B.I. Kronberg

The chemistry of some Brazilian soils: Element mobility during intense weathering

Abstract Chemical data for 56 elements in soils from three regions of Brazil (Bahia, Amazon, Goias) indicate that intense chemical weathering leads to a product soil essentially in the system, SiO2Al2O3Fe2O3H2O. The major minerals of the leached soils are quartz, kaolinite, gibbsite, goethite and hematite. Trace elements show a wide range of behaviour from those which are enriched as for some bio-important elements (B, Cl, Mo, Se, Sn, I) and refractory metals (Sc, Zr, Nb, Gd, Tm, Th) to the majority which are severely leached. Trace-element behaviour appears to be largely controlled by the dominant clay or Al2O3SiO2 minerals, or the degree of weathering. Consideration of the chemistry of lateritic soils and groundwaters indicates that the development of deep lateritic profiles requires times in the order of tens of millions of years in a very stable continental setting.

Upper Pleistocene Amazon deep-sea fan muds reflect intense chemical weathering of their mountainous source lands

Abstract Upper Pleistocene muds of the Amazon deep-sea fan contain an abundance of residual weathering products, as indicated by their bulk chemical and mineralogical compositions. The muds are chemically more mature than average shale, average pelagic mud, or the surrounding Demerara Abyssal sediments. They are, however, very similar to the bulk chemical compositions of contemporary suspended sediments of both the Amazon Estuary and those carried by the major Andean tributaries. The modern sediment compositions reflect the rapid weathering now occurring in the Andean source regions of the basin. The major-element geochemical signature of the Upper Pleistocene deep-sea fan sediments suggests that intense chemical weathering in the mountainous source lands has controlled the chemistry of Amazon River suspended sediments since at least the Upper Pleistocene. Minor- and trace-element signatures appear uniform throughout the fan and element concentrations are in general higher than in the intensely weathered Amazon lowland soils but lower than in deep-sea sediments.

Model for bauxite formation: Paragominas (Brazil)

Abstract Samples from a profile of the Paragominas bauxite (Brazil) were analyzed extensively. Principal minerals were determined as well as the concentrations of 56 elements. The major-element chemistry can be described in terms of the oxides and hydroxides of Al, Si, Ti and Fe, bound mineralogically as gibbsite, kaolinite and hematite. 30 of the 45 minor and trace elements analyzed often maintain their average crustal concentrations. Depletions are most obvious for the alkali metals, the alkaline earths and the metals of the first-row transition series. Enrichment is observed for B, Sc, Zr, Nb, Sn, Sb, I, Hf and Th. Differences in concentration patterns for some refractory metals (Al, Ti, Zr, Hf, etc.) lead to the suggestion that some (e.g., Ti, Zr, Hf) of these metals may be participating in biological reactions. The economic bauxite zone is bracketed above and below by kaolinitic horizons, and this succession of alteration could be explained by extraordinarily high flow rates of meteoric waters for millions of years through highly permeable terrains. The formation and preservation of these highly leached profiles also require a quiescent tectonic setting for 10–100 Ma. Such a model is consistent with conditions which may have prevailed in eastern Amazonia for at least 100 Ma.

Global tectonics and agriculture: A geochemical perspective

Abstract The global distribution of regions of sustained productive agriculture shows that an ample supply of fresh rock residues is vital to soil fertility. Once such rock debris is removed soil degradation will be evident in falling yields and trends within the clay fraction from chemically diverse smectite clays towards impoverished kaolinite-gibbsite soils. Increased erosion will follow the clay degradation. Reliance on N, P and K and organic residues alone will not be sufficient to counteract this trend unless Nature, through geological forces, or Man, through the application of appropriate rock dusts, intervenes to restore nutrient balance and a better clay spectrum. The geological evidence of natural weathering rates, and soil evidence of leaching rates, points to applications only of the order of 1 ton ha−1 year−1 being the amount of fresh rock replenishment required to hold the soil in balance for agriculture. Failure to achieve this makes way for a progressive trend towards soil degradation accelerating, as changes induced in the nature of the clay fraction enhance the leaching effect of the aerial environment. On a world scale these effects can best be seen by comparing the enhanced fertility of soils that have benefited from volcanic renewal near active plate boundaries or the fresh rock enrichment arising from the rock grinding involved in glaciation, with the almost ‘impossible’ soils typical of geologically quiescent areas, such as the Amazon and Congo basins where, merely to fertilise with N, P and K alone so impoverishes the soils of their critically limited supplies of other essential nutrients, that disaster is inevitable.

Semi-quantitative ESCA examination of coal and coal ash surfaces

Four North American coals and their ashes were examined using the direct, surface sensitive technique of X-ray photoelectron spectroscopy (ESCA). Two US National Bureau of Standards reference coals (SRM 1632a, SRM 1635) and one ash (SRM 1633a) were used for instrument calibration. These results verified the semi-quantitative nature of ESCA and its previously determined element detection level of ≈ 10−9 gm cm−2 of surface (≈0.1 bulk wt%). Thus, major elements and surface concentrated trace elements were detectable for these samples. ESCA detected elements present in the coal and/or ash in different chemical environments, for example sulphur as the sulphide or sulphate and carbon as graphite, carbonyl, carboxyl or hydrocarbon. The results of this preliminary study indicate that ESCA is useful to elucidate element siting within coal; information that is difficult to determine by analytical techniques requiring sample destruction prior to analysis. Several raw coals contained fluorine and chlorine near the one per cent level. Ashing effectively concentrates the mineral fraction of a coal by an order of magnitude resulting in additional elements being detected by ESCA. The resultant detection level of elements in the original coal is thus lowered to ≈0.01 bulk wt% by ashing. It was found that fluorine and sulphur were highly concentrated on several coal ash surfaces possible indicating surface Sorption reactions during combustion. Application of ESCA in coal geochemistry and mineralogy is indicated by this study. ESCA has the potential to measure elements at the minor and possibly trace level and confirm element siting within coal and coal ash. In addition, it may be possible to identify coals from different deposits by their unique broad scan ESCA spectra fingerprints.

Distributions of trace elements in Western Canadian coal ashes

Abstract Concentrations of 52 minor elements in coal ash were determined using spark source mass spectroscopy. Hg levels in raw coal were investigated by cold vapour atomic absorption spectrophotometry. The concentration of elements are compared to other available data and to levels in the Earths crust. F levels in coal ash exceed 500 μ g −1 and may be greater than 1 wt% in raw coal. Approximately half the elements (B, S, Ni, Zn, Ga, Ge, Se, Sr, Y, Mo, Sn, Sb, I, Ba, Pr, Nd, Sm, Eu, Ho, Hf, Pt, Hg, Pb, Tl, Bi, U) investigated are enriched in the coal ash with respect to the Earths crust. The ranges in minor element concentrations in coal ash and coal from different global regions are very similar.

SIMS imaging in the study of coal surfaces

Abstract With SIMS imaging of coal macerals it was possible to distinguish organic and inorganic domains. In the study presented here three phases could be identified—an aluminosilicate phase, an organic rich region and a second inorganic phase rich in Ca. The results show that SIMS imaging is a powerful technique for determining in situ element distributions.

Geochemistry and genesis of sulphide-anhydrite-bearing Archean carbonate rocks from Bahia (Brazil)

Abstract Archean carbonate rocks hosting sulphide-sulphate (anhydrite) phases are found in association with high-grade metamorphic rocks in the Caraiba Complex (central Bahia State, Brazil). The S-bearing rocks appear in drill-core samples in association with a meta-sedimentary (marble, calc-silicate, metapelite) sequence. δ 18 O values (+12‰ SMOW) are similar to those found in other Precambrian carbonates. δ 13 C values range from −6.5 to −12‰ PBD. A model for anhydrite genesis is not obvious. Low Fe levels, isotopic data and lithologic affinities with Archean carbonate sequences suggest the possibility of an evaporitic origin. Evidence of decarboxylation reactions and local accumulations of graphite suggest the possibility of sulphide formation by sulphate reduction. In any case, the extensive role of Archean life systems in the Caraiba paleoenvironment is evident.

PHOSPHATE RELEASE FROM SYNTHETIC GLASSES AND INHIBITION OF PHOSPHATE FIXATION ON FERRIC HYDROXIDE

Abstract Release of phosphate from glasses in the systems basalt-K3PO4 granite-K2SiO3 Na2SiO3-apatite has been studied. Equilibrium leachate concentrations in water are attained in 3-5 hr (K+, 5-60 μg cm-3; PO4 3-, 2-15 μg cm-3). Glass composition, pH, and grain size influence solubility. Phosphate fixation on freshly precipitated Fe(OH)3 and on iron-rich laterites appear to be inhibited when glasses are rich in Na-K silicates. This may be due to surface modification of Fe(OH)3 by the formation of sodium ferric silicates which are known to be stable at low temperatures.

Ferromanganese Nodules from Lake Ontario (Bay of Quinte): Minor Element Geochemistry

Abstract Spark source mass spectroscopy was used to analyze 61 elements in ten ferromanganese nodules found near Glenora in the Bay of Quinte at the eastern end of Lake Ontario. Most minor elements, including As, Pb, and Hg, have concentrations between 1–100 μg g−1. F, S, Co, Zn, and La have concentrations in 100 μg g−1 range. Ba and Sr are present at levels of 1% and 0.1% respectively. Compared to similar measurements on nodules found in the Great Lakes and in other parts of the globe, values reported here are generally lower. Compared to their marine equivalents, lake nodules appear to be inferior scavengers of minor elements. Examination of all available data corroborates the postulate that marine biological material is an important source of minor elements found in oceanic nodules.