Roger G. Burns

Applications of the Mössbauer effect to silicate mineralogy—I. Iron silicates of known crystal structure

Abstract Mossbauer parameters are reported for a number of silicates of known crystal struc- ture, containing high-spin iron in a range of co-ordination symmetries. The spectra are fitted by computer to Lorentzian line shapes and the component peaks in each spectrum are assigned to the various positions in the particular silicate structure. The results show that the chemical shift and the quadrupole splitting are sensitive not only to the oxidation state and electronic configuration, but also to the co-ordination number and site distortion. An attempt is made to relate the changes in quadrupole splitting to structural features for the six-co-ordinate Fe 2+ species. Line broadening in such iron compounds is discussed.

The uptake of cobalt into ferromanganese nodules, soils, and synthetic manganese (IV) oxides

Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and δ-MnO2. Previously, it was suggested that Co3+ ions in these minerals replace either Mn3+ or substitute for Fe3+ in incipient goethite epitaxially intergrown with δ-MnO2. Neither of these interpretations is now considered to be satisfactory on account of the large discrepancy of ionic radius between octahedrally coordinated low-spin Co3+ and high-spin Mn3+ or Fe3+ in oxide structures. The close agreement between the ionic radii of Co3+ and Mn4+ suggests that some cobalt substitutes for Mn4+ ions in edge-shared [MnO6] octahedra in many manganese(IV) oxide mineral structures. It is proposed that hydrated cations, including Co2+ ions, are initially adsorbed on to the surfaces of certain Mn(IV) oxides in the vicinity of essential vacancies found in the chains or sheets of edge-shared [MnO6] octahedra. Subsequently, fixation of cobalt takes place as a result of oxidation of adsorbed Co2+ ions by Mn4+ and replacement of the displaced manganese by low-spin Co3+ ions in the [MnO6] octahedra or vacancies.

Kinetics of high-pressure phase transformations: Implications to the evolution of the olivine → spinel transition in the downgoing lithosphere and its consequences on the dynamics of the mantle

Abstract The rate of a high-pressure phase transition increases exponentially with temperature (T) and overpressure or pressure beyond equilibrium (ΔP). It is also greatly promoted by introducing shear stress, diminishing grain size, and adding water or other catalysts to the reactants. For an isothermal and isobaric transition with no compositional change, if steady state of nucleation on grain surfaces is attained, the rate equation can be expressed: (1) before site saturation by: X = 1 − exp(−Kt4), where K = C 1 T 4 exp[− C 2 (C 3 + C 4 ΔP) 2 + C 5 T ][exp( −C 6 T )-exp −(C 7 + C 8 ΔP) T ] 3 and (2) after site saturation by: X = 1 − exp(−K′T), where K/t = C 9 T[exp( −C 6 T )-exp{t-(C 7 + C 8 ΔP T })] , where X is volume fraction of completion of transformation, t is time, and the Cs are characteristic constants. C1 and C9 are functions of grain size, C3 and C6 are functions of shear stress. All the Cs are almost independent of temperature and pressure. Thus, if X as a function of T, ΔP, and t over a narrow P-T range can be experimentally determined, the Cs can be calculated and the effect of grain size and shear stress on the rate of transformation can be evaluated. The isothermal and isobaric rate equations for a given composition, shear stress, and grain size are then experimentally determinable. The non-isothermal and non-isobaric rate equation can be calculated from the isothermal and isobaric ones if the rate of penetration into the metastability field is known. The important feature of the kinetics of high-pressure phase transitions predicted by these rate equations is that for a given rate of penetration into the metastability field, there can be defined a characteristic temperature, Tch, below which the rate of the transition is virtually zero no matter how metastable the material is. For the olivine → spinel transition in the mantle, this characteristic temperature may be as high as 700° C. Thus, in a fast moving downgoing slab, the temperature at its cold center may remain below Tch even down to depths in excess of 600 km, thereby greatly depressing the olivine—spinel phase boundary. At an early stage in the development of a downgoing slab, the plunging speed is slow. This allows the interior of the slab to heat up and the olivine → spinel transition to proceed rapidly and near equilibrium. As a result, the olivine—spinel phase boundary in the slab will be distorted upwards. The rising of the denser spinel phase then provides an additional driving force which accelerates the plate. Since the upper portion of the slab is pulled from below and the lower portion pushed from above, earthquakes of down-dip extension will occur in the upper mantle while those of down-dip compression will originate in the transition zone. Because the transformation occurs close to equilibrium, there will be an aseismic region separating the two seismic zones. When the plate velocity exceeds a certain limit, the temperature in the cold interior becomes low enough to depress the olivine → spinel transition. The phase boundary is then distorted downwards. The buoyant force thereby created will reduce the driving force, and the plunging speed of the plate will approach a steady state. In addition, the buoyant force will compress the slab from below and result in earthquakes of down-dip compression throughout the length of the slab. Now the olivine → spinel transition is so far from equilibrium that the reaction becomes implosive. A rise in frequency of deep earthquakes towards the implosion region in the lower transition zone is thus predicted. Therefore, as well as stabilizing the plate velocity, the olivine → spinel transition may also control earthquake distributions throughout the downgoing slab.

Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars

Abstract Ferric-bearing assemblages on Mars indicate that oxidative weathering of surface basalts has occurred during the evolution of the Red Planet. In aqueous environments chemical weathering proceeded through stages of dissolution of Fe 2+ -bearing silicate and sulfide minerals, oxidation of dissolved Fe 2+ to Fe 3+ ions, and hydrolysis of dissolved Fe 3+ to insoluble ferric-bearing oxide, oxyhydroxide, and hydroxysulphate phases. To determine when these ferrolysis reactions occurred on Mars and to estimate rates of chemical weathering of minerals in Martian surface rocks, experimental data for terrestrial olivines and pyroxenes with compositions resembling assemblages in SNC meteorites are reviewed. Dissolution of ferromagnesian silicates is generally stoichiometric under anoxic conditions. Nonstoichiometric dissolution of pyroxenes may occur initially, both as a result of preferential release of cations in the M2 sites and armoring of crystal surfaces by protective layers of insoluble ferric oxides when dissolution occurs under oxidizing conditions. Dissolution rates of Fe 2+ ions decrease in the order: olivine ≥ pyroxene M2 > pyroxene M1. Dissolution rates of these minerals are highest in acidic solutions, decrease with rising pH and at low temperatures, are least at near-neutral pH, and rise again when pH is in the alkaline range. Since low temperatures currently exist on Mars, dissolution rates of basaltic minerals are probably stoichiometric and extremely slow on the present-day Martian surface, but may have been much faster in the past, especially if acidic groundwater and a more temperate climate prevailed. Ferrous iron released during chemical weathering was metastable with respect to ferric iron when oxygenated groundwater on Mars exceeded pH 3.5. Rates of oxidation of dissolved Fe 2+ ions are strongly pH dependent; they are very slow in acidic solutions, in contrast to mineral dissolution rates. Rates of oxidation also decrease with increasing ionic strength and at low temperatures. However, in saline solutions, rates of oxidation eventually increase with rising ionic strength, including brines near eutectic temperatures that have been proposed on Mars. Rates of oxidation of Fe 2+ dissolved in slightly acidic brines at −25°C, for example, are estimated to be about 10 5 times slower than those of Fe 2+ occurring in terrestrial river water and bottom ocean water. Ferroan saponites precipitated from cold acidic saline solutions containing dissolved Mg 2+ , Fe 2+ , and silica are unstable when exposed to oxygen as a result of intracrystalline redox reactions, such as (Fe 2+ + OH − ) clay + 1 4 O 2 = (Fe 3+ + O −2 ) clay + 1 2 H 2 O . The partially dehydroxylated mixed-smectite plus interlayer ferric oxide assemblages that formed during exposure to the Martian atmosphere may account for difficulties in identifying clay silicates in remote-sensed reflectance spectral measurements of Mars due to loss of identifiable hydroxyl groups. Ferrolysis of dissolved Fe 2+ ultimately yielded hydrous ferric oxide deposits on Mars, the formation of which was influenced by pH, temperature, ionic strength, ion-pair formation, and the rate of oxidation in Martian groundwater. Deposition rates of hydrous ferric oxides into episodic ocean basins that occurred in the past on Mars were comparable to those that produced terrestrial Precambrian iron-formations. On present-day Mars, dissolved Fe 2+ ions may persist indefinitely, particularly in frozen permafrost. However, sublimation and evaporation of day-time equatorial melt-waters may cause localized oxidation of dissolved Fe 2+ ions, leading to the precipitation of nanophase ferric-bearing oxides, hydroxysulfate and partially dehydroxylated clay silicate assemblages that litter bright regions on the surface of Mars.

Kinetics of the olivine→spinel transition: Implications to deep-focus earthquake genesis

Abstract The rate of the olivine→spinel transition at high overpressure increases with diminishing grain size, or increasing shear stress, temperature, and possibly pressure. The transition rate is higher in Fe-rich compositions than in Mg-rich compositions, and it can be greatly increased by adding water or other mineralizers. Of all variables controlling the kinetics of the olivine→spinel transition in the mantle, temperature is the most critical. The olivine→spinel transition can be suppressed below 500°C in Mg-rich compositions, even in geological period of time. Since the temperature within a downgoing slab varies greatly according to different models of calculation, it is not clear at this stage whether the temperature is low enough to suppress the olivine→spinel transition. If the olvine→spinel transition cannot be suppressed, it may not be responsible for the genesis of deep-focus earthquakes. However, the rise of the olivine-spinel boundary in the cold interior of downgoing slabs provides an additional driving force for the plunging of these slabs. The distortion of the olivine-spinel boundary may also control the stress distribution in downgoing slabs and may be responsible for the observed alignment of principal stress axes of deep-focus earthquakes.

Fluxes of metals to a manganese nodule radiochemical, chemical, structural, and mineralogical studies

Fluxes of metals to the top and bottom surfaces of a manganese nodule were determined by combining radiochemical (230Th,231Pa,232Th,238U,234U) and detailed chemical data. The top of the nodule had been growing in its collected orientation at 4.7 mm Myr−1 for at least 0.5 Myr and accreting Mn at 200 μg cm−2 kyr−1. The bottom of the nodule had been growing in its collected orientation at about 12 mm Myr−1 for at least 0.3 Myr and accreting Mn at about 700 μg cm−2 yr−1. Although the top of the nodule was enriched in iron relative to the bottom, the nodule had been accreting Fe 50% faster on the bottom.232Th was also accumulating more rapidly in the bottom despite a 20-fold enrichment of230Th on the top. n nThe distribution of alpha-emitting nuclides calculated from detailed radiochemical measurements matched closely the pattern revealed by 109-day exposures of alpha-sensitive film to the nodule. However, the shape and slope of the total alpha profile with depth into the nodule was affected strongly by226Ra and222Rn migrations making the alpha-track technique alone an inadequate method of measuring nodule growth rates. n nDiffusion of radium in the nodule may have been affected by diagenetic reactions which produce barite, phillipsite and todorokite within 1 mm of the nodule surface; however, our sampling interval was too broad to document the effect. We have not been able to resolve the importance of nodule diagenesis on the gross chemistry of the nodule.

Infrared Study of the Hydroxyl Bands in Clinoamphiboles

Sharp single peaks in the fundamental and first-overtone bands of the O-H stretching vibration in pure Mg2+ and Fe2+ amphiboles split into a maximum of four sharp peaks, corresponding to hydroxyl groups linked to 3 Fe, 2 Fe + Mg, 2 Mg + Fe, and 3 Mg, in mixed Fe2+-Mg2+ amphiboles. Within any one solid-solution series, the frequencies of these peaks can be correlated with the electronegativity of ions in the Ml and M3 positions, and differences between series can be correlated with the size of ions in the M4 position. The O-H vector lies approximately normal to z in the (010) plane. The distribution of Fe2+ and Mg2+ ions between the (M1,M3) and (M2M4) positions in the cummingtonite-grunerite series, and between the (M1,M3) and M2 positions in the tremolite-ferroactinolite series, has also been estimated.

Mineral Mössbauer spectroscopy: Correlations between chemical shift and quadrupole splitting parameters

The variety of coordination numbers, symmetries, distortions and ligand environments in thermally-stable iron-bearing minerals provide wide ranges of chemical shift (δ) and quadrupole splitting (δ) parameters, which serve to characterize the crystal chemistries and site occupancies of Fe2+ and Fe3+ ions in minerals of terrestrial and extraterrestrial origins. Correlations between ferrous and ferric chemical shifts enable thermally-induced electron delocalization behavior in mixed-valence Fe2+-Fe3+ minerals to be identified, while chemical shift versus quadrupole splitting correlations serve to identify nanophase ferric oxides and oxyhydroxides in oxidized minerals and in meteorites subjected to aqueous oxidation before and after they arrived on Earth.

Post-depositional metal enrichment processes inside manganese nodules from the north equatorial Pacific

Interiors of manganese nodules from siliceous ooze beneath the Pacific equatorial high-productivity region, when examined by scanning electron microscopy (SEM) and electron microprobe, display post-depositional recrystallization textures and metalliferous oxide bands (diameter 1–10 μm, 30–40 wt.% Mn, 4–5% Ni, 3–4% Cu). SEM has revealed biogenic siliceous matter in all stages of degradation and dissolution within nodule interiors, creating cavities and voids. Often these miniature vugs contain authigenic phillipsite crystallites which are coated with delicate clusters of crystalline Mn-Fe oxides (todorokite) containing significant amounts of Ni and Cu. We postulate the following diagenetic processes and mechanism of uptake of transition metals inside manganese nodules: (1) palagonite + biogenic silica + pelagic clay → phillipsite + montmorillonite; (2) biogenic matter + amorphous FeOOH or δ-MnO2 → Feaq2+ and/or MnIIMnIV oxide (todorokite); (3) aerated seawater or δ-MnO2 + Feaq2+ → FeOOH and/or todorokite (deposited on phillipsite); (4) (NiII and CuII) organic chelates (adsorbed on clays, etc.) + amorphous FeOOH or δ-MnO2 → Ni-Cu-todorokite + phillipsite, etc. n nThis mechanism explains the well-known positive Mn-Ni-Cu and negative Fe-Ni, Fe-Cu correlations in nodules. By analogy with terrestrial todorokites, which require about 8 wt.% Mn to be in the divalent state to stabilize the crystal structure, as much as 8 wt.% (Ni + Cu) could be accommodated in todorokite-bearing deep-sea manganese nodules. However, although such nodules beneficiate Ni and Cu with respect to marine sediments and seawater, they remain undersaturated in these divalent cations.

Applications of the Mössbauer effect to silicate mineralogy—II. Iron silicates of unknown and complex crystal structures

Abstract Measurements have been made of the Mossbauer spectra of howieite, deerite and two sapphirines, for which the crystal structures are unknown and zussmanite and the alkali amphibole crocidolite. Computer analyses of the spectral data for each silicate have yielded values for the chemical shift and quadrupole splitting for each doublet in the Mossbauer spectra. These parameters are correlated with those for iron ions in silicates of known crystal structures to give information on the oxidation state, electronic configuration, and co-ordination site of each iron atom in the structures. High spin iron is found in all minerals studied. All minerals except one sapphirine contain at least one structurally distinct type of Fe2+ ion in sixfold co-ordination, while deerite contains Fe2+ ions in fourfold co-ordination. Howieite, deerite, and crocidolite contain Fe3+ in sixfold co-ordination, while sapphirines contain Fe3+ in fourfold co-ordination. Computer calculated areas in the Mossbauer spectra have been used to estimate the fraction of each iron species in a structure giving rise to absorption, assuming that the area under a peak is directly proportional to the amount of iron in a site. The results for Fe2+ and Fe3+ containing minerals agree well with chemical analysis values. The area data has allowed Fe2+ site populations to be determined in the crocidolite structure.