Constituent Grains Research Articles

Optical properties of analogs of Titan’s aerosols produced by dusty plasma

Analogs of Titan’s aerosols are produced in the laboratory as grains in a gas mixture, or as layers on a substrate. This production procedure enables the methane-nitrogen mixture composition to be changed. The aim of this paper is to understand the variations observed on the linear polarization of the scattered light as a function of the production conditions. The influence of the concentration of methane injected in the plasma will be discussed and compared with the previous work of Hadamcik et al. (2009a). The diameter of the grains are measured by SEM-FEG images. The decrease of absorption with increasing wavelength, measured by spectroscopic ellipsometry on layers, is observed for a decreasing initial methane ratio and analyzed in terms of an increasing ‘amine’ content in the materials. The phase function parameters of the linear polarization of the scattered light are discussed in terms of the diameters of the aggregates and of the constituent grains, and the variation of the refractive indices (mainly absorption). The polarization is found to be highly correlated with the constituent grain size. Finally, the experimental results are compared to polarization measurements from space of the Titan’s atmosphere.

Towards an optimization theory for deforming dense granular materials: Minimum cost maximum flow solutions

We use concepts and techniques of network optimization theory to gain a better understanding of force transmission in dense granular materials. Specifically, we represent a deforming granular material over the different stages of a quasi-static biaxial compression test as a series of representative flow networks, and analyze force transmission through these networks. The forces in such a material are transmitted through the contacts between the constituent grains. As the sample deforms during the various stages of the biaxial test, these grains rearrange: while many contacts are preserved in this rearrangement process, some new contacts form and some old contacts break. We consider the maximum flow problem and the minimum cost maximum flow (MCMF) problem for the flow networks constructed from this evolving network of grain contacts. We identify the flow network bottleneck and establish the sufficient and necessary conditions for a minimum cut of the maximum flow problem to be unique. We also develop an algorithm to determine the MCMF pathway, i.e. a set of edges that always transmit non-zero flow in every solution of the MCMF problem. The bottlenecks of the flow networks develop in the locality of the persistent shear band, an intensively-studied phenomenon that has long been regarded as the signature failure microstructure for dense granular materials. The cooperative evolution of the most important structural building blocks for force transmission, i.e. the force chains and 3-cycles, is examined with respect to the MCMF pathways. We find that the majority of the particles in the major load-bearing columnar force chains and 3-cycles consistently participate in the MCMF pathways.

Ripples in intertidal mud—a conceptual explanation

On protected mudflats and along sheltered tidal channel margins, wave- and current-generated ripples are frequently observed on surficial and subsurface mud beds, although such bedforms are generally not thought to occur in cohesive sediments. In this paper, examples of such ripple marks in the German Wadden Sea (back-barrier tidal flats of Spiekeroog island) and also along the west coast of Korea (Baeksu tidal flats) are documented and analyzed. The mud ripples are 5–8 cm in spacing and 0.3–0.8 cm in height, and are composed of slightly sandy to virtually pure mud (80–98% mud content). For the Spiekeroog study area, a comparison of in situ particle-size measurements of suspended matter and of dispersed mud collected from the ripples shows that the former consists of low-density flocs which are considerably larger than the constituent grains of the latter. To assess local wave effects, near-bed orbital velocities and orbital diameters were calculated on the basis of standard wave theory using estimated wave parameters at the time of the study (June 2004) as well as wave data recorded nearby within the back-barrier tidal basin. The relationships between grain size, morphometric ripple parameters, and the near-bed orbital diameter show the wave-generated mud ripples to be of the orbital post-vortex type. It is demonstrated that only short-period shoaling (intermediate water depth) waves with periods of 1.5–2.5 s and heights of 0.1–0.5 m are able to generate and maintain such ripples. Corresponding near-bed orbital velocities range from 8–32 cm s–1 and near-bed orbital diameters from 6.25–10 cm. It can be anticipated that increased current shear and turbulence associated with higher and longer waves prevent ripple formation due to the resuspension of settled mud, and the breakdown of suspended flocs and aggregates into smaller particles which then tend to remain in suspension. The most plausible explanation for the formation of the mud ripples is that mud flocs and aggregates deposited from suspension around high-water slack tide under moderate weather conditions initially respond as single (non-cohesive) particles which are hydraulically equivalent to ambient very fine sands. During exposure at low tide, gradual loss of water transforms the rippled mud into increasingly more cohesive mud drapes which are more resistant to erosion. Unless destroyed during high-energy events, the mud ripples may remain intact long enough to become buried and thereby preserved. Indeed, occasional but persistent observations of ripples in sub-Recent to ancient mudrocks document their preservation potential.

The importance of stress percolation patterns in rocks and other polycrystalline materials

A new framework for thinking about the deformation behavior of rocks and other heterogeneous polycrystalline materials is proposed, based on understanding the patterns of stress transmission through these materials. Here, using finite element models, I show that stress percolates through polycrystalline materials that have heterogeneous elastic and plastic properties of the same order as those found in rocks. The pattern of stress percolation is related to the degree of heterogeneity in and statistical distribution of the elastic and plastic properties of the constituent grains in the aggregate. The development of these stress patterns leads directly to shear localization, and their existence provides insight into the formation of rhythmic features such as compositional banding and foliation in rocks that are reacting or dissolving while being deformed. In addition, this framework provides a foundation for understanding and predicting the macroscopic rheology of polycrystalline materials based on single-crystal elastic and plastic mechanical properties.

Growth of (111) One-Axis-Oriented Bi(Mg1/2Ti1/2)O3 Films on (100)Si Substrates

Films of a high-pressure perovskite phase, Bi(Mg1/2Ti1/2)O3, were prepared on (111)c-oriented SuRuO3-coated (111)Pt/TiO2/SiO2/(100)Si substrates. The perovskite Bi(Mg1/2Ti1/2)O3 films had a (111) one-axis orientation, because their constituent grains were epitaxially grown on (111)c-oriented perovskite SrRuO3 ones. The remanent polarization and piezoelectric constant measured at an applied electric field of 600 kV/cm were about 30 µC/cm2 and 40 pm/V, respectively. A remarkable phase transition was not observed from room temperature to 350 °C in a (111) one-axis-oriented Bi(Mg1/2Ti1/2)O3 film, suggesting that the Curie temperature of this film is above 350 °C.

Атомная структура и энергия тройных стыков границ наклона в никеле

Atomic structure and energy of triple junctions of tilt boundaries <111> and <100> in nickel were studied by the method of molecular dynamics. It is shown that the formation energy of equilibrium triple junctions of tilt boundaries as a special defect is practically zero, and a triple junction is, in fact, a part of its constituent grain boundaries. It is noted that in the center of a triple junction of low-angle tilt boundaries a defect-free region often formed.

A breakup model of Jetting formation of Explosively Loaded Granular Shells

ABSTRACTThis paper investigates the underlying physics governing the explosion driven expansion and fragmentation of spherical beds packed of partially saturated sand with varying mass fractions of interstitial oil. The breakup onset of the sand shells is characterized by the formation of fragments/agglomerates consisting of a large number of constituent grains, which in later time present themselves as prolific and regular jetting streams. Test data show a postponed jetting formation when sand shells are subject to the explosion with a higher detonation velocity, meanwhile a reduced jet mass scale is observed. A kinetic energy driven breakup model is proposed based on the instability criterion involving the opposing forces of stabilizing inertial pressures and destabilizing viscous resistance. This analytical model is capable of predicting the onset of granular material fragmentation as well as the characteristic fragment size, which is consistent with the experimental results.

Engineering Nanostructures of Inorganic Materials for Optical and Chemical Applications

Considerable interest is being exhibited in the novel and enhanced properties of nanostructured materials. These materials, with their constituent phase or grain structures modulated on a length scale of less than 100 nm, are now artificially synthesized by awide variety of physical, chemical, and mechanical methods. Today, scientists and engineers have access to a wealth of technologies such as self-assembly, chemical template, electron-beam lithography, focusedion-beam (FIB) lithography, nanoimprint, nanomolding, and scanning-probe lithography to build nanostructures at unprecedented scales, resolution, and throughput. The art of developing functionalized nanostructured materials exploiting unusual interfacial properties seems to have produced hitherto unknownman-made materials. Exploitations of nanostructured metal and oxide materials with many excited optical, chemical, and mechanical properties will lead to the broad range of potential applications including catalyst, drug discovery, sensing, nanoimaging, spectroscopy, optoelectronics, data storage, and structured-materials with enhanced mechanical properties. In this special issue, a series of contributed papers is focused on the function-motivated nanostructure design, fabrication, characterization, and application including metal, oxide, and ceramics. A wide range of recent progress on the chemical and physical fabrication methods of metal nanostructures, their optical properties, and application has been reviewed by G. Kawamura’s and Y. Yang’s groups. C. T. Torres et al. reported the transmittance modulation of optical signals in a nanocomposite integrated by two different silver doped zinc oxide thin solid films. T. K. Lee et al. reported polyol-free synthesis of highly loaded Pt catalysts on sulfuric-acid-treated graphene oxide (SGO) and their catalytic properties. Y. H. Lin’s group reported Fe-doped NiO nanofibers synthesized by electrospinning method and studied their ferromagnetic behaviors. The surface modification of SiC ceramics and the optical application have been reviewed by F. Jiang. The nanostructures of Si, GaN, and TiO 2 and their optical-electric properties were studied by D. Li et al., S. P. Chang et al., and L. Cheng et al., respectively. Kasuga’s group demonstrated aluminum silicate nanotube coating of siloxane-poly (lactic acid)-vaterite composite fibermats by electrospinning. This approach may provide a new method of improving the surface of polymer-based biomaterials.

Lithofacies attributes, depositional system and diagenetic properties of the Permian Gharif Formation from Haushi–Huqf area, Central Oman

Over 70 m thick interbedded sandstone, siltstone and claystone of the upper member of the Gharif Formation are exposed in western Huqf area in Oman Interior Sedimentary Basin. The Gharif Formation, particularly its upper member hosts major hydrocarbon reservoir in the subsurface of the Oman Interior Sedimentary Basin. The upper member of the Gharif Formation is comprised of interbedded thick sandstone, siltstone, carbonaceous clays and intraformational conglomerates. The sandstone lithofacies, on average, constitute 10 m thick multistoreyed sequences, which are composed internally of 2–3 m thick and 100 s of metres across vertically and laterally amalgamated sandstone bodies. Two major types of sandstones (types 1 and 2) are identified on the basis of their lithofacies association and internal architecture. The type 1 sandstone constitutes the lower part of the member and is comprised of pebbly to coarse-grained, planar and trough cross-bedded sandstone, plane bedded sandstone and pebble lags at the base of major sandstone bodies. The cross-beds are, on average, 30 cm thick exhibiting a dominant paleoflow direction towards NW (280–300° N). It is interpreted to be deposited by low sinuosity braided streams. The type 2 sandstone constitutes the upper part of the member and is comprised of medium-grained sandstone, trough to low angle plane bedding associated with lateral accretion surfaces. It is commonly interbedded with carbonaceous clays. Silicified plant fragments are commonly distributed in the upper part of the sandstone. Interbedded clays and siltstones are red, mottled and extensively bioturbated due to root burrows. It is interpreted to be deposited by high sinuosity meandering streams. In the uppermost part of the section, several dark grey to black carbonaceous clay/coal beds with plant matter are interbedded with sandstone and red clay indicating development of swampy conditions during onset of the coastal setting in the uppermost part of the formation. About 30 cm thick bioclastic sandstone deposited by the marine coastal bars mark transition from the Gharif Formation to carbonate dominated Khuff Formation. The sandstone of the Gharif Formation is arkosic in composition. Very small amount of cement and negligible compaction of constituent grains in sandstone indicates shallow burial before uplift.

Microbialite formation in southern Sinai (Egypt)

Ongoing microbialite formation is described at two previously unreported sites in southern Sinai, Egypt. Samples were collected in the peritidal tropical environment of Nabq Bay and Hidden Bay (southern Sinai, Egypt). Field observations and sample analyses show evidence of both sediment trapping and biostabilization in bacterial mucilaginous sheaths and microbially induced mineralization, producing a suite of increasingly lithified material: from agglomerated, consolidated sand to lithified crusts and oncoids. Thin-sections show evidence of bacteria (cyanobacteria and sulphate-reducing bacteria) among the constituent grains in the form of gelatinous filaments (green and red sheaths), and microalgal colonies along the outer edge, accompanied by a very high grade of clast alteration. The alternation of planar to irregular dark, superposed layers, and clastic layers is visible at the surface and/or inside some crusts. Widespread filaments of Schizothrix among grains have been identified, as well as extracellular polymeric substances (EPS), sheaths stabilizing particles, and calcareous tubular encrustations around cyanobacteria filaments. Carbonate precipitates include diffuse micrite, microcrystals of high-Mg calcite precipitating in the EPS matrix, and acicular aragonite as isopachous rims around grains. Cementation is accompanied by partial dissolution and progressive alteration of original grain boundaries. We describe four microbialite categories on the basis of their macroscopic morphology combined with different texture and lithification grade. The occurrence of the southern Sinai microbialites is explained by the interplay of local sedimentary dynamics and accommodation space in a peritidal tropical environment undergoing large temperature and salinity variations.

Non-invasive characterization of particle morphology of natural sands

Particle morphologies, i.e. particle sizes and shapes, have a marked influence on the mechanical response of granular materials including soils. Until now most investigations of particle shape have been two-dimensional and size has been most often assessed using sieving. This paper makes use of recent developments in three-dimensional imaging technologies to characterize the internal features of a soil in 3D including quantification of particle morphology. The soil investigated was Reigate sand (from Southeast England), a geologically old sand, which in its intact state exhibits significant interlocking amongst the constituent grains. Intact and reconstituted specimens having similar densities were tested under triaxial compression. The specimens were impregnated with an epoxy resin at three different stages of shear deformation and small cores from each specimen were scanned using X-ray micro-tomography. Different systems and scanning parameters were explored in order to obtain 3D, high-resolution, images with a voxel size of 5μm (0.018×d50). The morphology measurements were compared with sieve data and measurements obtained using a 2D, image based, laser system. The sieve size is shown to correlate well with the intermediate principal axis length. Clear differences are noted between the 2D and 3D shape measurements. Breakage of fractured grains, along existing fissures, occurs both during reconstitution and shearing of the intact soil, a phenomenon that cannot be observed using invasive techniques such as sieve analysis.

Amorphous calcium carbonate associated with biofilms in hot spring deposits

Calcium carbonate nanoparticles are intimately associated with crystalline calcite and aragonite in the Eryuan, Gongxiaoshe, and Zhuyuan hot springs (water temperature >75°C), which are located in Yunnan Province, China. The nanoparticles, <1μm long, are spherical to disc-shaped and commonly fuse together into small clusters. Their general appearance and lack of crystal faces or edges indicate that they are noncrystalline. Morphologically, these nanoparticles are similar to calcified nannobacteria or the constituent grains in amorphous calcium carbonate (ACC), which can be formed by various biogenic and abiogenic processes. In the Chinese hot springs, the ACC is always found under, in, or on top of biofilms, commonly in close proximity to crystalline calcite and/or aragonite. Textural evidence indicates that the ACC probably developed in microdomains that develop in the complex biofilm hydrogels. Critically, there is no evidence to support the notion that the nanoparticles are calcified nannobacteria. In the Chinese springs, ACC appears to play a formative role in the development of wheat-sheaf arrays of aragonite crystals and some of the calcite crystals. Hollow cores in some of the aragonite bundles probably formed as ACC was dissolved and many of the aragonite crystals appear to have developed as ACC recrystallized. Similarly, layers of ACC that coat the surfaces of some calcite crystals could be diagenetically transformed into calcite. The development of ACC in hot spring systems may be widespread and may play a critical but transitory role in the development of crystalline CaCO3 in these high temperature environments.

Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains

Collisions of mm-size dust aggregates play a crucial role in the early phases of planet formation. We developed a laboratory setup to observe collisions of dust aggregates levitating at mbar pressures and elevated temperatures of 800 K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller than 25 \mum in size. We find that for all impact energies in the studied range sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the sticking probability decreases with increasing impact parameter. The sticking probability increases with energy density (impact energy per contact area). We also observe collisions of aggregates that were formed by a previous sticking of two larger aggregates. Partners of these aggregates can be detached by a second collision with a probability of on average 19.8 \pm 4.0%. The measured accretion efficiencies are remarkably high compared to other experimental results. We attribute this to the rel. large dust grains used in our experiments, which make aggregates more susceptible to restructuring and energy dissipation. Collisional hardening by compaction might not occur as the aggregates are already very compact with only 54 \pm 1% porosity. The disassembly of previously grown aggregates in collisions might stall further aggregate growth. However, owing to the levitation technique and the limited data statistics, no conclusive statement about this aspect can yet be given. We find that the detachment efficiency decreases with increasing velocities and accretion dominates in the higher velocity range. For high accretion efficiencies, our experiments suggest that continued growth in the mm-range with larger constituent grains would be a viable way to produce larger aggregates, which might in turn form the seeds to proceed to growing planetesimals.

Numerical Simulation of Tire-Ground System Considering Soft Ground Characteristics

The dynamic property and behavior of the soft ground are greatly affected by the constituent grains' shape. The consideration of the elastic-plastic property of the soft ground is also important for the multipass calculation of the tire on the soft ground. We developed the efficient interactive model of the tire and the soft ground that were composed of the distributed lumped mass-spring model as the tire, Discrete Element (DE) model as the upper soft ground, and the mass-spring model as the lower soft ground. In the present study, we have improved the previous soft ground model by considering the grains' shape and the elastic-plastic property of the soft ground, and have confirmed these effects through several numerical simulations.

Macroscopic elastic anisotropy in tough ceramics from the single crystal elastic behaviour

The modern techniques for engineering analysis are based on a deep understanding of the proportional relationship between stress and strain and the description in terms of isotropic elastic constants, in many cases, is enough. In anisotropic materials the knowledge of the elastic constants is important for the manufacturing process and for micro mechanical modelling behaviour of the material to develop the new composite materials. Theoretical calculations of elastics constants and anisotropies are applied to tough ceramics ZrO2, HfO2 and in rutile TiO2 and SnO2. Their dependence of the residual hydrostatic stress in the crystal are given. The microscopic elastic anisotropy determined in this work, could be applied in the macro scale in the case the constituent grains have a preferred orientation in the crystal lattice or the grains shape is not spherical (faceted) and it is aligned to a common crystal axis. To cover the case where the grains are distributed at randomly, the Voigt-Reuss-Hill polycrystalline approach is applied to obtain the average values of Young, bulk, shear modules, Poisson coefficient and sound velocity in the transversal and longitudinal modes. These theoretical results, could be useful in the interpretation of experimental results obtained with the method known as diffraction elastic constant (DEC).

Jamming II: Edwards’ statistical mechanics of random packings of hard spheres

The problem of finding the most efficient way to pack spheres has an illustrious history, dating back to the crystalline arrays conjectured by Kepler and the random geometries explored by Bernal in the 1960s. This problem finds applications spanning from the mathematician’s pencil, the processing of granular materials, the jamming and glass transitions, all the way to fruit packing in every grocery. There are presently numerous experiments showing that the loosest way to pack spheres gives a density of ∼55% (named random loose packing, RLP) while filling all the loose voids results in a maximum density of ∼63%–64% (named random close packing, RCP). While those values seem robustly true, to this date there is no well-accepted physical explanation or theoretical prediction for them. Here we develop a common framework for understanding the random packings of monodisperse hard spheres whose limits can be interpreted as the experimentally observed RLP and RCP. The reason for these limits arises from a statistical picture of jammed states in which the RCP can be interpreted as the ground state of the ensemble of jammed matter with zero compactivity, while the RLP arises in the infinite compactivity limit. We combine an extended statistical mechanics approach ‘a la Edwards’ (where the role traditionally played by the energy and temperature in thermal systems is substituted by the volume and compactivity) with a constraint on mechanical stability imposed by the isostatic condition. We show how such approaches can bring results that can be compared to experiments and allow for an exploitation of the statistical mechanics framework. The key result is the use of a relation between the local Voronoi volumes of the constituent grains (denoted the volume function) and the number of neighbors in contact that permits us to simply combine the two approaches to develop a theory of volume fluctuations in jammed matter. Ultimately, our results lead to a phase diagram that provides a unifying view of the disordered hard sphere packing problem and further sheds light on a diverse spectrum of data, including the RLP state. Theoretical results are well reproduced by numerical simulations that confirm the essential role played by friction in determining both the RLP and RCP limits. The RLP values depend on friction, explaining why varied experimental results can be obtained.

The influence of particle characteristics on the behaviour of coarse grained soils

This paper describes an experimental study examining the influence of the mechanical and geometrical properties of the constituent grains on the overall material response of cohesionless granular materials. Glass ballotini were used as an analogue soil; their relatively simple geometry allowed the influence of particle shape and inter-particle friction to be examined independently. Techniques were developed to control the surface roughness of the ballotini to facilitate a parametric study. The particle shape was also varied by crushing the ballotini. At the micro-scale, the particle characterisation included accurate measurements of inter-particle friction, contact stiffness, particle surface roughness and particle shape. At the macro-scale the sensitivity of overall material response to changes in surface roughness and geometry was characterised using triaxial tests and oedometer tests on smooth spherical ballotini, roughened ballotini and crushed angular ballotini. Compression tests indicated that the initial load deformation response at particle–particle contact points is significantly softer than previously believed. Optical interferometry of particles after single particle–particle shearing tests confirmed that plastic strains occurred at the contact point, which were related to plastic yield. A Hertzian response was only seen at higher contact loads. A clear relationship between the inter-particle friction and the particle surface roughness was found. However, the macro-scale experiments indicated that while the material response may be slightly dependent on the surface roughness and friction, the influence of particle shape is very much more significant.

Observations and analyses of floc size and floc settling velocity in coastal salt marsh of Luoyuan Bay, Fujian Province, China

In coastal environments, fine-grain sediments often aggregate into large and porous flocs. Electro-Magnetic Current Meters (EMCM) and Laser In Situ Scattering and Transmissometry (LISST-ST) have been deployed within a Spartina alterniflora marsh of the Luoyuan Bay in Fujian Province, China, to measure the current velocity, the floc size and the settling velocity between 15 and 22 January 2008. During the observations, the near-bed water was collected in order to obtain the suspended sediment concentration (SSC) and constituent grain size. Data show that: (1) the nearbed current velocities vary from 0.1 to 5.6 cm/s in the central Spartina alterniflora marsh and 0.1–12.5 cm/s at the edge; (2) the SSCs vary from 47 to 188 mg/dm3. The mean grain size of constituent grains varies from 7.0 to 9.6 µm, and the mean floc sizes (MFS) vary from 30.4 to 69.4 µm. The relationship between the mean floc size and settling velocity can be described as: ws=adb, in which ws is the floc settling velocity (mm/s), a and b are coefficients. The floc settling velocity varies from 0.17 to 0.32 mm/s, with a mean value of 0.26 mm/s, and the floc settling velocity during the flood tide is higher than that during the ebb tide. The current velocity and the SSC are the main factors controlling the flocculation processes and the floc settling velocity.

509 軟弱地盤特性を考慮したタイヤー地盤系の運動解析(車両の境界問題,モビリティへの応用,OS-19 マルチボディダイナミクス,総合テーマ「伝統を,未来へ!」)

The dynamic property and behavior of the soft ground are greatly affected by the constituent grains' shape. The consideration of the elastic-plastic property of the soft ground is also important for the multipass calculation of the tire on the soft ground. We developed the efficient interaction model of the tire and the soft ground that were composed of the distributed lumped mass-spring model as the tire, Discrete Element (DE) model as the upper soft ground, and the mass-spring model as the lower soft ground. In the present study, we have improved the previous soft ground model by considering the grains' shape and the elastic-plastic property of the soft ground, and have confirmed these effects through several numerical simulations.

Impact of Seasonal Changes on the Formation and Accumulation of Soft Siliceous Sediments on the Discharge Apron of Geysir, Iceland

Abstract Unconsolidated, highly porous, permeable siliceous sediments found in shallow-water pools on the middle and distal parts of Geysir’s discharge apron are formed of cryogenic opal-A (COA), opal-A spheres, opal-A cement, silicified microbes, and detrital lithic grains. Silicified leaves and silica-coated grass stems and twigs are locally present with these pool sediments, and microbial mats commonly cover the sediment surface. Partially consolidated layers (< 0.5 cm thick) form when the constituent grains are bound together by filamentous microbes, exopolysaccharides, and/or opal-A cement. Most components of the sediment can be related to processes that are controlled largely by seasonal variations. Low temperatures in winter promote freezing of the medial to distal discharge waters and lead to formation of COA in ice. When the ice melts, the released COA grains become part of the pool sediments. Microbial mats trap and bind sediment and provide templates for silica precipitation during spring and summer, but become (semi-)dormant during the constant darkness that Iceland experiences during winter. When the waters are sufficiently supersaturated, opal-A precipitates as spheres in the water column and cement in pool deposits. In some pools, seasonal changes are evident from the alternation of “soft” and “hard” (partly cemented) laminae. Elsewhere, however, the distinction between the different time-dependent components is lost because they become superimposed on each other or as aperiodic changes in flow patterns affect different pools.