Chemistry of iron sulfides.

Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Halloysite clay minerals — a review

The geology of northern and central Africa is characterized by broad plateaux, narrower swells and volcanism occurring from ∼45 Myr ago to the present. The greatest magma volumes occur on the >1,000-km-wide Ethiopian and east African plateaux, which are transected by the Red Sea, Gulf of Aden and east African rift systems, active since the late Oligocene epoch. Evidence for one or more mantle plumes having impinged beneath the plateaux comes from the dynamic compensation inferred from gravity studies, the generally small degrees of extension observed and the geochemistry of voluminous eruptive products1,2,3,4. Here we present a model of a single large plume impinging beneath the Ethiopian plateau that takes into account lateral flow and ponding of plume material in pre-existing zones of lithospheric thinning5. We show that this single plume can explain the distribution and timing of magmatism and uplift throughout east Africa. The thin lithosphere beneath the Mesozoic–Palaeogene rifts and passive margins of Africa and Arabia guides the lateral flow of plume material west to the Cameroon volcanic line and south to the Comoros Islands. Our results demonstrate the strong control that the lithosphere exerts on the spatial distribution of plume-related melting and magmatism.

Cenozoic magmatism throughout east Africa resulting from impact of a single plume

Aquatic surface chemistry: Edited by Werner Stumm. Wiley, New York. 1987. $69.95 (ISBN 0471822951)

Silicon (Si) is the second-most abundant element in the earth's crust. In the pedosphere, however, huge spans of Si contents occur mainly caused by Si redistribution in soil profiles and landscapes. Here, we summarize the current knowledge on the different pools and fluxes of Si in soils and terrestrial biogeosystems. Weathering and subsequent release of soluble Si may lead to (1) secondarily bound Si in newly formed Al silicates, (2) amorphous silica precipitation on surfaces of other minerals, (3) plant uptake, formation of phytogenic Si, and subsequent retranslocation to soils, (4) translocation within soil profiles and formation of new horizons, or (5) translocation out of soils (desilication). The research carried out hitherto focused on the participation of Si in weathering processes, especially in clay neoformation, buffering mechanisms for acids in soils or chemical denudation of landscapes. There are, however, only few investigations on the characteristics and controls of the low-crystalline, almost pure silica compounds formed during pedogenesis. Further, there is strong demand to improve the knowledge of (micro)biological and rhizosphere processes contributing to Si mobilization, plant uptake, and formation of phytogenic Si in plants, and release due to microbial decomposition. The contribution of the biogenic Si sources to Si redistribution within soil profiles and desilication remains unknown concerning the pools, rates, processes, and driving forces. Comprehensive studies considering soil hydrological, chemical, and biological processes as well as their interactions at the scale of pedons and landscapes are necessary to make up and model the Si balance and to couple terrestrial processes with Si cycle of limnic, fluvial, or marine biogeosystems.

http://wwwuser.gwdg.de/~kuzyakov/Sommer_JPNSS_2006_Si_Review.pdf

Silicon pools and fluxes in soils and landscapes—a review

Mineralogy and geochemistry of a sulfuric acid spring water with a pH of 3.37 to 2.89 were investigated to verify the formation processes of iron minerals and the effects of bacteria on their formation. To estimate the solubility of schwertmannite, experimental dissolution in 10.0 mM H2SO4 was conducted and this solubility data was used for geochemical modeling. Experimental incubation of the spring water containing bacteria was also performed and compared with a simulated abiotic system to evaluate the role of bacteria in the mineral formation. The spring water seeps through cracks of hydrothermally altered andesitic rocks containing pyrite, and precipitates schwertmannite and jarosite. Schwertmannite appears as a film-like thin layer floating on the water surface and composed of aggregates of spherical particles with diameters of 1 to 5 μ m. Jarosite is produced as a precipitate on submerged rock surfaces. The precipitate contains well crystallized jarosite spheres 5 to 10 μ m in diameter. Some ellipsoidal to rod shaped bacteria covered or decorated by poorly ordered iron minerals are also present in close association with the schwertmannite spheres. Results of the experimental incubation demonstrate that the oxidation rates of Fe2+ are 5.3 × 103 to 7.2 × 103 times greater than those of the simulated abiotic system, suggesting that the formation of the iron minerals is promoted by bacterial oxidation of Fe2+. The dissolution experiment indicates that the solubility product of the schwertmannite having an average chemical composition of Fe8O8(OH)5.9(SO4)1.05 is approximately log K s = 7.06 ± 0.09. Using this data, geochemical modeling reveals that the spring water is supersaturated with respect to schwertmannite and also goethite and jarosite, but undersaturated with respect to ferrihydrite. Additionally, it is confirmed that the bulk solution chemistry deviates slightly into the stability field of goethite rather than jarosite. This suggests that the aquatic environments in contact with the rock surfaces may be more acidic and/or enriched in SO2−4 relative to the bulk solution, which may eventually lead to the formation of jarosite instead of goethite.

Geochemical modeling of bacterially induced mineralization of schwertmannite and jarosite in sulfuric acid spring water

Smectite and zeolites were formed from a volcanic glass as the products of reaction with NaOH solution at 90°C and 100°C under atmospheric pressure. Formation conditions of smectite and various zeolites were determined by the ratio of the amounts of volcanic glass (g) to NaOH (g) in the solution. Smectite was formed under the condition that the values of weight of volcanic glass (g)/(NaOH(g)/40) are between 0.5 and 6. Fe was an important constituent of the octahedral layer of smectite.

Synthesis of smectite from volcanic glass at low temperature

Experimental alteration of obsidian was performed in 0.001 to 0.5 N NaOH and KOH solutions at 150 and 200 degrees C for 1 to 30 d. The products were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). Changes in chemical composition and pH value of solutions during the reactions were also measured. As the pH of reacting solutions was increased, smectite, phillipsite and rhodesite crystallized progressively in NaOH solutions, while smectite, merlinoite and sanidine grew successively in KOH solutions. In addition, a small amount of less-soluble, poorly ordered boehmite was present as products of all the experiments. Smectite mainly appeared at slightly high pH, Si/Al and Na/K conditions, whereas rhodesite should be produced in extremely high pH, Na/K and Si/Al conditions. Sanidine was also formed in conditions of very high pH and Si/Al and very low Na/K. In intermediate conditions of pH and Si/Al, crystallization of phillipsite was stimulated in solutions of Na/K>10, while formation of merlinoite was favored in conditions of Na/K<1.

Experimental Study on the Formation of Zeolites from Obsidian by Interaction with NaOH and KOH Solutions at 150 and 200 °C

Microbial biomineralization in a weathered volcanic ash deposit from the 1914 to 1915 a.d. eruption of Sakurajima volcano was investigated by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). The solution chemistry of pore water was also analyzed to elucidate saturation conditions. In addition, experimental incubations of bacteria collected from the volcanic ash were performed to confirm bacterial mineralization. TEM revealed that the weathered volcanic ash contains significant amounts of spherical to rod-shaped bacteria ranging from 1.3 × 108 to 2.6 × 108 cell/g, most of which have cell wall surfaces that are completely covered or decorated by either massive aggregates of allophane-like granular materials or irregular aggregates of smectite-like fibers and/or flakes. EDX confirmed that the granular minerals have chemical compositions similar to proto-imogolite allophane, whereas the smectite-like fibers and/or flakes show a wide range of chemical compositions corresponding to the compositional field between allophane and nontronite. The volcanic ash contains about 22 wt% of pore water, which is slightly acidic, relatively low redox potential, and enriched in Si, Na, Cl−, and SO42– ions. The saturation indices (SI) calculated by the PHREEQC geochemical code indicate that the pore water is almost saturated with respect to amorphous Al(OH)3, ferrihydrite, amorphous silica, and cristobalite, and significantly oversaturated with respect to silicate minerals in the order: halloysite < kaolinite < montmorillonite < allophane < nontronite. The allophane-like granular minerals seems to be preferentially precipitated by bacterial interaction with Al and Si ions in the pore water as a metastable phase. The poorly ordered smectite-like fibers and/or flakes may be transformed from the allophane-like materials as a intermediate phase between allophane and nontronite by the driving force originated from the greatest SI value of nontronite. The experimental incubation confirmed that amorphous silica containing a small amount of Fe is formed on the bacterial cell surfaces in liquid media with both Fe and Si ions. Likewise, beidellite-like smectite associated with the bacterial surfaces is produced in liquid media containing both Al and Si ions. However, no minerals are produced in the same media containing no metal ions or no bacteria. These results imply that bacteria play an important role in the accumulation of metal ions and in the formation of silicate minerals during weathering of volcanic ash.

/pdf/microbial-biomineralization-in-weathered-volcanic-ash-3o6scf7mwk.pdf

Microbial biomineralization in weathered volcanic ash deposit and formation of biogenic minerals by experimental incubation

The rehydration properties and behavior of interlayer cations of Ca-, Mg-, Na-, and K-saturated homoionic saponite and vermiculite heated at various temperatures were examined and their rehydration mechanisms elucidated. The most notable features of saponite were (1) except for the Mg- saturated specimen, all saponite samples rehydrated until the crystal structure was destroyed by heating; (2) the rehydration rate in air after heating decreased in the order: K + > Na § > Ca 2+ > Mg2+; (3) the interlayer cations apparently migrated into hexagonal holes of the Sit4 network on thermal dehydration; and (4) the b-parameter expanded on thermal dehydration. The rehydration properties and behavior of interlayer cations of vermiculite were: (1) except for the K-saturated specimen, all vermiculite samples rehydrated until the crystal structure was destroyed by heating; (2) the rehydration rate in air after heating decreased in the order: Mg 2+ > Ca ~+ > Na + > K+; (3) the interlayer cations apparently did not migrate into the hexagonal holes, but remained at the center of the interlayer space, even after thermal dehydration; and (4) except for the K-saturated specimen, the b-parameters of the samples contracted on thermal dehydration. The different rehydration properties of saponite and vermiculite were apparently due to the behavior of the interlayer cations during thermal dehydration. For rehydration to occur, the interlayer cations of saponite had to migrate out of the hexagonal holes. Consequently, saponite saturated with a large cation rehydrated rapidly, whereas saponite saturated with a small cation rehydrated slowly. On the other hand, the interlayer cations of vermiculite remained in the interlayer space; therefore, the rehydration properties of vermiculite were strongly affected by the hydration energies of the interlayer cations. Fur- thermore, electron diffraction patterns suggested that the saponite and vermiculite consisted of random stacking and ordered stacking of adjacent 2:1 layers, respectively. The nature of the stacking of the minerals seemed to be the most important factor controlling the behavior of interlayer cations in the thermal dehydration process.

Motoharu Kawano

Papers

Geochemical modeling of bacterially induced mineralization of schwertmannite and jarosite in sulfuric acid spring water

Synthesis of smectite from volcanic glass at low temperature

Experimental Study on the Formation of Zeolites from Obsidian by Interaction with NaOH and KOH Solutions at 150 and 200 °C

Microbial biomineralization in weathered volcanic ash deposit and formation of biogenic minerals by experimental incubation

Dehydration and rehydration of saponite and vermiculite