Natural speciation of Ni, Zn, Ba, and As in ferromanganese coatings on quartz using X-ray fluorescence, absorption, and diffraction

The mineralogy of natural ferromanganese coatings on quartz grains and the crystal chemistry of associated trace elements Ni, Zn, Ba, and As were characterized by X-ray microfluorescence, X-ray diffraction, and EXAFS spectroscopy. Fe is speciated as ferrihydrite and Mn as vernadite. The two oxides form alternating Fe- and Mn-rich layers that are irregularly distributed and not always continuous. Unlike naturally abundant Fe-vernadite, in which Fe and Mn are mixed at the nanoscale, the ferrihydrite and vernadite are physically segregated and the trace elements clearly partitioned at the microscopic scale. Vernadite consists of two populations of interstratified one-water layer (7 Å phyllomanganate) and two-water layer (10 Å phyllomanganate) crystallites. In one population, 7 Å layers dominate, and in the other 10 Å layers dominate. The three trace metals Ni, Zn, and Ba are associated with vernadite and the metalloid As with ferrihydrite. In vernadite, nickel is both substituted isomorphically for Mn in the manganese layer and sorbed at vacant Mn layer sites in the interlayer. The partitioning of Ni is pH-dependent, with a strong preference for the first site at circumneutral pH and for the second at acidic pH. Thus, the site occupancy of Ni in vernadite may be an indicator of marine vs. continental origin, and in the latter, of the acidity of streams, lakes, or soil pore waters in which the vernadite formed. Zinc is sorbed only in the interlayer at vacant Mn layer sites. It is fully tetrahedral at a Zn/Mn molar ratio of 0.0138, and partly octahedral at a Zn/Mn ratio of 0.1036 consistent with experimental studies showing that the ^VIZn/^IVZn ratio increases with Zn loading. Barium is sorbed in a slightly offset position above empty tetrahedral cavities in the interlayer. Arsenic tetrahedra are retained at the ferrihydrite surface by a bidentate-binuclear attachment to two adjacent iron octahedra, as commonly observed. Trace elements in ferromanganese precipitates are partitioned at a few, well-defined, crystallographic sites that have some elemental specificity, and thus selectivity. The relative diversity of sorption sites contrasts with the simplicity of the layer structure of vernadite, in which charge deficit arises only from Mn⁴⁺ vacancies (i.e., no Mn³⁺ for Mn⁴⁺ substitution). Therefore, sorption mechanisms primarily depend on physical and chemical properties of the sorbate and competition with other ions in solution, such as protons at low pH for Ni sorption.     

Mineral paragenesis of the Kalahari managanese field, South Africa

The mineralogy of the managanese ores of the giant Paleoproterozoic Kalahari manganese deposit of the Transvaal Supergroup has been subject of many studies and up to now 135 different ore and gangue minerals have been described, of which eight represent new mineral species. Through correlation of different mineral assemblages with specific geological events and by determining relative ages of minerals in outcrop, hand specimen and under the microscope an attempt is made in this publication to construct a paragenetic sequence for this complex mineral assemblage. Sedimentation and early diagenesis of the Hotazel Formation, composed of interbedded iron formation and braunite lutite, was followed by low-grade metamorphism and associated stratabound metasomatism. Braunite lutite of sedimentary origin, comprising 97% of the total ore reserve, is composed of braunite, hematite and kutnahorite, and abundant early diagenetic kutnahorite and manganoan calcite forming laminae and ovoids. Fluid flow during late diagenesis or lower greenschist facies metamorphism led to stratabound metasomatic oxidation of Mn-bearing carbonates to hausmannite and Mn-poor calcite. Three structurally controlled hydrothermal alteration events succeed metamorphism. These events are referred to as Wessels, Mamatwan and Smartt events. The Wessels alteration event is the oldest of the three events and it is of great economic importance because virtually all of the high-grade ore (> 42% Mn), 3% of the total ore reserve, formed during this event through alteration of carbonate-rich low-grade Mamatwan-type ore (braunite lutite) to high-grade, carbonate-poor Wessels-type manganese ore. This Wessels hydrothermal alteration event took place in the northwestern part of the Kalahari deposit, associated with a system of major north-south- and minor east-west-striking normal faults. The Mamatwan alteration event is observed throughout the Kalahari manganese deposit but alteration is very localized. Reduction halos and discolouration of braunite lutite around fracture or joint-hosted sulphide-carbonate mineralization are typical of the Mamatwan alteration event. In contrast, the Smartt alteration event is characterized by oxidation of braunite lutite and the formation of todorokite and manganomelane. Syn- and Post-Kalahari supergene alteration has taken place below the suboutcrop of the Hotazel Formation against the calcretized sediments of the Cenozoic Kalahari Formation. Cryptomelane and pyrolusite are the predominant products of surficial weathering.