant successfully quantified illite components in prepared mixtures of 1M illite and 2M 1 muscovite and 1M d illite and 2M 1 muscovite. We show examples to validate IAA from prepared mixtures of end-members with known ages and also to obtain end-member ages of highly illitic clay fault gouge. Using K-Ar ages and PolyQuant to quantify the 1M and 2M 1 components from mixtures of RM-30 (25 ± 1 Ma) and a 2M 1 muscovite from Wards (428 ± 9 Ma), IAA determined end-member ages of 19.9 Ma (0-44 Ma) and 429 Ma (408-459 Ma)-in reasonable agreement with the measured results. In addition, we use clay gouge from the Canadian Rockies to date movement along thrust faults. Regional geologic data indicate movement along these faults occurred between 73 and 48 Ma. IAA determined the ages of faulting to be from 79.2 to 53.5 Ma, in good agreement with the known range of fault dates. Our results indicate that neoformation of illite in the fault zone must have modified the ages to younger dates.

Mean thickness measurements and crystal-thickness distributions (CTDs) of illite particles vary systematically with changes in hydrothermal alteration type, fracture density, and attendant mineralization in a large acid-sulfate/Mo-porphyry hydrothermal system at Red Mountain, near Lake City, Colorado. The hydrothermal illites characterize an extensive zone of quartz-sericite-pyrite alteration beneath two deeply rooted bodies of magmatic-related, quartz-alunite altered rock. Nineteen illites from a 3000 ft vertical drill hole were analyzed by XRD using the PVP-10 intercalation method and the computer program MudMaster (Bertaut-Warren-Averbach technique). Mean crystallite thicknesses, as determined from 001 reflections, range from 5-7 nanometers (nm) at depths from 0-1700 ft, then sharply increase to 10-16 nm at depths between 1800-2100 ft, and decrease again to 4-5 nm below this level. The interval of largest particle thickness correlates strongly with the zone of most intense quartz-sericite-pyrite alteration (QSP) and attendant high-density stockwork fracturing, and with the highest concentrations of Mo within the drill core. CTD shapes for the illite particles fall into two main categories: asymptotic and lognormal. The shapes of the CTDs are dependent on conditions of illite formation. The asymptotic CTDs correspond to a nucleation and growth mechanism, whereas surface-controlled growth was the dominant mechanism for the lognormal CTDs. Lognormal CTDs coincide with major through-going fractures or stockwork zones, whereas asymptotic CTDs are present in wallrock distal to these intense fracture zones. The increase in illite particle size and the associated zone of intense QSP alteration and stockwork veining was related by proximity to the dacitic magma(s), which supplied both reactants and heat to the hydrothermal system. However, no changes in illite polytype, which in other studies reflect temperature transitions, were observed within this interval.

The Waukesha Illite is an excellent example of the illites found in argillaceous rocks, typical for Paleozoic shales that have undergone significant burial diagenesis during their geologic history. It consists of a mixture of detrital 2M 1 , interpreted to be a residuum of karstification within Silurian carbonates, and diagenetic 1M and 1M d illite. The chemistry and the age of the illite polytypes are different. Extrapolating to 100%, the 1M and 1M d polytypes have an apparent diagenetic age between 295 and 325 Ma. The chemistry of the 1M polytype could not be determined because of its low abundance. The approximate chemical composition of the 1M d polytype is 0.67 K, 3.6 Si, and 1.9 Al per half unit cell. The 2M 1 polytype has an apparent detrital age between 440 and 520 Ma, and an approximate chemical composition per half unit cell of 0.78 K, 3.4 Si, and 2.1 Al, all within our margin of error. X-ray diffraction (XRD) results of both random powder and oriented preparations both indicate that the Waukesha Illite consists of a mixture of illites. The XRD patterns of the random powder preparation indicate it is a physical mixture of three different illite polytypes. This result was confirmed using 3 different methods: (1) by measuring illite polytype-specific reflections; (2) by mixing illite polytype reference samples; and (3) by mixing WILDFIRE calculated XRD patterns. Decomposition of the illite 001 XRD peak from oriented preparations also indicates mixtures of illites. However, the proportions of the three illitic components derived from the oriented 001 peak decomposition differ from those results derived from the analysis of the random powder data. Therefore, the shape of the 001 reflection of the Waukesha Illite cannot be explained by mixing the three different illite polytypes.

The isotopic composition of boron in illite-smectite (I-S) can be important for monitoring fluid/rock interactions in sedimentary basins. Boron substitutes for Si during reaction of smectite to illite and can preserve information about paleofluid B-isotopic composition. Boron is enriched in oilfield brines, therefore the isotopic composition of those brines may be recorded during illitization and represent a monitor of hydrocarbon maturity and migration. We re-examined previously published experimental results on B-isotope fractionation between I-S and water. By separating B from two crystallographic sites of I-S (tetrahedral and interlayer), we found differences in the δ 11 B that might be used as a single-mineral geothermometer. Boron incorporation in I-S follows a non-linear kinetic pathway. Maximum interlayer-B incorporation occurs during R1-ordering. R3-ordering approaches equilibrium with expulsion of interlayer-B leaving only tetrahedral layer-B. The important discovery is that tetrahedral layer δ 11 B does not change between R1 and R3 ordering. Boron substitutes in an equilibrium ratio early in the crystallographic reordering of I-S. Natural I-S samples were tested from Gulf Coast mudstones, increasingly illitized with burial depth. Diagenetic reaction kinetics differ from hydrothermal experiments, but still reveal large δ 11 B differences (up to 40‰) between the interlayer and tetrahedral layer. Interlayer δ 11 B decreases with increasing temperature and illitization. We propose that interlayerδ 11 B values represent metastable equilibrium, whereas tetrahedral layer B represents a temperature-dependent equilibrium. If this is true, then the B-isotope geochemistry of I-S can be used to determine paleotemperatures and monitor the influence of hydrocarbons on pore fluids associated with diagenetic I-S.

We report a method to physically separate smectite from illite in natural shale samples. This method is based upon the large contrast in the density of mixed-layer clay minerals reported in the literature. Our objective was to investigate the behavior of separate density fractions of mixedlayer smectite and illite during burial diagenesis. Samples were obtained from shale cuttings hand-picked from a well drilled offshore Louisiana. Each sample was separated into 5 fractions: “pure” end-member smectite (EMS), smectiterich mixed-layer clays (SML), illite-rich mixed-layer clays (IML), “pure” end-member illite (EMI), and quartz (FGQ). The mineralogy of each clay-mineral separate as determined by XRD was reasonably consistent at all depths, although the abundance of the mixed-layer separates varied. The illite-rich mixed-layer fraction increased in abundance with depth at the expense of the smectiterich mixed-layer fraction. The fine-grained quartz fraction showed an increase in abundance, a decrease in average grain size, a loss of K-feldspar, and a heavier isotopic signature with depth. We did not, however, find a correlation with depth in the amount of the end-member clay fractions. The isotopic signature of the end-member smectite shows evidence of equilibration with depth, but the end-member illite does not. We conclude that the discrete illite fraction is detrital in origin and was not involved in the clay-mineral transformation at the depths sampled in our well. It reflects the sediment provenance. The results of this study illustrate the efficacy of the newly developed density separation technique for physically isolating clay mineral species from one another.

The role of clay diagenesis in pressure-ramp (overpressure) development is evaluated from Mesozoic mudstone well cuttings in the Egersund Basin, Norwegian Continental Shelf. Major changes in the mineral assemblage over the range of increasing geopressure include ~50% decrease in detrital kaolinite in bulk material, and increases in the proportions of illite, I-S, and percent of nonexpandable illite-like layers in I-S. SEM and TEM observations confirm that pore space is defined primarily by intersecting clay-mineral packets at the scale of tens or hundreds of nanometers (0.01-0.1 μm). TEM data document the reactions of kaolinite and detrital mica to form I-S. The principal reaction that occurred over the geopressure ramp was dissolution of kaolinite and precipitation of neoformed I-S in pore space of these mudstones. These changes also correspond to changes in <0.1 μm particle populations, as determined by Pt/C shadowed TEM measurements, as well as high-resolution TEM lattice-fringe observations. The neoformed I-S has dimensions similar to those of the pore space in the mudstones. The results support a mineralogical model of precipitation of diagenetic clay, resulting in severe permeability reduction, which can be related to the observed pressure ramp. These observations and results provide an integrated petrologic framework for modeling pressure-ramp development, which had been difficult to achieve using conventional basin models of porosity and permeability evolution based on mechanical compaction.

Late Jurassic sandstones of the shallow-marine Sundance Formation contain authigenic chlorite minerals that occur as rosette-like pore fillings of interstratified berthierine/chamosite (B-C) and honeycomb-like pore linings of corrensite and discrete chlorite. B-C is nearly ubiquitous in Sundance sandstones, but is absent near the top of the formation, whereas corrensite and discrete chlorite were detected only in uppermost Sundance sandstones, within 4 m of the contact with the overlying nonmarine Morrison Formation. Glauconite grains are common and occur as laminae along bedding planes and cross-beds, indicating reworking and deposition as clasts in tidally influenced regimes. The mineralogical, chemical, and morphological properties of the B-C and corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals, odinite in the case of B-C, and saponite in the case of corrensite and discrete chlorite. Odinite has been recognized in numerous shallow-marine sands of the Holocene verdine facies, and the shallowmarine conditions associated with Sundance deposition would have been ideal for odinite formation. Saponite commonly forms in aeolian and evaporitic environments, implying that the saponite precursor to corrensite and discrete chlorite formed in uppermost Sundance sands that were exposed to an influx of oxidizing groundwater following regression of the Sundance sea. X-ray diffraction (XRD) indicates that the proportion of 7 Å B layers in B-C ranges from 5 to 28%, and chemical analysis by scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) indicates positive correlation between %B and Fe/(Fe + Mg). The polytype of the B-C is Ibb, and the corrensite and discrete chlorite are disordered IIb. These are the expected polytypes and %B for sandstones exposed to burial diagenetic conditions of 3000-4000 m and temperatures 90-120 °C.

Ilmenite grains from weathering profiles developed on granite and ultramafic chlorite schist in the Georgia Piedmont were studied for evidence of morphological and chemical alteration. Ilmeniterich concentrates from the fine sand (90-150 μm) component were studied to test the assumption that there is no difference between ilmenite in the parent rock and that in colluvium delivered to primary drainage systems. Ilmenite grains in the granite profile are rounded to subhedral, and commonly contain hematite exsolution blebs. Dissolution pits are observed along the boundaries of the exsolution blebs, with goethite occurring as an alteration product. Ilmenite grains in the schist profile occur as fractured anhedral grains with uncommon lamellae of rutile. Grain fractures are filled with goethite and hematite, particularly in the B-horizon. Ilmenite from the granite profile is Mn rich (7-15 mol% MnTiO 3 ), whereas ilmenite from the schist profile contains only 1-2 mol% MnTiO 3 and up to 8 mol% MgTiO 3 . Two populations of grains develop in both profiles. Grains with abundant exsolution blebs and fractures alter through a proposed two-step reaction mechanism. It is proposed that ilmenite first undergoes a solid-state transformation to pseudorutile via an anodic oxidation mechanism. Oxidized Fe and Mn diffuse from the structure and precipitate as goethite and MnO 2 . Pseudorutile is ephemeral and undergoes incongruent dissolution to form anatase, hematite, and goethite. The second population of grains experienced only slight oxidation and dissolution on grain surfaces, and they persist through the weathering profile. The Fe 2+ content of competent ilmenite grains is somewhat lower in the C-horizon, compared with grains in the host rock. In horizons above the C-horizon, the Fe 2+ contents of the ilmenite are similar to those in the host rock. This study shows that using ilmenite minor-element chemistry as a tracer for sediment provenance is a valid technique, however, textural features of ilmenite in colluvium may be distinct from those in the parent rock. Also, the production of secondary phases, such as anatase, goethite, and hematite, in soil profiles results in part from the alteration of ilmenite.

Numerous studies have demonstrated the presence of at least two distinct kaolinites in individual kaolinite samples, one a low-defect material and the other a moderate- to high-defect material. Other studies have shown that some kaolinites contain the lowest-defect material in the coarsest size fractions whereas others contain the lowest-defect kaolinite in the finest fractions. In an attempt to clarify possible mechanisms for producing such kaolinite samples, we have used powder X-ray diffraction to study the effects of mechanical grinding on the nature of layer stacking in the >40 μm fraction of the American Petroleum Institute kaolinite standard no. 9 from Mesa Alta, New Mexico. This material is relatively rich in a low-defect kaolinite. Hand grinding for 10 min plus grinding under acetone for up to an additional 34 min in an automatic agate grinder produced significant changes in its diffraction pattern. However, further dry grinding in a ball mill for 10 min produced material that was almost totally disordered, based on measures such as the Hinkley index. The diffraction patterns of the wet-ground materials showed evidence of increasing disorder that could be modeled best as a physical mixture of low- and high-defect material, consistent with a physical mixture of the original ordered phase with varying amounts of a highly disordered material. Disorder in the high-defect kaolinite is caused by the interstratification of normal kaolinite layers with their enantiomorphs. Contrary to expectations, grinding of kaolinite does not produce a progressive increase in disorder for all of the crystallites present in a sample. Instead, grinding apparently creates increased amounts of a disordered kaolinite that coexist with relatively unaffected material. There is no evidence for the occurrence of an intermediate disordered phase. Contrary to previous reports, disorder caused by physical stress does not include random layer displacements of ±b/3.

The structure of heavy-metal sorbed synthetic birnessites (MeBi) was studied by powder X-ray diffraction (XRD) using a trial-and-error fitting procedure to improve our understanding of the interactions between buserite/birnessite and environmentally important heavy metals (Me) including Pb, Cd, and Zn. MeBi samples were prepared at different surface coverages by equilibrating at pH 4 a Na-rich buserite (NaBu) suspension in the presence of the desired aqueous metal. Two main types of experimental XRD patterns were obtained as a function of the Me cations sorbed from solution, which exert a strong control on layer stacking sequence, as well as on the location and coordination of Me: (1) CdBi and PbBi samples correspond to a one-layer hexagonal (1H) structure, AbC b'A' C'b' AbC…, and (2) ZnBi exhibits a one-layer monoclinic (1M) structure in which adjacent layers are shifted by +a/3, AbC b'A'c' BcA c'B'a' CaB a'C'b' AbC. Simulated XRD patterns show that octahedral layers contain 0.833 Mn cations (Mn 4+ and Mn 3+ ) and 0.167 vacant octahedra; Mn 3+ interlayer and adsorbed Me interlayer compensate for the layer charge deficit. Mn 3+ interlayer is octahedrally coordinated in all samples and is located above or below vacant layer octahedra sharing three O layer with neighboring Mn layer octahedra to form a triple-corner surface complex ( VI TC sites). In ZnBi and CdBi samples, Me interlayer is also located in TC sites; all Cd is octahedrally coordinated whereas about 30% of Zn is tetrahedrally coordinated ( IV TC sites). In PbBi samples, all Pb is octahedrally coordinated, most of these cations (~75%) being located in TC sites. Additional Pb is located above or below empty tridentate cavities, sharing three edges with neighboring Mn layer octahedra ( VI TE sites). Structural formulae calculated for each sample show that during the NaBu-to-MeBi structural transformation, interlayer Na + and Mn 2+ are replaced by Me and H + adsorbed from solution, whereas Mn 3+ interlayer resulting from the equilibration of NaBu at low pH is less affected. Sorption of divalent Me above and below vacant layer sites provides optimal conditions for local charge compensation in MeBi.

Selected-area electron diffraction (SAED) and energy-dispersive analysis were used to study the structure of synthetic heavy-metal sorbed birnessites (MeBi). Samples were prepared by equilibrating a suspension of Na-rich buserite (NaBu) at pH 4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu. Five main types of SAED patterns were observed. Types I and II were observed only for ZnBi micro-crystals, and they both consist of two super-cell reflection networks related by a mirror plane parallel to the a*-c* plane. In direct space, these twinned networks correspond to hexagonal supercells with A H = B H = √7b/√3, and A H = B H = √7b, for ZnBi type I and II, respectively. The supercells in the two varieties result from an ordered distribution of vacant layer octahedra capped by interlayer Zn in ZnBi layers. This distribution is described by a hexagonal cell with A H = √7b. In ZnBi micro-crystals of type I, interstratified twinned right- and left-handed fragments are similar to chalcophanite (ZnMn 3 O 7 ·3H 2 O; Wadsley 1955; Post and Appleman 1988), and distributions of vacant layer octahedra from adjacent layers are regularly shifted with respect to each other by 1/3 of the long diagonal of the hexagonal layer unit cell. In ZnBi micro-crystals of type II, distributions of vacant layer octahedra are not regularly shifted from one layer to the adjacent one. SAED patterns of types III and IV occur for PbBi, ZnBi, and CdBi micro-crystals and contain super-cell reflections distributed parallel to [100]* with a periodicity that is not commensurate with that of the MeBi sub-structure (a*/2.15 and a*/5.25, respectively). The super-cell reflections result from the ordered distribution within MeBi layers of vacant layer sites capped by Me as pairs along the a axis. Within each pair, vacant sites are separated by 2a for type III, and by 5a for type IV. In one-layer monoclinic structures, the apparent incommensurability arises from the +a/ 3 shift between adjacent layers having a similar one-dimensional periodic distribution of interlayer Me located above and below vacant octahedra sharing three corners with Mn layer octahedra (TC sites). Tetrahedral coordination of these Me cations in TC sites, as in ZnBi, leads to the formation of strong H-bonds between adjacent layers. A similar incommensurate effect occurs in one-layer hexagonal MeBi if octahedrally coordinated Me cations periodically distributed along the a axis are located above and/or below empty tridentate cavities sharing three edges with Mn layer octahedra ( VI TE sites, PbBi). SAED patterns of type V contain only sub-cell reflections and were observed mostly for PbBi and CuBi micro-crystals. Three different conditions can lead to the absence of super-cell reflections: (1) a low amount of sorbed Me (PbBi); (2) the presence of Me having a scattering power similar to that of Mn on a single side of vacant layer sites (CuBi); or (3) a random distribution of interlayer species.

The structure of a synthetic analogue of Na-birnessite (NaBi) was studied by powder X-ray diffraction (XRD). It is shown that NaBi has a one-layer triclinic structure with sub-cell parameters: a P = 2.9513(4) Å, b P = 2.9547(4) Å, c P = 7.334(1) Å, α P = 78.72(2)°, β P = 101.79(1)°, γ P = 122.33(1)°, and space group P1̄. This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 Å, b = 2.848 Å, c = 7.334 Å, α = 90.53°, β = 103.20°, and γ = 90.07°. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mnbearing octahedral layers whose negative charge arises mostly from the substitution of Mn 3+ for Mn 4+ . The departure from the hexagonal symmetry of layers results from Jahn-Teller distortion of Mn 3+ octahedra, which are elongated along the a axis, segregated in Mn 3+ -rich rows parallel to the b axis, and separated from each other along the a axis by two Mn 4+ -rows. Structural sites of interlayer Na cations and H 2 O have been determined as well as their occupancies. The sub-cells of the two NaBi modifications described by Drits et al. (1997) as types I and II likely contain four sites for interlayer species, two of which are occupied by Na and the other two by H 2 O molecules. In the two NaBi varieties, these pairs of sites are split along the c axis and related by a center of symmetry. This splitting is consistent with the modulated structure of both NaBi types, which arises from the periodic displacement of interlayer species along the b axis with a periodicity λ = 6b (Drits et al. 1997).

The “classical” modeling of powder X-ray diffraction (XRD) patterns of lamellar structures, such as phyllosilicates, assumes that the samples are composed of “crystals” having various thickness and well-defined translations between layers. This model is able to describe the high-angle domain of XRD patterns but sometimes fails in the low-angle region. The new model proposed here considers the samples to be composed of “particles” that have larger sizes than crystals and contain defects such as cracks, inner-porosity, bent layers, edge dislocations, etc. These defects induce variations in the d-spacings, introduced in the calculation by distributions of the d-spacings. For phyllosilicates, this model is consistent not only with XRD, but also with small-angle X-ray scattering (SAXS) data, transmission electron microscopy (TEM) results, and high-resolution transmission electron microscopy (HRTEM) observations.

Electron back-scattering pattern (EBSP) analysis is shown to be capable of identifying polytypic groups (subfamilies) of hydrous phyllosilicates in a scanning electron microscope with spatial resolution far better than X-ray diffraction and with easier sample preparation than transmission electron microscopy. Polytypic groups are distinguished by the intensity distribution in the pattern. However, identification of individual polytypes in each group is difficult because the Kikuchi bands characteristic of each polytype are weak. In the case of 1:1 phyllosilicates and one-layer chlorite, the four and six groups, respectively, can be distinguished by the Kikuchi bands corresponding to reflections with h ≠ 3n and k = 3n (orthohexagonal cell setting). Practical application for identifying the polytypic groups distributed in a small crystal of cronstedtite, an iron-bearing trioctahedral 1:1 phyllosilicate, is described.

Orientational order-disorder behavior of NH 4 + is a well-established phenomenon in certain ammonium halides and ammonium salts. Anticipating similar behavior of ammonium in the interlayer site of tobelite, synthetic ND 4 /NH 4 tobelite has been studied by infrared spectroscopy from room temperature down to 20 K. Both ND 4 and NH 4 are found to occupy the interlayer site in this mica structure. The spectra show noticeable changes on cooling. Autocorrelation analysis reveals distinct changes in the line width of the central autocorrelation peak. This is attributed to the transition of the ND 4 + group from an orientationally disordered state at higher temperatures to a relatively ordered state below a critical temperature of 140 K. The order parameter for the transition follows classical second-order behavior as a function of temperature.

This first X-ray Photoelectron Spectroscopy (XPS) study of loellingite (FeAs 2 ) reveals a strong Fe 2p 3/2 singlet peak at 707.0 eV signifying that fully coordinated Fe of the bulk phase has the same oxidation state of bulk Fe in marcasite, arsenopyrite, and pyrite. The As 3d peak of loellingite (41.1 eV) demonstrates that As atoms of As dimers in loellingite have the same oxidation state as As atoms of As-S dimers in arsenopyrite. These XPS data consequently corroborate previous Mössbauer studies and confirm theoretical considerations; Fe of loellingite, arsenopyrite, and marcasite is divalent (formally) rather than being trivalent (arsenopyrite) or tetravalent (loellingite). Arsenic is present as As 1- (formal oxidation state) in both loellingite and arsenopyrite. The presence of an intense Fe 2p 3/2 singlet peak for loellingite, rather than a multiplet set of peaks, demonstrates that all valence electrons of Fe in the bulk phase are paired; loellingite is a diamagnetic material thus resolving issues related to magnetic studies of the mineral. A multiplet set of peaks explains the weak high binding energy tail of the Fe 2p 3/2 spectrum. This set of peaks is derived from Fe surface species. As for pyrite surfaces, Fe surface sites are of C 4v symmetry due to loss of a ligand during fracture. The symmetry stabilizes the d z² orbital leading to an intermediate spin state with unpaired electrons in Fe valence orbitals and a multiplet set of peaks. A second, weak set of multiplet peaks, probably representing Fe 3+ bonded to As are also present, suggesting that a minor proportion of surface Fe 2+ and arsenic undergo an auto-redox reaction in response to fracture. The As 3d spectrum includes two spin-orbit-split doublets. One doublet represents As of As-As dimers located within the bulk phase (fully coordinated). The second is located at somewhat greater binding energy than the bulk contribution and may represent a surface polymeric As contribution.

A single-pass flow-through experiment was conducted to induce serpentinization, and to relate serpentine mineralogy to reaction progress during open-system alteration of San Carlos olivine by air-saturated water. The fluid flowed at 300 °C and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell. Chemical analysis of the fluids indicated a steady-state composition with respect to Mg and pH after 550 hours, which was interpreted to reflect saturation with brucite and one or more serpentine-group minerals. The alteration products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by TEM using platinum/carbon replica techniques. After reaction, olivine fragments displayed clear signs of dissolution, and growth of neoformed phases. Brucite and magnetite were distributed uniformly through the reaction cell, although magnetite was more abundant. There is, however, a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell. Lizardite is the dominant serpentine phase at the inlet, whereas fibrous serpentines are predominant near the outlet. Lizardite crystals are more numerous (but smaller) in the middle of the cell, and are topotactically aligned along a single preferential crystallographic orientation of the olivine. Cylindrical chrysotile formed predominantly on the surfaces of lizardite, conical or polygonal serpentine. The morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions, and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation.

The bulk diffusivity of dissolved carbon dioxide (CO 2 and CO 3 2- ) in NaAlSi 3 O 8 + nNa 2 O (n = 0- 6.87 wt%) and in NaAlSi 3 O 8 + nH 2 O (n = 0-2 wt%) melts was investigated at 1523 K and 0.5 GPa using the diffusion couple technique. CO 2 contents of the starting glass pairs varied between 0 and 0.2 wt%. Symmetrical concentration-distance profiles of bulk CO 2 were determined by infrared spectroscopy. An error function was fitted to the profiles to obtain apparent chemical diffusion coefficients of bulk CO 2 . In the investigated compositional range, the diffusivity of bulk CO 2 increases exponentially with Na 2 O and H 2 O content and thus exponentially with the ratio of nonbridging oxygen atoms per tetrahedral cations (NBO/ T). The bulk CO 2 diffusivity increases from logD CO₂ = -11.38 (D CO₂ in m2/s) in NaAlSi 3 O 8 melt to logD CO₂ = -10.92 in NaAlSi 3 O 8 melts containing 6.87 wt% Na 2 O excess, and to logD CO₂ = -10.91 in NaAlSi 3 O 8 melts containing 2 wt% H 2 O. These data imply that either: (1) the diffusivities of the CO 2 species (molecular CO 2 and CO 3 2- ) are very similar, or (2) the speciation of CO 2 in the quenched glasses is very different from the speciation in the melt.

This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled f O₂ in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen sensor membrane made of platinum to measure f H₂ , and therefore f O₂ , and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been tested successfully at temperatures up to 1250 °C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at geologically relevant conditions. We used the new technique to determine the effect of f O₂ on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water solubility with decreasing f O₂ from MnO-Mn 3 O 4 to QFM buffer conditions in the pressure range 50-200 MPa. Kinetic problems in crystallization experiments in basaltic systems and the duration necessary to attain equilibrium Fe 2+ /Fe 3+ ratio in the charge are discussed.

Intracellular magnetite (Fe 3 O 4 ) crystals produced by magnetotactic bacteria strain MV-1 are in the single-domain size range, and are chemically pure. We have previously suggested that they exhibit an unusual crystal habit described as truncated hexa-octahedral. Such a crystal morphology has not been demonstrated for any inorganic population of magnetite, nor would it be expected, given considerations of symmetry and free energy. By inference, this morphology is a physical signature of a biological origin. Here we report data from transmission electron microscope (TEM) tomography of such crystals isolated from magnetotactic bacteria, which confirm the unusual geometry, originally proposed from classical TEM tilt imaging.