Phyllosilicate Mineralogy Research Articles

Linkage between aggregate stability of granitic soils and the permanent gully erosion in subtropical China

As an integrated parameter for soil structure and function, aggregate stability is crucial to soil quality and erodibility, resulting in extensive studies on the formation and stability mechanisms of aggregates, especially for heavy-textured soils. Granitic soils with a wide range of particle composition are concentrated by large-scale permanent gullies in subtropical China, while limited information is available on the aggregate structure of granitic soils. To investigate the formation mechanism of these gullies, granitic soils were sampled systemically from different genetic horizons from temperate (no gully erosion) and subtropic (concentrated by gully erosion) areas, and the aggregate size distribution and stability of these soil were measured by Yoder method with sand correction. The results showed that macroaggregate (especially the 0.25 ~ 2 mm-sized fraction) occupied 56.92% of the aggregation. The disaggregation reduction (DR) ranging from 0.18 to 1.12 mm was shown to be more reasonable than the mean weight diameter in characterizing the aggregate stability of granitic soils. Additionally, macroaggregate and DR were unveiled to increase with the increase of weathering degree, implying that the humid climate with high rainfall conditions in the subtropic areas was responsible for the formation of the permanent gullies at a regional scale. The formation and stability of aggregates were generally determined by clay content (R2 = 30.6%), soil organic carbon (R2 = 20.1%), free aluminum oxides (R2 =11.1%), and free iron oxides (R2 =6.7%). Among these influential factors, clay fraction associated with aggregates in a size of > 2 mm was a promising variable for predicting DR (R2 =75%, p < 0.001). Collectively, the aggregate structure of granitic soils is directly determined by inorganic cementing matters controlled by the variation of phyllosilicate mineralogy under different rainfall conditions, while the coarse texture of granitic soils emphasizes the contribution of organic cementing agent (SOC). This study facilitates the understanding of aggregate structure and its stability mechanism of coarse-grained soils as well as erosion control in granitic soil areas.

Quantifying the contribution of phyllosilicate mineralogy to aggregate stability in the East Asian monsoon region

Aggregate stability strongly influences soil function and has been a key indicator of soil quality and susceptibility to erosion. Although clay mineralogy has been recognized to have substantial effects on aggregate stability, little quantitative information is available on the specific contribution of various phyllosilicates in clay fraction. This study aimed to quantify the individual and combined effects of phyllosilicate minerals and their interaction with other soil properties on aggregate stability and breakdown mechanisms. A wide range of soil types in heavy textures from quaternary sediments were sampled under three land uses (forest, grass and arable) and at different genetic horizons along the mid-temperate to tropical climate gradient in the East Asian monsoon region. Soil aggregate stability against slaking (MWDFW), mechanical breakdown (MWDWS) and differential swelling (MWDSW) was determined, as well as phyllosilicate mineralogy by X-ray diffraction and a series of soil properties related to porosity, texture, organic matter, sesquioxides, and exchangeable cations. Aggregate stability generally exhibited a unimodal geographic trend (MWDSW in 1.85 ~ 3.49 mm, MWDWS in 0.76 ~ 3.22 mm and MWDFW in 0.14 ~ 2.74 mm), which became more remarkable with increased disruptive forces (coefficient of variation in 9 ~ 59%). Correspondingly, the dominant mechanisms of aggregate breakdown shifted from both slaking and mechanical breakdown to only slaking with increased soil weathering. The profile variation of aggregate stability and the land use effect differed with soil type and disruptive force. For phyllosilicate mineralogy, vermiculite had the largest explanatory power in aggregate stability (R2 = 54%, p < 0.001), followed by kaolilite and 1.4 nm intergrade mineral (R2 = 28 ~ 32%, p < 0.01), while illite explained little variance (R2 = 6%, p > 0.05), demonstrating the more prominent role of phyllosilicate mineralogy than other soil properties (R2 < 43%) in aggregate stabilization. Besides the dominant effect of vermiculite, soil organic carbon, exchangeable sodium and calcium exerted a complimentary effect (R2 = 12%). Collectively, aggregate stability is predominantly influenced by phyllosilicate mineralogy (especially swelling minerals) at the regional scale and by soil organic matter and exchangeable cations at the soil pedon scale.

Bound to rock

Geochemistry Organic matter binds to phyllosilicates, a process which affects both its transport and chemical stability. How does that affect the fate of terrestrial organic carbon that enters the ocean? Blattmann et al. show that organic carbon derived from soils is stripped from mineral surfaces upon discharge and dispersal into the ocean, whereas organic matter derived from ancient rocks is preserved there. Their results show that preservation of continentally derived organic matter in marine sediments is controlled largely by phyllosilicate mineralogy. Science , this issue p. [742][1] [1]: /lookup/doi/10.1126/science.aax5345

Mineralogical control on the fate of continentally derived organic matter in the ocean

First-order relationships between organic matter content and mineral surface area have been widely reported and are implicated in stabilization and long-term preservation of organic matter. However, the nature and stability of organomineral interactions and their connection with mineralogical composition have remained uncertain. In this study, we find that continentally derived organic matter of pedogenic origin is stripped from smectite mineral surfaces upon discharge, dispersal, and sedimentation in distal ocean settings. In contrast, organic matter sourced from ancient rocks that is tightly associated with mica and chlorite endures in the marine realm. These results imply that the persistence of continentally derived organic matter in ocean sediments is controlled to a first order by phyllosilicate mineralogy.

A 40,000 yr record of clay mineralogy at Lake Towuti, Indonesia: Paleoclimate reconstruction from reflectance spectroscopy and perspectives on paleolakes on Mars

Sediment deposited within lake basins can preserve detailed records of past environmental conditions on planetary surfaces, including both Earth and Mars. Establishing how to best characterize these paleoclimate records is thus critical for understanding the evolution of past planetary climates. Here, we present an ∼40 k.y. lake sediment record from Lake Towuti, Indonesia, developed using visible to near-infrared (VNIR) reflectance spectroscopy. Source sediment from the main river input to Lake Towuti, the Mahalona River, is spectrally dominated by Mg-rich serpentine; however, we also identify a distinct Al-phyllosilicate component, which we interpret as kaolinite, that increases in relative proportion to serpentine with decreasing grain size. Sink sediment from two cores collected at the distal margins of the Mahalona River delta has similar spectral signatures to the input source sediment. The cores capture systematic variations in the proportion of Al-phyllosilicate to serpentine over time, which is also expressed in changes in bulk elemental chemistry of the sediment. We show that the abundance of serpentine relative to Al-phyllosilicate increases dramatically during the globally cooler, regionally drier climate of the Last Glacial Maximum. This change records the grain size−dependent mineralogy of deltaic sediment, which is ultimately driven by forced delta progradation and river incision during lake lowstands. Our analyses show that VNIR reflectance spectroscopy offers a rapid, nondestructive, and effective method for developing paleoenvironmental records from sedimentary phyllosilicate mineralogy. Exposed paleolake deposits on Mars should preserve similar paleoenvironmental information that can be accessed through detailed remote sensing observations of stratigraphy and VNIR reflectance spectroscopy in a source-to-sink framework.

Does phyllosilicate mineralogy explain organic matter stabilization in different particle-size fractions in a 19-year C3/C4 chronosequence in a temperate Cambisol?

Finely divided minerals play a key role on the stabilization of soil organic matter (SOM). Among them, phyllosilicates are known to be particularly efficient due to their surface properties. However, how the wide variety of phyllosilicates with contrasted surface properties results in differing abilities to stabilize SOM is still unclear. In this study, we used a C3/C4 chronosequence established in a temperate Cambisol to compare the quantity of organic C and the incorporation of maize-derived C associated to various particle-size fractions with contrasting phyllosilicate mineralogy. We separated sand, silt and clay fractions in samples from 0–20-cm and 45–60-cm horizons. In each fraction, we measured organic C, total N and C isotope signatures. Phyllosilicate mineralogy of fine-silt, coarse-clay, medium-clay, and fine-clay fractions (2–20μm, 0.2–2μm, 0.2–0.05μm, <0.05μm) was determined using X-ray diffraction. Although phyllosilicate mineralogy differed between fractions, no clear relation was observed between mineralogy and organic C among the various particle-size fractions in any horizon. Interestingly, coarse-clay fractions providing a lower specific surface area had the greatest content of organic C and the lowest incorporation of fresh maize-C. This supported the hypothesis that a significant part of organic C stabilized in these fractions is due to the presence of very stable clay-size aggregates (resistant to sonication) that encapsulate organic C. The differing ability of the various phyllosilicates to stabilize organic C should therefore be evaluated according to both their ability to adsorb organic C and to form stable clay-size aggregates. This may prevent from finding simple relationships between phyllosilicate mineralogy and organic C stabilization.

Toward an understanding of phyllosilicate mineralogy in the outer main asteroid belt

Proposed mineralogical linkages between CM/CI carbonaceous chondrites and outer Main Belt asteroids remain uncertain due to a dearth of diagnostic absorptions in visible and near-infrared (∼0.4–2.5μm) spectra of the two sets of objects. Absorptions near 3μm in both sets hold promise for illuminating the potential linkages. Spectral comparisons of meteorites and asteroids have been challenging because meteorite spectra have usually been acquired in ambient terrestrial environments, and hence were contaminated by atmospheric water. In this study, we compare near-infrared spectra of chondrites measured in the laboratory under asteroid-like conditions (Takir, D. et al. [2013]. Meteorit. Planet. Sci. 48, 1618–1637) and spectra of asteroids measured with the long-wavelength cross-dispersed (LXD: 1.9–4.2-μm) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF) (Takir, D., Emery, J.P. [2012]. Icarus 219, 641–654). Using the 3-μm band shape, we find that spectral Group 2 CM and CI (Ivuna) chondrites are possible meteorite analogs for asteroids with the sharp 3-μm features, which are predominately located in the 2.5<a<3.3AU region. Spectral Group 2 CM chondrites contain phyllosilicate phases intermediate between endmembers Fe-serpentine and Mg-serpentine, with a petrological subtype ranging from 2.2 to 2.1 (Takir, D. et al. [2013]. Meteorit. Planet. Sci. 48, 1618–1637). No meteorite match was found for asteroids showing a rounded 3-μm feature, which tend to be located farther from the Sun (3.0<a<4.0AU), or for asteroids with distinctive spectra like 1 Ceres or 52 Europa. The study of the 3-μm band in meteorites and asteroids has implications for the understanding of phyllosilicate mineralogy and its distribution in the outer Main Belt region.

Metamorphic evolution of the Karakaya Complex in northern Turkey based on phyllosilicate mineralogy

The Triassic Karakaya Complex (KC) of the Sakarya Composite Terrane in northern Turkey is traditionally subdivided into two units. The Lower Karakaya Complex (LKC) consists of a tectonic melange with blocks of metabasic rocks, metacarbonates, meta-arenites and metapelites that have been affected by high pressure/low temperature metamorphism. It is followed by a low pressure/low temperature metamorphic overprint; the latter is the only metamorphic event in some tectonic slivers of the LKC. The Upper Karakaya Complex (UKC) units are primarily composed of diagenetic to low-grade metamorphic rocks, comprising Late Permian and Triassic cherts and blocks of OIB-type volcanic rocks interfingering with Anisian limestones. LKC slide-blocks of variable sizes are frequently observed within the UKC. Phyllosilicates of LKC and UKC were examined for their abundance, crystallinity, polytype and b cell dimension. Trioctahedral chamositic chlorites have IIb polytype and phengites 2 M1 polytypes in the LKC units and 2 M1 + 1 M + 1Md polytypes of phengitic dioctahedral illites in the UKC units. Kubler index data correspond to the low anchizone and epizone for the LKC units, and to the high diagenesis-low anchizone, and in part to the epizone for the UKC units. The b values of illites are consistent with a high-pressure facies series for the LKC, but only intermediate-pressure facies for the UKC. According to textural features, mineral paragenesis, clay transformations, index minerals, and b values, the lower-middle parts of the LKC represent an accretional tectonic setting, whereas the UKC units reflect pressure temperature conditions of an extensional basin affected by high heat flow.

Nature and degree of aqueous alteration in CM and CI carbonaceous chondrites

AbstractWe investigated the petrologic, geochemical, and spectral parameters that relate to the type and degree of aqueous alteration in nine CM chondrites and one CI (Ivuna) carbonaceous chondrite. Our underlying hypothesis is that the position and shape of the 3 μm band is diagnostic of phyllosilicate mineralogy. We measured reflectance spectra of the chondrites under dry conditions (elevated temperatures) and vacuum (10−8 to 10−7 torr) to minimize adsorbed water and mimic the space environment, for subsequent comparison with reflectance spectra of asteroids. We have identified three spectral CM groups in addition to Ivuna. “Group 1,” the least altered group as determined from various alteration indices, is characterized by 3 μm band centers at longer wavelengths, and is consistent with cronstedtite (Fe‐serpentine). “Group 3,” the most altered group, is characterized by 3 μm band centers at shorter wavelengths and is consistent with antigorite (serpentine). “Group 2” is an intermediate group between group 1 and 3. Ivuna exhibits a unique spectrum that is distinct from the CM meteorites and is consistent with lizardite and chrysotile (serpentine). The petrologic and geochemical parameters, which were determined using electron microprobe analyses and microscopic observations, are found to be consistent with the three spectral groups. These results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy. High‐quality ground‐based telescopic observations of Main Belt asteroids can be expected to reveal not just whether an asteroid is hydrated, but also details of the alteration state.

The influence of phyllosilicate mineralogy on the rheology of mineral slurries

Phyllosilicate minerals have been shown to present some rheological challenges during ore beneficiation, with difficulties such as reduced flotation rates and complex tailings treatment arising. This study focuses on two phyllosilicate minerals which commonly exist as major gangue components in many industrial ores: muscovite with its thin platy structure and vermiculite which exhibits concertina-like swelling behaviour. The surface charge distribution of the minerals is evaluated using electrokinetic zeta potential measurements and potentiometric titrations. Rheology tests give the plastic viscosities and Bingham yield stresses of the suspensions. The results show that muscovite and vermiculite particles have an apparent negative charge due to the high aspect ratio of the negatively charged tetrahedral faces to the edges within a tetrahedral-octahedral-tetrahedral (T–O–T) layer configuration. The charge separation between the edges and faces results in the non-typical stacking behaviour of muscovite and vermiculite particles, forming suspensions with higher Bingham yield stresses than quartz, a non-phyllosilicate isotropic mineral with a regular non-platy morphology. However, upon yielding, these structures are destroyed and the resultant suspensions are characterised by low Bingham viscosities similar to isotropic minerals. This study provides a preliminary understanding of the flow behaviour of muscovite and vermiculite.

Significance of phyllosilicate mineralogy and mineral chemistry in an epithermal environment. Insights from the palai-islica Au-Cu deposit (Almería, SE Spain)

AbstractPhyllosilicate mineralogy is key to understanding hydrothermal processes within accepted epithermal deposit models but little information has been published about the mineral chemistry of epithermal deposits. X-ray diffraction, optical and electronic microscopy (scanning and transmitted), electron microprobe, and Fourier transform infrared spectroscopy were used in this work to study phyllosilicates in the Palai-Islica Au-Cu epithermal, volcanic-hosted deposit, in order to link phyllosilicate mineralogy and mineral chemistry to ore genesis. Different phyllosilicate assemblages are characteristic of two types of mineralization, and related hydrothermal alteration. Chlorite and mica appear in polymetallic quartz veins with sulfides, and in the related chloritic and sericitic hydrothermal alteration. These minerals have notable textural and chemical differences (i.e. Fe/(Fe+Mg), Si and Al in chlorite and illitic and phengitic components in mica) amongst veins and altered rocks, revealing different genetic conditions. These chemical features also distinguish propylitic and regional, non ore-related, low-temperature alteration. Hot hydrothermal fluids of near-neutral pH are responsible for vein mineralization and alteration. Illite, interstratified illite-smectite, kaolinite, and pyrophyllite are characteristic, with a distribution pattern by zones, of the intermediate argillic and advanced argillic alteration around areas of silicification. In the latter, native gold appears associated with interstratified illite-smectite, suggesting a relatively low-temperature formation. Hot, low-pH fluids are responsible for this mineralization and alteration assemblage. The present study contributes to epithermal models showing the co-existence of two different alteration styles in the same hydrothermal system.

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

Mineralogical and geochemical investigation of clay-rich mine tailings from a closed phosphate mine, Bartow Florida, USA

Clay-rich mine tailings from phosphate mine operations in Florida are a major environmental and economic problem. Options for reclamation and restoration for these tailings are very limited and are fundamentally restricted by poor physical properties such as low mechanical strength, low hydraulic conductivity, and heavy metal content. The major control on these bulk physical properties is the mineralogy of the materials. Eight continuous push borings were obtained to investigate stratigraphy, mineralogy, aspects of geochemistry, and bulk properties of a deposit of clay-rich mine tailings from a phosphate mine near Bartow, Florida that ceased operations in the early 1970s. Stratigraphy is dominated by laminated clay-rich sediment with minor units of silt and sand. An intact kaolinite liner occurs near the impoundment walls and the impoundment floor has approximately 4 m of relief. Moisture content varies from 4.35 to 57.40 wt% and organic content varies from 0.41 to 9.53 wt%. Bulk XRF investigation indicates that the P2O5 concentrations vary from approximately 4 to 21 wt%. A very strong correlation (r 2 = 0.92) between CaO and P2O5 indicates that apatite is a major control on the phosphate. The strong correlation (r 2 = 0.77) of Al2O3 and TiO2 suggests that the source materials for this deposit are comparatively uniform. A number of heavy metal elements and trace elements occur. Cr, V, Ni, Cu are interpreted to be in phosphate minerals, largely apatite. Sr and Pb are interpreted to be in both phyllosilicates and phosphate minerals. Two populations of apatite were observed in the clay-sized fraction, one that was Fe and Si- bearing and another that was only Si-bearing. Fe-bearing apatite had Fe2O3 contents that varied from 0.38 to 5.32 wt% and SiO2 contents that varied from 0.90 to 3.32 wt%. The other apatite population had a wider range of SiO2 contents that varied from 0.77 to 8.80 wt%. TEM imaging shows that apatite grains are dominantly single crystals with lesser amounts of aggregates. Wavellite commonly occurs as individual or clusters of lath-like crystals and the chemical composition differs from the pure aluminium phosphate end member with average concentrations of components being that of CaO (1.57 wt%), Fe2O3 (1.98 wt%), SiO2 (5.94 wt%). In the clay-sized phosphate minerals investigated no fluorine was found above detection limit (approximately 0.15 wt%), nor was any uranium, radium, heavy metal, or REE element detected. The phyllosilicate mineralogy of the deposit is dominated by smectite (montmorillonite with lesser amounts of nontronite), palygorskite, illite and kaolinite. No systematic variation in the relative proportions of phyllosilicates was observed in the clay deposit. Energy dispersive spectroscopy EDS analysis indicates that chemical compositions of phyllosilicates are somewhat typical but overall are enriched with respect to Fe compared to theoretical end members. The relative enrichment of Fe is interpreted to be a primary sedimentary feature. Ca content in smectite minerals is high and may inhibit stabilization using lime or similar methods. The high percentages of montmorillonite and palygorskite explain the high bulk water contents observed. This investigation provides fundamentally new details regarding clay tailing deposits from closed phosphate mines in central Florida which can be used in restoration and reclamation efforts.

Vegetation effects on pedogenetic forms of Fe, Al and Si and on clay minerals in soils in southern Switzerland and northern Italy

The older forest type Quercetum– Betuletum (oak/birch; Q-type vegetation) in southern Switzerland and northern Italy was to a large extent replaced by chestnut forests ( Castanea sativa; C-type vegetation) in roman times. When laurophylloid vegetation (L-type vegetation) invaded some of these chestnut systems during the last few decades, it caused detectable changes in organic chemistry. The invasion of the L-type vegetation was predominately due to increased winter temperatures. We tested whether these vegetation changes led to measurable long-term and short-term responses of the mineral matrix by comparing soils under Q-type with C-type vegetation (probing for long-term effects; > 100–2000 years) and soils under C-type with L-type vegetation (short-term effects; a few decades). To do so, we examined soil characteristics including the dithionite-, oxalate- and pyrophosphate-extractable forms of Fe, Al and Si as well as the phyllosilicate mineralogy of the clay fraction with a pair-wise comparison procedure. On L-type patches, short-term changes resulted in higher contents of secondary, poorly crystalline Fe. The accumulation of pedogenetic Fe probably results from a lower availability of organic complexing moieties under L-type vegetation and thus leading to a reduced removal. As soil acidity did not change with the vegetation types, a strong effect of organic compounds on Fe and Al chemistry must be assumed. A correlation analysis confirmed that metal binding to organic matter was different between L-type stands (correlation of Fe and Al with organic matter was less significant) and C-type stands. The differences in clay phyllosilicate assemblage between the different vegetation sites were rather small. There was, however, a trend towards higher contents of hydroxy-interlayered vermiculite (HIV) under L-type vegetation when compared to C-type sites. It seems that Al-polymers fixation in interlayers of 2:1 clay minerals was increased or their removal hindered at sites having L-type vegetation. Changes in the long-term (response to C-type vegetation) were only measurable for the pyrophosphate-extractable Si content (formation of phytolithe?). The colonisation of laurophyllous species led in the short-term to significant alterations of the soil system that were even more pronounced than the long-term effect of chestnut on soil quality.

Halloysite versus gibbsite: Silicon cycling as a pedogenetic process in two lowland neotropical rain forest soils of La Selva, Costa Rica

Halloysite and gibbsite, although known to require quite different conditions for their formation, commonly occur together in the same horizon in oxisols derived from andesitic parent materials in tropical Costa Rica. We selected two soils of similar parent material, but of different ages and soil moisture regimes to identify possible clues to the coexistence of these two minerals. We employed selective dissolution procedures, X-ray fluorescence analysis and X-ray diffraction on field moist and air dry bulk soil samples to investigate how mineralogy changes with depth. We further separated the size fraction < 2 μm by means of sedimentation after organic matter and iron oxide removal to obtain more specific information on the phyllosilicate mineralogy of the clay size fraction. We found both soils to be depleted of primary minerals and pedogenesis to have progressed to advanced weathering stages particularly in the subsoils. Gibbsite XRD signal intensities were linearly and significantly related to weathering indices, corroborating the residual nature of gibbsite as an endproduct of weathering processes. The Si-bearing quartz and kaolinite-group minerals were enriched in the topsoils, indicating (i) their independence from a primary mineral Si source and (ii) the existence of a mechanism capable of protecting them against the continuous tropical weathering pressure. As we found no indications for retrospective additions of soil material through mass movement or aeolian additions, we believe a vegetation dependent, biological pumping mechanism to be the most plausible explanation for the presence of silica bearing minerals in the La Selva topsoils. The vertical distribution of 1.0 nm halloysite and its accumulation in the lower reaches of the wetter alluvial soil suggest that this metastable mineral forms as a result of Si enrichment where the residence time of the pore water is long enough to allow for Si concentrations to exceed the halloysite precipitation threshold. Taken together, our evidence indicates gibbsite in the La Selva soils to be the endproduct of intense tropical weathering, while the presence of hydrated halloysite seems to have mainly kinetic reasons and is most probably coupled to the contemporary soil moisture regime.

Sodium hypochlorite oxidation reduces soil organic matter concentrations without affecting inorganic soil constituents

SummaryOxidative treatment can isolate a stable organic matter pool in soils for process studies of organic matter stabilization. Wet oxidation methods using hydrogen peroxide are widely used for that purpose, but are said to modify poorly crystalline soil constituents. We investigated the effect of a modified NaOCl oxidation (pH 8) on the mineral composition of 12 subsoils (4.9–38.2 g organic C kg−1) containing varying amounts of poorly crystalline mineral phases, i.e. 1.1–20.5 g oxalate‐extractable Fe kg−1, and of different phyllosilicate mineralogy. Post‐oxidative changes in mineral composition were estimated by (i) the determination of elements released into the NaOCl solution, (ii) the difference in dithionite‐ and oxalate‐extractable Si, Al and Fe, and (iii) the specific surface areas (SSAs) of the soils. The NaOCl procedure reduced the organic C concentrations by 12–72%. The amounts of elements released into the NaOCl extracts were small (≤ 0.14 g kg−1 for Si, ≤ 0.13 g kg−1 for Al, and ≤ 0.03 g kg−1 for Fe). The SSA data and the amounts of dithionite‐ and oxalate‐extractable elements suggest that the NaOCl oxidation at pH 8 does not attack pedogenic oxides and hydroxides and only slightly dissolves Al from the poorly crystalline minerals. Therefore, we recommend NaOCl oxidation at pH 8 for the purpose of isolating a stable organic matter pool in soils for process studies of organic matter stabilization.

Patents

As an integrated parameter for soil structure and function, aggregate stability is crucial to soil quality and erodibility, resulting in extensive studies on the formation and stability mechanisms of aggregates, especially for heavy-textured soils. Granitic soils with a wide range of particle composition are concentrated by large-scale permanent gullies in subtropical China, while limited information is available on the aggregate structure of granitic soils. To investigate the formation mechanism of these gullies, granitic soils were sampled systemically from different genetic horizons from temperate (no gully erosion) and subtropic (concentrated by gully erosion) areas, and the aggregate size distribution and stability of these soil were measured by Yoder method with sand correction. The results showed that macroaggregate (especially the 0.25 ~ 2 mm-sized fraction) occupied 56.92% of the aggregation. The disaggregation reduction (DR) ranging from 0.18 to 1.12 mm was shown to be more reasonable than the mean weight diameter in characterizing the aggregate stability of granitic soils. Additionally, macroaggregate and DR were unveiled to increase with the increase of weathering degree, implying that the humid climate with high rainfall conditions in the subtropic areas was responsible for the formation of the permanent gullies at a regional scale. The formation and stability of aggregates were generally determined by clay content (R2 = 30.6%), soil organic carbon (R2 = 20.1%), free aluminum oxides (R2 =11.1%), and free iron oxides (R2 =6.7%). Among these influential factors, clay fraction associated with aggregates in a size of > 2 mm was a promising variable for predicting DR (R2 =75%, p < 0.001). Collectively, the aggregate structure of granitic soils is directly determined by inorganic cementing matters controlled by the variation of phyllosilicate mineralogy under different rainfall conditions, while the coarse texture of granitic soils emphasizes the contribution of organic cementing agent (SOC). This study facilitates the understanding of aggregate structure and its stability mechanism of coarse-grained soils as well as erosion control in granitic soil areas.

Fluid flow, alteration and polysulphide mineralisation associated with a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium

In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone. The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts. It is concentrated in quartz–sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H 2O–CO 2–CH 4–NaCl–(KCl) composition and a minimum temperature of 250–320 °C. The δ 18O values of quartz vary between +12.3‰ and +14.5‰ SMOW, and δD compositions of the fluid inclusions in the quartz crystals range from −65‰ to −35‰ V-SMOW. The δD and the calculated δ 18O values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids. The δ 34S values, between +4.7‰ and +10.6‰ CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt.

Transformations of 2:1 phyllosilicates in 41-year-old soils under oak and pine

The large, unconfined lysimeters at the San Dimas Experimental Forest, in southern California, provide a unique setting in which to study decade-scale vegetation effects on mineral weathering. We investigated the 2:1 phyllosilicate mineralogy of lysimeter soils under 41-year-old monocultures of scrub oak (Quercus dumosa Nutt.) and Coulter pine (Pinus coulteri B. Don), and compared the results to archived original fill material. X-ray diffraction showed that mica increased relative to vermiculite in the clay and medium silt fractions of A horizons under both oak and pine, compared to the archived original fill material. The increase, however, was far greater under oak than under pine. No mineralogical differences were observed in the subsurface horizons of either oak or pine, compared to archived material. Nonexchangeable K increased by 23% in the clay fraction of the oak A horizon, and increased by 5% in the clay fraction of the A horizons under pine, relative to archived parent material. Strong evidence supports biocycling as the basis for observed decade-scale mineral transformations. We conclude that the return of K to the soil surface through litter decomposition, and subsequent fixation by vermiculite, resulted in increased mica contents in A horizons. More K may have been fixed by vermiculite in the oak A horizon compared to pine due to greater K concentration in the oak litter pool; earthworm-mediated mineralization of K from organic matter under oak; and presence of fewer roots at the surface under oak, and, consequently, less plant removal of K from the A horizon.