Jeffrey E. Post

Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge

Pyroxene water contents measured by Fourier transform infrared spectrometry for Mexican and Simcoe (WA, USA) spinel-peridotite xenoliths range from 140 to 528 ppm in clinopyroxenes and 39 to 265 ppm in orthopyroxenes. Correlations between these water contents and major-element compositional data for the pyroxenes, associated spinels, and whole-rock xenoliths demonstrate that these water contents record mantle values that have not been perturbed since the xenoliths were brought to the surface by their host magmas. Broad positive correlations of pyroxene water contents with whole-rock Al2O3 are consistent with water behaving as an incompatible element during peridotite melting. The main control on the range of pyroxene water contents, however, appears to be the redox state of the peridotite, because estimates of oxygen fugacity from Mo « ssbauer (Simcoe) and microprobe data (Mexico) on spinels are negatively correlated with water contents. This is consistent with the dominant mechanism of H incorporation into pyroxene, which is dependent on the oxidation-reduction of iron. Metasomatism of sub-arc mantle-wedge peridotites by oxidized fluids or melts rising from the slab raises the oxygen fugacity of the peridotites, and where temperature is high enough, induces them to partially melt. The oxidation, in turn, lowers the solubility of water in the peridotite minerals, causing more than half of the original water to be expelled. That water enters the hydrous partial melts and these ascend through the lithosphere to feed the arc magmatic system in the upper crust. Low water contents in pyroxenes from sub-arc mantle-wedge peridotites, such as those from Simcoe and some western Mexican sites, therefore appear to be complementary to the high water contents that characterize subductionzone magmas and fuel their explosive eruptions. An estimate of water budget in subduction zones, however, indicates that the amount of water coming from the dehydration of mantle-wedge anhydrous minerals probably accounts for less than 5% of the total water present in subduction-related magmas. The high water contents of arc magmas thus are mainly attributed to fluids or melts from the slab proper. The relatively dry sub-arc mantle wedge appears to be an effective medium through which subducted water is transported from slabs toward the surface. Published by Elsevier Science B.V.

MANGANESE MINERAL FORMATION BY BACTERIAL SPORES OF THE MARINE BACILLUS, STRAIN SG-1 : EVIDENCE FOR THE DIRECT OXIDATION OF MN(II) TO MN(IV)

Abstract The spores of a marine Bacillus bacterium, strain SG-1, are able to oxidize Mn (H) over a wide range of temperatures (0–80°C) and Mn (II) concentrations ( 25 mM), in both low ionic strength N- (2-hydroxyethyl) piperazine-N′-ethanesulfonic acid (HEPES) buffer (HB) and in HEPES-buffered seawater (SW). Using SG-1 spores as a catalyst for manganese mineral formation, and by varying the temperature and Mn (II) concentration at pH 7.4–8.0, a variety of manganese oxide and manganate minerals were formed under environmentally relevant conditions in HB and SW. In general, mixed phases of lower valence state minerals (hausmannite, Mn304; feitknechtite, βMnOOH; and manganite, γMnOOH) formed in HB and SW at high Mn (II) concentrations (10 mM initial), or at high temperatures (70°C), by two weeks. βMnOOH was favored at low temperatures (3°C) and Mn3O4 at higher temperatures (55–70°C). After 1 year of aging, yMnOOH became the dominant or only mineral present at 25 and 55°C. At lower Mn (II) concentrations (initial concentrations ≤100 μM in HB and ≤1 MM in SW), Mn(IV) minerals precipitated. In HB the Mn(IV) minerals most often resembled sodium buserite, evidenced by collapse of a 10 to 7 A phase with air drying at room temperature. In SW both buserite and Mg-rich noncollapsible 10 A manganates were formed. The Mg-rich 10 A manganates did not collapse to 7 A even with baking at 100°C. The oxidation state of the minerals were generally higher in SW (as high as 3.7) than in HB (3.2). Mn (IV) minerals also formed at higher Mn (II) concentrations in SW than in HB. These observed differences between SW and HB may have resulted from differences in the chemical milieu, or because of the marine adapted physiology of the bacterial spores. Under a variety of conditions (HB and SW, 3–55δC) Mn (IV) mineral formation often occurred at pH and Mn (II) concentrations too high to be favorable for the disproportionation of Mn304, or βMnOOH to Mn (IV). The results strongly suggest direct oxidation of Mn(II) to Mn(IV) by SG-1 spores without lower valence intermediates. Considering the environmental relevance of these experiments, direct oxidation of Mn (II) to Mn (IV) by microbes is probably a common process in natural environments.

Stability of 124, 123, and 247 superconductors

Abstract The 124 phase YBa2Cu4O8 is synthesized by solid state reaction at 930°C in P(O2)>62;30 bar, while the 247 phase Y2Ba4Cu7O15−x is favored in the narrow range of P(O2) between 10 and 30 bar. The 123 phase YBa2Cu3O7−δ forms at P(O2) 62;6.6) may be thermodynamically unstable to decomposition into 124 (plus non-superconducting compounds) at all temperatures, with synthesis of 124 at low temperatures and oxygen pressure limited only by kinetics. Since very high oxygen pressures are not required, synthesis of the 124 and 247 phases is relatively easy, this may have considerable practical importance.

THE WIDESPREAD DISTRIBUTION OF A NOVEL SILICA POLYMORPH IN MICROCRYSTALLINE QUARTZ VARIETIES

An x-ray examination of more than 150 specimens of fine-grained quartz varieties from around the world has revealed that more than 10% and as much as 80% of the silica in many samples is actually moganite, a little-known silica polymorph. Rietveld refinements of 50 powder x-ray diffraction patterns produced by fibrous quartz (agate, chalcedony) and nonfibrous quartz (chert, flint) indicate that the concentrations of moganite within each subgroup are widely distributed. The large amount of moganite (>30%) found in cherts from arid, alkaline environments may resurrect length-slow silica as an indicator of evaporitic regimes, and the absence of moganite in weathered and hydrothermally altered silica samples may be a useful measure of fluid-rock interaction.

Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy

Abstract X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.

Synchrotron powder X-ray diffraction study of the structure and dehydration behavior of palygorskite

Abstract Rietveld refinements using synchrotron powder X-ray diffraction data were used to study the crystal structure and dehydration behavior of sepiolite from Durango, Mexico. The room-temperature (RT) sepiolite structure in air compares well with previous models but reveals an additional zeolitic H2O site. The RT structure under vacuum retained only ~1/8 of the zeolitic H2O and the volume decreased by 1.3%. Real-time, temperature-resolved synchrotron powder X-ray diffraction data and Rietveld refinements were used to investigate the behavior of the sepiolite structure from 300 to 925 K. Rietveld refinements revealed that most of the zeolitic H2O is lost by ~390 K, accompanied by a decrease in the a and c unit-cell parameters. Above ~600 K the sepiolite structure folds as one-half of the crystallographically bound H2O is lost. Rietveld refinements of the “anhydrous” sepiolite structure reveal that, in general, unit-cell parameters a and b and volume steadily decrease with increasing temperature; there is an obvious change in slope at ~820 K suggesting a phase transformation coinciding with the loss of the remaining bound H2O molecule.

Neutron and temperature-resolved synchrotron X-ray powder diffraction study of akaganéite

Abstract Rietveld refinements using neutron powder diffraction data were used to locate H atom positions and obtain a more precise crystal structure refinement for akaganéite [Fe3+7.6Ni2+0.4O6.35 (OH)9.65Cl1.25·nH2O]. Difference Fourier maps clearly showed H atoms positions near those O atoms at the midpoints of the tunnel edges. The O-H vectors point toward the Cl sites at the center of the tunnel, and weak hydrogen bonds likely form between the framework O atoms and Cl. The Cl position is near the center of a prism defined by the eight hydroxyl H atoms. The Cl atoms fill ~2/3 of the tunnel sites, suggesting an ordering scheme in a given tunnel with every third tunnel site vacant. Such an arrangement allows the Cl anions to increase their separation distance along a tunnel by displacing away from one another toward their respective adjacent vacancies. The Fe-O octahedra in akaganéite are distorted with Fe-(O, OH) distances ranging from 1.94 to 2.13 Å and show three longer and three shorter Fe-O distances; as expected the longer distances are associated with the OH- anions. Temperature-resolved synchrotron X-ray powder diffraction data and Rietveld refinements were used to investigate changes in the akaganéite structure and its transformation into hematite as it was heated from 26 to 800 °C. Rietveld refinements revealed surprising consistency in all unit-cell parameters between room temperature and ~225 °C, resulting in nearly zero thermal expansion of the akaganéite structure over a 200 °C interval. Above ~225 °C, the unit-cell volume gradually decreased, primarily in response to decreases in c and b, and an increase in the b angle. The a parameter remained nearly constant until ~225 °C and increased thereafter. Akaganéite started to transform to hematite in the temperature range 290 to 310 °C with no evidence for maghemite as an intermediate phase.

VIBRATIONAL ANALYSIS OF PALYGORSKITE AND SEPIOLITE

Lattice dynamic calculations for the sepiolite and palygorskite structures using polarized Raman and FTIR spectra provide a fundamental basis for interpreting spectral features by assigning vibrational modes. The Si-O stretch and O-Si-O bond bending force constants determined for palygorskite are similar to equivalent values calculated previously for other phyllosilicates. The Mg-O bond stretch values, on the other hand, are about half of those determined for the equivalent Al-O and Mg-O bond stretch environments in other phyllosilicates, suggesting that the bonding within the octahedral ribbons in palygorskite and sepiolite is weaker than that in the continuous octahedral sheets in micas. The weaker bonding allows more flexible octahedral environments in palygorskite and sepiolite, giving rise to higher probabilities for cation substitutions and vacancies relative to the micas. Above ∼700 cm−1 in the IR and 750 cm−1 in the Raman spectra, the eigenmodes are dominated by atomic displacements within the silicate sheets. Below 700 cm−1 the eigenmodes become mixed with motions among the Mg octahedra and the silicate sheets; the eigenmodes assigned to the most prominent peaks in the Raman spectra (near 700 cm−1) belong to this group. As mode frequencies decrease, the corresponding eigenmodes evolve from more localized Mg-O stretch, O-Mg-O bend and O-Si-O bend motions to longer-range motions such as silicate sheet deformations caused by silicate tetrahedra rotation and silicate sheet shearing around the Mg-octahedral sheets.

Time-resolved structural analysis of K- and Ba-exchange reactions with synthetic Na-birnessite using synchrotron X-ray diffraction

Abstract Time-resolved Rietveld refinements using synchrotron X-ray diffraction (XRD) have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. Potassium- and Ba-birnessite, like Na-birnessite, were found to have triclinic symmetry. Rietveld analyses of the XRD patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and β unit-cell parameters, with a decrease of 1.7 and 0.5%, respectively, in unit-cell volume relative to Na-birnessite. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Split electron density maxima with centers at (0 0 0.5) were present for Na, K, and Ba end-members; however, with increased substitution of K+ for Na+, the axis connecting the split-site maxima rotated from an orientation parallel to the b-axis to along the a-axis. Substitution of Ba2+ for Na+ did not result in rotation, but splitting of the interlayer site was more pronounced.

Water in the interlayer region of birnessite: Importance in cation exchange and structural stability

Abstract Birnessite is an important scavenger of trace metals in soils and aqueous environments. The basic birnessite-type structure consists of sheets of Mn octahedra separated by ~7 or ~10 Å (“buserite”) interlayer regions filled with cations and water. Synthetic birnessite-like structures were produced through cation exchange reactions with synthetic Na-birnessite. The unheated, synthetic Mg2+, Ca2+, and Ni2+ layer structures have an ~10 Å interlayer spacing, whereas the other cation-exchanged synthetic birnessites and the related mineral chalcophanite have an interlayer spacing of ~7 Å. The Li+, Na+, K+, Cs+, and Pb2+ synthetic birnessites each contain two to three structurally different water sites, as evidenced by multiple H2O bending and stretching modes in the infrared spectra. The complexity of the water bands in these spectra is likely related to disordering of cations on the interlayer sites. H-birnessite contains structural water and either hydroxyl, hydronium (H3O+), or both. The small difference in the width of the water stretching modes between room temperature and -180 °C indicates that the water molecules in birnessite-like structures are predominantly structurally, rather than dynamically, disordered. Most of the synthetic birnessites, including Na- and K-birnessite, undergo significant water loss at temperatures below 100 °C. There is a linear relationship between the temperature at which most of the water is lost from a given cation-exchanged birnessite and the heat of hydration of the interlayer cation. This finding implies that the interlayer water is strongly bound to the interlayer cations, and plays an important role in the thermal stability of birnessite-like structures.