Virginia Mee Burns

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. The 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. Diffusion 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.

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. This 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.

The mineralogy and crystal chemistry of deep-sea manganese nodules, a polymetallic resource of the twenty-first century

The relatively high concentrations of cobalt, nickel, and copper in deep-sea manganese nodules, such as those occurring on the sea-bed beneath the north equatorial Pacific Ocean, indicate that these marine sediments are potential ore deposits. In order to explain the strong enrichments of Ni, Cu, and Co in the nodules, the crystal chemistries and structures of the host manganese oxide minerals must be understood. Over twenty manganese(IV) oxide minerals are known, but only three predominate in manganese nodules. They are todorokite, birnessite, and delta-MnO2. All MnIV oxides contain edge-shared MnO6 octahedra linked in diverse ways, leading to a hierarchy of structure-types somewhat resembling the classification of silicates. Todorokite is deduced to contain chains of edge-shared MnOe octahedra enclosing huge tunnels, thus resembling hollandite and psilomelane. Birnessite has a layered structure with essential vacancies in the sheets of edge-shared MnO6 octahedra, while δ-MnO2 is a disordered birnessite. The uptake of Co into manganese nodules involves replacement of low-spin Co3+ for Mn4+ ions in the structures, whereas Ni2+ and Cu2+ substitute for Mn2+ ions in octahedra located in the chains or between layers of edge-shared MnO6 octahedra.

Geochemistry and bonding of thiospinel minerals

The thiospinel group of minerals present a variety of interesting problems in mineral chemistry. Carrollite (CuCo2S4), linnaeite (Co3S4), siegenite [(Co, Ni)3S4], polydymite (Ni3S4) and violante (FeNi2S4) occur in ore deposits, daubreelite [(Fe, Mn, Zn)Cr2S4] is present in meteorites and greigite (Fe3S4) is found in lacustrine sediments. This paper illustrates how the crystal chemistry, geochemistry and certain physical properties may be interpreted by molecular orbital and band theories of the chemical bond. In the spinel structure, transition metal ions occur in tetrahedral and octahedral coordinations, and 3d electrons of cations bonded to sulphur atoms are distributed amongst non-bonding and anti-bonding molecular orbitals. The composition ranges, geochemistry, and variations of cell parameters, microhardness, reflectivities and relative stabilities of thiospinels are directly related to the numbers of electrons in antibonding molecular orbitals. Linnaeite, which is the most stable thiospinel mineral in the system Cu-Co-Ni-Fe-S, has the smallest number of antibonding electrons. It has the smallest cell edge, highest reflectivity and largest microhardness. With increasing electron occupancy of the antibonding orbitals, each of these physical properties, together with the thermal stabilities, decrease along the linnaeite-siegenite-polydymite, linnaeite-carrollite and violarite-polydymite series. In each of these minerals, the unusually small cell edge may be correlated with the occurrence of transition metal ions in low-spin states. The metallic conductivity and Pauli-paramagnetism of many of these minerals is related to electron delocalization in antibonding molecular orbitals. Iron cations occur in high-spin states in greigite, giving rise to increased numbers of electrons in antibonding orbitals. As a result greigite has a larger cell edge and lower thermal stability than other thiospinel minerals. The semiconducting and ferrimagnetic properties of greigite indicate that the 3d electrons are more localized on the cation than sulphospinels in the Cu-Co-Ni-Fe-S system. Absence of solid-solution between violarite and greigite is attributed to differing spin-states of octahedral Fe(II) ions, which are low-spin in violarite and high-spin in greigite. The high thermal stability of daubreelite is due to the large octahedral site preference energy of Cr3+ in the structure. The preference of transition metal ions for octahedral coordination is reflected by the ease in which thiospinel phases transform to a cation defect NiAs structure at elevated pressures and temperatures. As a result, polymorphic transitions involving thiospinel phases are postulated in the mantle and in shocked meteorites.

Mineralogy of chromium

Abstract The minerals of chromium are tabulated together with sources of data on their crystal structures and on new or recent occurrences. Cr 3+ ions in octahedral coordination with oxygen and tetrahedral [Cr VI O 4 ] 2− ions predominate in minerals on Earth, whereas lunar minerals contain Cr 2+ and Cr 3+ ions. Meteorites also contain additional rare sulfide and nitride minerals of chromium.