Geology topics

David M. Sherman

Publications and source records attributed to David M. Sherman.

11 recordsLinked to original sources

USGS2012

The nature of the pressure-induced metallization of FeO and its implications to the core-mantle boundary

The pressure and temperature-induced metallization of FeO discovered by Knittle et al [1986] is here argued to result from a Mott transition associated with increased Fe(3d)-Fe(3d) orbital overlap at high pressures. The metallic bonding in the Fe(3d) t 2g band may account for the 4% volume decrease of FeO associated with the metallization transition. If so, a structural change (B1→B2 or B1→B8) or spin-pairing transition may not need to be invoked to explain the high pressure phase transition in FeO. Below the Neel temperature of FeO, antiferromagnetic ordering of Fe spins forces the Fe (t 2g ) electrons to be localized. Since the Neel temperature increases with pressure, no metallization transition of FeO was observed by Yagi et al. [1985] in their high-pressure measurements at 300K. Neither (Mg, Fe)O and (Mg,Fe)SiO 3 can undergo a Mott transition at high pressure and temperature. Consequently, it is here argued that a lower mantle containing only these phases should be electrically insulating. Finally, the formation of itinerant d-electrons in FeO may be a necessary, if not sufficient, condition for the apparent alloying of FeO with Fe. Such alloying may allow oxygen to be incorporated into the outer core.

Geophysical Research Letters↗

USGS2012

The high-pressure electronic structure of magnesiowustite (Mg, Fe)O: applications to the physics and chemistry of the lower mantle

The electronic structure of magnesiowustite is investigated using self-consistent field X α scattered wave (SCF- X α-SW) molecular orbital calculations on (FeO 6 ) 10− and (FeMg 12 O 14 ) 2− clusters. Calculated one-electron transition energies are used to interpret the optical spectrum of (Mg, Fe)O. The results are applied to the electrical and thermal conductivity of the lower mantle. The spin pairing of Fe 2+ and the effect of pressure on bonding in magnesiowustite, with some inferences regarding the incorporation of oxygen in the outer core, is also addressed. The approach used here appears to give a reliable description of the energy and pressure dependence of the spin-allowed 5 T 2 g → 5 E g ligand field transition and the spin-pairing transition of Fe 2+ in (Mg, Fe)O. However, the oxygen to metal charge transfer transitions in (Mg, Fe)O are not as reliably determined insofar as the p - d band gap varies with cluster size and the energies of the charge transfer states cannot be found without including configurational interaction. Nevertheless, it is argued that the charge transfer transitions that are intrinsic to (Fe, Mg)O are of a sufficiently high energy to be irrelevant to the electrical and thermal conductivity of the lower mantle. This is especially true if Fe 2+ adopts the low-spin configuration. The geophysically significant properties of (Fe, Mg)O probably result from defect Fe 3+ .

Journal of Geophysical Research↗

USGS2012

Chemical bonding in the outer core: high-pressure electronic structures of oxygen and sulfur in metallic iron

From its density the outer core is believed to be an alloy of iron and a light element such as sulfur or oxygen. The nature of the light element in the core is an important constraint for theories of the Earth's formation. In this paper the electronic structures of oxygen and sulfur impurities in metallic iron are investigated to determine if pressure, temperature, and composition-induced changes in bonding might affect phase equilibria along the Fe-FeS and Fe-FeO binaries. The electronic structure of sulfur in metallic iron is consistent with the miscibility between Fe and FeS liquids. Volume compression strengthens the Fe-S bond, and it is expected that at sufficiently high pressure, sulfur can substitute for Fe and give solid solution behavior between Fe and FeS. In contrast, the electronic structure of oxygen in metallic iron shows that oxygen cannot act as a substitutional impurity (replacing Fe). This explains the observed miscibility gap on the Fe-FeO binary at 1 atm pressure. Volume compression does not greatly change the electronic structure if oxygen substitutes for iron in bcc and fcc iron. Iron-oxygen bonding does occur, however, if oxygen occupies interstitial sites. Insofar as the molar volume of FeO incorporated as interstitial oxygen in metallic iron is smaller than that of pure FeO, the incorporation of oxygen into metallic iron may be favored under the pressures of the Earth's core.

Journal of Geophysical Research↗

USGS1995

Mineralogical and geochemical evolution of micas from miarolitic pegmatites of the anorogenic pikes peak batholith, Colorado

A suite of 29 micas from miarolitic pegmatites associated with granitic units of the anorogenic Pikes Peak batholith (1.08-1.02 Ga), Colorado range in composition, and follow in paragenetic sequence, from 1M siderophyllite (N = 1), and 3T or 2M1 lithian biotite (N = 5) to 1M zinnwaldite (N = 20) and 1M ferroan lepidolite (N = 1). Locally, 1M (?) phlogopite (N = 1) and ferroan 2M1 muscovite (N = 1) are also present. Pervasive, late-stage hydrothermal alteration along with possible supergene weathering of many of these micas produced vermiculite. Additionally, some vugs and cavities were filled with chlorite and/or smectite. Early crystallized micas form tapered columnar crystals in graphic pegmatite, growing toward, and adjacent to the miarolitic cavity zone which contains the later crystallized micas. Principal associated minerals are quartz, microcline perthite (mostly amazonite), and albite, with local topaz or fluorite, and rarely tourmaline (schorl-elbaite). Progressively younger micas of the main crystallization sequence display increasing Si, Li, F, and Al/Ga, and decreasing total Fe, Mg, and octahedral occupancy. The zinc content of all micas is considerably elevated, whereas Mn, Rb, Cs, and Sc are moderate and T1 is very low. Early siderophyllite and lithian biotite show a narrow range of FeO/Fe2O3 (5.6-8.0), whereas later zinnwaldite is much more variable (2.4-40.3). Annite of the host granite and early graphic pegmatite is compositionally homogeneous, but most mica crystals from cavities show remarkable compositional and abrupt, sharp and distinct color zoning. Most cavity-grown zinnwaldite crystals show a decrease, from core to rim, in total Fe and Mg, whereas Si, Li and F increase and Mn, Rb, Cs and Na are essentially constant. A few to more than 100 color zones have been identified in some mica crystals. The zones are well correlated with the Ti content (<0.2 wt. % TiO2 colorless, 0.4-0.6 wt.% TiO2 red-brown). The total Fe content may or may not correlate with color zoning, whereas Zn variations (up to 1.1 wt. %) are entirely independent. The dark color zones probably reflect Fe-Ti charge transfer. The mica composition sequence described here is typical of the extreme fractionation observed in pegmatites of the NYF family, associated with anorogenec granites. Elevated Fe, Zn, and enhanced Sc contents are characteristic of this family. Strong enrichment in Li, Rb, and F is present, particularly in the micas of the miarolitic cavities. Sharp color zonation and compositional variation in cavity-grown zinnwaldite and ferroan lepidolite crystals suggest rapid changes in the intensive parameters, particularly the f(O2), of the parent fluid during the final stages of pegmatite consolidation ?? 1995 Springer-Verlag.

Mineralogy and Petrology↗

USGS1992

High-temperature heat capacity of Co3O4 spinel: thermally induced spin unpairing transition

A strong anomaly was found in the heat capacity of Co3O4 between 1000 K and the decomposition temperature. This anomaly is not related to the decomposition of Co3O4 to CoO. The measured entropy of transition, ??S=46??4 J mol-1 K-1 of Co3O4, supports the interpretation that this anomaly reflects a spin unpairing transition in octahedrally coordinated Co3+ cations. Experimental values of heat capacity, heat content and entropy of Co3O4 in the high temperature region are provided. The enthalpy of the spin unpairing transition is 53??4 kJ mol-1 of Co3O4. ?? 1992 Springer-Verlag.

Physics and Chemistry of Minerals↗

USGS1992

Evidence for ammonium-bearing minerals on Ceres

Spectra obtained from recent telescopic observation of 1-Ceres and laboratory measurements and theoretical calculations of three component mixtures of Ceres analog material suggest that an ammoniated phyllosilicate is present on the surface of the asteroid, rather than H 2 O frost as had been previously reported. The presence of an ammoniated phyllosilicate, most likely ammoniated saponite, on the surface of Ceres implies that secondary temperatures could not have exceeded 400 kelvin.

Science↗

USGS1990

Molecular orbital (SCF-X-α-SW) theory of Fe 2+- Mn 3+ , Fe 3+- Mn 2+ , and Fe 3+- Mn 3+ charge transfer and magnetic exchange in oxides and silicates

Metal-metal charge-transfer and magnetic exchange interactions have important effects on the optical spectra, crystal chemistry, and physics of minerals. Previous molecular orbital calculations have provided insight on the nature of Fe2+-Fe3+ and Fe2+-Ti4+ charge-transfer transitions in oxides and silicates. In this work, spin-unrestricted molecular orbital calculations on (FeMnO10) clusters are used to study the nature of magnetic exchange and electron delocalization (charge transfer) associated with Fe3+-Mn2+, Fe3+-Mn3+, and Fe2+-Mn3+ interactions in oxides and silicates.

American Mineralogist↗

USGS1987

Molecular orbital (SCF-Xα-SW) theory of metal-metal charge transfer processes in minerals

A number of mixed valence iron oxides and silicates (e.g., magnetite, ilvaite) exhibit thermally induced electron delocalization between adjacent Fe 2+ and Fe 3+ ions and optically induced electronic transitions which are assigned to Fe 2+ →Fe 3+ intervalence charge transfer. In this paper, the mechanism of electron delocalization (i.e., polarons versus itinerant electrons) and the nature of optically induced intervalence charge-transfer in minerals are investigated using molecular orbital theory. SCF-Xα-SW molecular orbital calculations were done for several mixed-valence (Fe 2 O 10 ) 15− clusters corresponding to edgesharing Fe 2+ and Fe 3+ coordination polyhedra. A spinunrestricted formalism was used so that the effect of ferromagnetic versus antiferromagnetic coupling of adjacent Fe 2+ and Fe 3+ cations could be determined. The molecular orbital results can be related to the polaron theory of solid state physics and the perturbation theory formalism used by Robin and Day (1967) and others to describe electron transfer in mixed valence compounds. Intervalence charge-transfer results from the overlap of Fe(3d) orbitals across the shared edges of adjacent FeO 6 polyhedra to give weak Fe-Fe bonds. Electron delocalization, however, requires that adjacent Fe cations be ferromagnetically coupled. Antiferromagnetic coupling results in distinguishable Fe 2+ and Fe 3+ cations. Electronic transitions between the Fe-Fe bonding and Fe-Fe antibonding orbitals results in the optically-induced intervalence charge transfer bands observed in the electronic spectra of mixed valence minerals. Such transitions are predicted to be polarized along the metal-metal bond direction, in agreement with experimental observations.

Physics and Chemistry of Minerals↗

USGS1987

Molecular orbital (SCF-Xα-SW) theory of metal-metal charge transfer processes in minerals - II. Application to Fe 2+ --> Ti 4+ charge transfer transitions in oxides and silicates

A molecular orbital description, based on Xα-Scattered wave calculations on a (FeTiO 10 ) 14− cluster, is given for Fe 2+ → Ti 4+ charge transfer transitions in minerals. The calculated energy for the lowest Fe 2+ → Ti 4+ metal-metal charge transfer transition is 18040 cm −1 in reasonable agreement with energies observed in the optical spectra of Fe-Ti oxides and silicates. As in the case of Fe 2+ → Fe 3+ charge transfer in mixed-valence iron oxides and silicates, Fe 2+ → Ti 4+ charge transfer is associated with Fe-Ti bonding across shared polyhedral edges. Such bonding results from the overlap of the Fe( t 2 g ) and Ti( t 2 g ) 3 d orbitals.

Physics and Chemistry of Minerals↗

USGS1986

Cluster molecular orbital description of the electronic structures of mixed-valence iron oxides and silicates

A molecular orbital description, based on spin-unrestricted X α-scattered wave calculations, is given for the electronic structures of mixed valence iron oxides and silicates. The cluster calculations show that electron hopping and optical intervalence charge-transger result from weak FeFe bonding across shared edges of FeO 6 coordination polyhedra. In agreement with Zener's double exchange model, FeFe bonding is found to stabilize ferromagnetic coupling between Fe 2+ and Fe 3+ cations.

Solid State Communications↗

USGS1985

Electronic spectra of Fe3+ oxides and oxide hydroxides in the near IR to near UV.

Optical absorption and diffuse reflectance spectra of several Fe2O3 and FeOOH polymorphs (hematite, maghemite, goethite, lepidocrocite) in the near-IR to near-UV spectral regions (2000-200 nm) are presented. The spectra consist primarily of Fe3+ ligand field and ligand-to-metal charge-transfer transitions.-J.A.Z.

American Mineralogist↗