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IntroductionThe importance of silicon for soil functioning has only recently received the attention it deserves in soil science (and ecology). To identify the actual effects of Si on ecosystem functioning, a precise knowledge about different Si soil fractions (plant-available, amorphous Si, and Si bound to organic matter, short-range ordered aluminosilicates (SROAS), or iron oxides/hydroxides) is required. Sequential extraction techniques are a common tool to quantify different soil fractions in soils of humid-temperate climates, whereas their reliability in other climatic zones or contrasting soils remains unclear. This study tested to what extend sequential extraction is applicable on soils from the High Andes and Amazon Basin, two contrasting regions with distinct soil formation processes.MethodsTo ensure method validity, we analyzed Si, iron (Fe), and aluminum (Al) in extracts and conducted X-ray diffraction (XRD) to detect mineral changes pre- and postextraction. Amorphous Si was more abundant in the High Andes soils, yet plantavailable Si was unexpectedly lower compared to Amazon Basin soils.ResultsSilicon occluded in Fe oxides/hydroxides or SROAS showed no differences between the two contrasting soils. XRD results revealed the following limitations: dithionite extraction did not completely remove Fe from hematite in Andean soils, and powder XRD lacked precision for amorphous Si quantification. Among the tested methods, CaCl2 extraction (for plant-available Si) and a two-step protocol—density separation for biogenic Si (bASi), followed by Tiron extraction for total amorphous Si (tASi)—were the most effective for these soils.DiscussionThese findings, with different methods not being able to characterize the soil used, underscore the need to refine Si extraction techniques for environments. Expanding methods originally developed in temperate soils is critical to understanding Si’s role in biogeochemical cycles and its broader significance for different ecosystem performance.

The morphology, chemical, and mineralogical composition of subsurface filamentous fabrics (SFF) from the Deccan Volcanic Province (DVP) were investigated to determine the origin of these spectacular aggregates. SFF occur in a wide variety of morphologies ranging from pseudo-stalactites to irregular fabrics and are classified as SFFIr (irregular) or SFFMa (matted). The SFF samples exhibit a thread-like (or filament-like) center from which mineral precipitation starts to form the final macroscopic morphologies. Detailed investigations revealed organic material (fungal chitin) in the innermost filamentous core, which may have acted as an initial nucleus for the mineralization processes. The morphometric characteristics of certain filamentous fabrics are very similar to those of microbial filaments and the fabrics formed from them but are clearly distinct from similar types of non-biological precipitates (fibrous minerals, speleothems, and “chemical gardens”). These features indicate that the filamentous cores might be products of microbial communities that were active in the basaltic cavities. The SFF cross-sections display similar concentric layers of the mineral succession and reach thicknesses of several centimeters with spectacular lengths up to 100 cm and constant diameters. The typical mineralization sequence points to temporal variation in the chemical composition of the mineralizing fluids from Fe(Mg)-rich (Fe-oxides/-hydroxides, Fe-rich sheet silicates such as celadonite and di-/tri-smectite) to Ca-dominated (Ca-rich zeolites) and finally pure SiO2 (opal-CT, chalcedony, and macro-crystalline quartz). Assuming biological activity at least during the early mineralization processes, circumneutral pH conditions and maximum temperatures of 100–120 °C were supposed. The formation of filamentous cores including Fe-bearing phyllosilicates probably occurred near the surface after cooling of the lava, where the elements necessary for mineral formation (i.e., Si, Mg, Al, Fe) were released during alteration of the volcanic host rocks by percolating fluids.

Agate/chalcedony samples of different origin were investigated by performing Raman, X-ray diffraction (using Rietveld refinement), and cathodoluminescence measurements. These analyses were performed to measure the content and spatial distribution of the silica polymorph moganite, which is considered to represent periodic Brazil-law twinning of alpha -quartz at the unit-cell scale in agate/chalcedonies. Homogeneous standard samples including the nearly alpha -quartz free moganite type material from Gran Canaria were analysed in order to compare results of the X-ray diffractometry and Raman spectroscopy techniques and to provide a calibration curve for the Raman results. However, due to the different length scales analysed by the two techniques, the \"moganite content\" in microcrystalline SiO sub(2) samples measured by Raman spectroscopy (short-range order) was found to be considerably higher than the \"moganite content\" measured by X-ray diffractometry (long-range order). The difference is explained by the presence of moganite nanocrystals, nano-range moganite lamellae, and single Brazil-law twin-planes that are detected by vibrational spectroscopy but that are not large enough (in the sense of coherently scattering lattice domains) to be detected by X-ray diffractometry. High resolution Raman analysis provides a measure of the moganite content and its spatial variation in microcrystalline silica samples with a lateral resolution in the mu m-range. Variations in the moganite-to-quartz ratio are revealed by varying intensity ratios of the main symmetric stretching-bending vibrations (A sub(1) modes) of alpha -quartz (465cm super(-1)) and moganite (502cm super(-1)), respectively. Traces of Raman microprobe analyses perpendicular to the rhythmic zoning of agates revealed that the moganite-to-quartz ratio is often not uniform but shows a cyclic pattern that correlates with the observed cathodoluminescence pattern (colour and intensity). Data obtained from an agate sample from a fluorite deposit near Okorusu, Namibia and from a volcanic agate from Los Indios, Cuba were selected for detailed presentation. Variations of cathodoluminescence and Raman data between single bands in agates suggest alternating formation of fine-grained, highly defective chalcedony intergrown with moganite, and coarse-grained low-defect quartz. Multiple zones indicate dynamic internal growth during a self-organizational crystallization process from silica-rich fluids.

Hydrothermally altered rocks hosting precious metal deposits frequently contain stacking disordered layer silicates. X-ray diffraction analysis using the Rietveld method can be used to determine mineral abundances in these rocks if suitable disorder models are applied. It is shown here that disorder models of kaolinite and pyrophyllite can be described by a recursive calculation of structure factors. This permits the physically sound refinement of real structure parameters of these disordered minerals and the determination of mineral abundances. Even mixtures containing two disordered Si–Al layer silicates can be quantified reliably. The developed disorder models can now be implemented in routine phase analysis, allowing the quantification of large numbers of samples to identify mineralogical gradients surrounding ore deposits.

In the present study, the behavior and properties of plasma-sprayed hydroxyapatite coatings [Ca 10 (PO 4 ) 6 (OH) 2 , HAp] were investigated in relation to the spraying process. The experiments were focused on the influence of type of feedstock and spray power on the phase composition and distribution within the coatings. Depth profiles of the coatings were investigated before and after incubation in revised simulated body fluid (SBF) by X-ray diffraction and infrared spectroscopy. Besides HAp, the coatings contain oxyapatite (OAp) and carbonate apatite (CAp). Additionally, tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), CaO, and an amorphous phase were detected in the coatings. The HAp content directly depends on the used spray powder and spray power, where the influence of spray powder is much higher than the influence of the spray power. The grain size range of the spray powder strongly influences the HAp content in the coating and the formation of CaO. The in vitro behavior of the coatings in simulated body fluid mainly depends on the contents of CaO and amorphous calcium phosphate, respectively. The formation of portlandite due to the reaction of the coating with the SBF is strongly influenced by the porosity of the coatings and can be used as an indicator for the depth of interaction between fluid and coating.

The release of engineered titanium dioxide nanoparticles in the environment is nowadays continuously increasing due to their wide range of industrial applications. Their potential toxicity effects became of major concern, and several assessment studies in natural waters were already undertaken. However, no consensus arose about the environmental factors influencing their hazardous impact, but rather contrary conclusions were drawn. In this study, the acute toxicity of commercial TiO 2 nanoparticles suspensions at different concentrations on microcrustacean ( Daphnia magna ), marine rotifers ( Brachionus plicatilis ), and marine microalgae ( Phaeodactylum tricornutum ) under environmental conditions, in synthetic fresh and marine water, was investigated. Factors driving TiO 2 adverse effects on aquatic environment, such as allotropic form, primary particle size, surface area, particle concentration, and agglomerate size, were studied. A thorough characterization of both surface and bulk properties of nano‐sized TiO 2 particles was therefore performed. Our results showed that Daphnia magna test is the most sensitive test for assessing toxicity of TiO 2 samples on aquatic organisms. For anatase samples, toxicity toward aquatic organisms depends (i) on the primary particle size and the extent of agglomeration (mass median diameter d 50 ), and consequently on surface reactivity (total surface site concentration, specific surface area, pH IEP ) (ii) on the presence of rutile impurities in the sample. Toxicity results of rutile and anatase samples of comparable primary particle size (70–500 nm) are of same order of magnitude and remained less toxic than nanometric particles (10–20 nm). Rutile agglomeration was found to be higher than anatase agglomeration, toxicity results obtained for rutile could be attributed to the shape of particles, or it could be due to the presence of BaTiO 3 impurities. This work emphasized the importance of studying the effects of bulk and surface parameters of engineered TiO 2 nanoparticles to understand their reactivity toward micro‐organisms under environmental conditions.

Agate/chalcedony samples of different origin were investigated by performing Raman, X-ray diffraction (using Rietveld refinement), and cathodoluminescence measurements. These analyses were performed to measure the content and spatial distribution of the silica polymorph moganite, which is considered to represent periodic Brazil-law twinning of α-quartz at the unit-cell scale in agate/chalcedonies. Homogeneous standard samples including the nearly α-quartz free moganite type material from Gran Canaria were analysed in order to compare results of the X-ray diffractometry and Raman spectroscopy techniques and to provide a calibration curve for the Raman results. However, due to the different length scales analysed by the two techniques, the \"moganite content\" in microcrystalline SiO2 samples measured by Raman spectroscopy (short-range order) was found to be considerably higher than the \"moganite content\" measured by X-ray diffractometry (long-range order). The difference is explained by the presence of moganite nanocrystals, nano-range moganite lamellae, and single Brazil-law twin-planes that are detected by vibrational spectroscopy but that are not large enough (in the sense of coherently scattering lattice domains) to be detected by X-ray diffractometry. High resolution Raman analysis provides a measure of the moganite content and its spatial variation in microcrystalline silica samples with a lateral resolution in the μm-range. Variations in the moganite-to-quartz ratio are revealed by varying intensity ratios of the main symmetric stretching-bending vibrations (A1 modes) of α-quartz (465 cm-1) and moganite (502 cm-1), respectively. Traces of Raman microprobe analyses perpendicular to the rhythmic zoning of agates revealed that the moganite-to-quartz ratio is often not uniform but shows a cyclic pattern that correlates with the observed cathodoluminescence pattern (colour and intensity). Data obtained from an agate sample from a fluorite deposit near Okorusu, Namibia and from a volcanic agate from Los Indios, Cuba were selected for detailed presentation. Variations of cathodoluminescence and Raman data between single bands in agates suggest alternating formation of fine-grained, highly defective chalcedony intergrown with moganite, and coarse-grained low-defect quartz. Multiple zones indicate dynamic internal growth during a self-organizational crystallization process from silica-rich fluids.