Silica Phase Transitions Research Articles

Provenance and Sedimentary Context of Clay Mineralogy in an Evolving Forearc Basin, Upper Cretaceous-Paleogene and Eocene Mudstones, San Joaquin Valley, California

Mudstone samples from the Moreno (Upper Cretaceous-Paleocene) and Kreyenhagen (Eocene) formations are analysed using X-ray diffraction (XRD) and X-ray fluorescence (XRF) to determine their mineralogy. Smectite (Reichweite R0) is the predominant phyllosilicate present, 48% to 71.7% bulk rock mineralogy (excluding carbonate cemented and highly bio siliceous samples) and 70% to 98% of the <2 μm clay fraction. Opal CT and less so cristobalite concentrations cause the main deviations from smectite dominance. Opal A is common only in the Upper Kreyenhagen. In the <2 μm fraction, the Moreno Fm is significantly more smectite-rich than the Kreyenhagen Fm. Smectite in the Moreno Fm was derived from the alteration of volcaniclastic debris from contemporaneous rhyolitic-dacitic magmatic arc volcanism. No tuff is preserved. Smectite in the Kreyenhagen Fm was derived from intense sub-tropical weathering of granitoid-dioritic terrane during the hypothermal period in the early to mid-Eocene; the derivation from local volcanism is unlikely. All samples had chemical indices of alteration (CIA) indicative of intense weathering of source terrane. Ferriferous enrichment and the occurrence of locally common kaolinite are contributory evidence for the intensity of weathering. Low concentration (max. 7.5%) of clinoptilolite in the Lower Kreyenhagen is possibly indicative of more open marine conditions than in the Upper Kreyenhagen. There is no evidence of volumetrically significant silicate diagenesis. The main diagenetic mineralisation is restricted to low-temperature silica phase transitions.

High-pressure silica phase transitions: Implications for deep mantle dynamics and silica crystallization in the protocore

AbstractThe subsolidus phase diagram of silica in the 80–220 GPa pressure range was determined by density functional theory (DFT). The transition pressures calculated using the generalized gradient approximation (GGA) in the static limit (at 0 K, without zero point vibrational energy) for the β-stishovite (CaCl2-structure) to seifertite and the seifertite to pyrite-type transitions are 95 and 213 GPa, respectively. These are in good agreement with those calculated using hybrid functionals, giving transition pressures of 96 and 215 GPa. This indicates that previous local density approximation (LDA) results underestimate the transition pressure by 10–15 GPa. Density functional perturbation theory calculations, carried out using GGA within the quasi-harmonic approximations, give Clapeyron slopes of 5.4 and −2.8 MPa/K for the β-stishovite to seifertite and seifertite to pyrite-type transitions, respectively. This suggests that the seifertite-forming transition occurs at 109 GPa (470 km above the core-mantle boundary, CMB) at an ambient mantle geotherm, whereas the pyrite-type transition occurs at 200 GPa (620 km below the CMB) at 4700 K, which is close to the core adiabat. We also calculate the equation of state and show that the stability of seifertite in the lowermost mantle contributes negative buoyancy to recycled oceanic crust, although not as much as in some previous studies. Nevertheless, the increased density of seifertite over β-stishovite may lead to layers with elevated proportions of basaltic material within the large low S-wave velocity provinces. The seifertite to pyrite-type silica transition in the outer core will affect the silica liquidus surface in the system Fe-Si-O and forms a basis for further investigations of silica crystallization in the protocore.

Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn-Na-W/SiO2 Catalyst.

The work here presents a thorough evaluation of the effect of Mn-Na-W/SiO2 catalyst surface parameters on its performance in the oxidative coupling of methane (OCM). To do so, we used microporous dealuminated β-zeolite (Zeo), or mesoporous SBA-15 (SBA), or macroporous fumed silica (Fum) as precursors for catalyst preparation, together with Mn nitrate, Mn acetate and Na2WO4. Characterizing the catalysts by inductively coupled plasma-optical emission spectroscopy, N2 physisorption, X-ray diffraction, high-resolution scanning electron microscopy-energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and catalytic testing enabled us to identify critical surface parameters that govern the activity and C2 selectivity of the Mn-Na-W/SiO2 catalyst. Although the current paradigm views the phase transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right surface properties. Specifically, the SBA-based catalyst, characterized by relatively closed surface porosity, demonstrated low activity and low C2 selectivity. By contrast, for the same composition, the Zeo-based catalyst showed an open surface pore structure, which translated up to fourfold higher activity and enhanced selectivity. By varying the overall composition of the Zeo catalysts, we show that reducing the overall W concentration reduces the size of the Na2WO4 species and increases the catalytic activity linearly as much as fivefold higher than the SBA catalyst. This linear dependence correlates well to the number of interfaces between the Na2WO4 and Mn2O3 species. Our results combined with prior studieslead us to single out the interface between Na2WO4 and Mn2O3 as the most probable active site for OCM using this catalyst. Synergistic interactions between the various precursors used and the phase transition are discussed in detail, and the conclusions are correlated to surface properties and catalysis.

Diagenesis of spiculites and carbonates in a Permian temperate ramp succession – Tempelfjorden Group, Spitsbergen, Arctic Norway

AbstractThis paper explores little investigated diagenesis of spicule‐dominated sediments, based on Permian spiculites and cool‐water carbonates of the Tempelfjorden Group in central Spitsbergen. Field observations, petrography, stable isotope geochemistry, and mineralogical and chemical analyses reveal that the strata have been subjected to multistage diagenesis as the result of silica phase transitions at medium burial depths and deep‐burial overprinting. The growth of silica concretions occurred during the opal‐A/opal‐CT conversion and was controlled by the content and distribution of clay and spicules in the sediment, resulting in a variety of megascopic silica fabrics. Opal‐CT was subsequently dissolved, and all silica is now in a stable quartz stage. Petrographically, the rocks are characterized by a variety of chalcedony and quartz cements which perfectly preserve precursor textures. Most cements precipitated from silica‐oversaturated fluids, and their shapes reflect the silica saturation state and geometry of the pore space. Some microquartz and cryptoquartz also formed by a solid–solid inversion (recrystallization) of chalcedony. The cements have δ18O values between +30‰ and +20‰ Standard Mean Ocean Water and display a systematic depletion in 18O from the first to the last crystallized, interpreted to reflect a gradual increase in temperature during burial. The precipitation of quartz cements started in the Middle Triassic when the strata passed the 19°C isotherm at burial depths of ca 600 m, and was completed in the mid‐Cretaceous, 2·3 km beneath the sea floor at temperatures of 75°C. Late diagenetic overprinting of the chert includes fracturing, brecciation and cementation with carbonate cements having δ18O values between +2‰ and −30‰ Pee Dee Belemnite and δ13C values between +4‰ and −14‰ Pee Dee Belemnite; they are linked to hot solutions introduced during Cretaceous volcanism or Palaeogene tectonism. This study illustrates the diagenetic pathway during burial of spicule‐rich sediments in a closed system and thereby provides a baseline for studies of more complexly altered chert deposits.

Impact of three common post-depositional environmental settings on siliceous sinter diagenesis: An eight year experiment

Samples of newly-formed, opal-A, filamentous siliceous sinter were placed inside three contrasting environments to assess their effect on sinter diagenesis: (1) an actively discharging fumarole; (2) buried in steaming vegetation; (3) buried in soil in a non-thermal area. Each setting was chosen to represent common, sinter post-depositional environments. The sinter remained in these settings for eight years, during which time, samples were taken from the sinter and examined using X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). These techniques enabled changes in the silica phase mineralogy and morphology to be tracked over time. While all three sinters remained as opal-A during the eight years, minor but significant silica phase changes did take place and were reflected by the increase in maximum intensity value and shift in apex position on the XRPD trace. Silica phase transitions were greatest in the sinter buried in the steaming vegetation, followed by the sinter in the fumarole. The sinter buried in the non-thermal garden, revealed almost no change. Throughout the duration of the experiment, in all three settings, the filamentous nature of the sinter was preserved. Botryoidal clusters and new generations of opal-A spheres were observed in samples, from all three sites. At various time periods, in all environments, silicified filaments formed smooth, non-porous ridges separated by a more open arrangement of silicified filaments.Major, minor and no dissolution textures were observed in the sinter buried in the steaming vegetation, non-thermal soil and the fumarole settings respectively. Rows on aligned, opal-A spheres were only present in the fumarole sinter at 33weeks. Tracking sinter diagenesis in three commonly found, post-sinter formation environments, contributes to our understanding of the preservation potential of siliceous sinters and the influence steam, meteoric water and organics have on sinter diagenesis.

Phase transition of silica in the TMB–P123–H2O–TEOS quadru-component system: A feasible route to different mesostructured materials

Various siliceous structures were obtained using a nonionic block copolymer (Pluronic P123) surfactant and trimethylbenzene (TMB) as a hydrophobic additive by hydrolysis and condensation of tetraethoxysilane (TEOS) in a sol–gel process. The resultant materials were characterized by small-angle X-ray diffraction (SAXRD), nitrogen adsorption analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed the structure transformation from hexagonal structure (HEX) to multilamellar vesicles (MLVs) and then to mesocellular foams (MCFs) in the TMB–P123–H2O–TEOS quadru-component system. The morphology of the mesoporous silica was mainly controlled by the mass ratio of TMB/P123 resulted from the increasing volume of the hydrophobic chain of micelle of P123 that caused by more amount of TMB dissolved in the PPO segment of polymer. The fact that the occurrence of rod-like particles with curved ends and the coexistence of the MLVs and the HEX structure indicates that the MLVs are developed from the ends of HEX structures, rather than formed by a direct cooperative self-assembly mechanism. Further increasing of packing parameter of surfactant resulted from TMB addition transforms lamellar micelles to reversed micelles, leading to the formation of MCFs.

Silica phase-transformations during diagenesis within petrified woods found in fluvial deposits from Thailand–Myanmar

Transitions in petrified wood samples from several Thailand–Myanmar sites included opal-A to opal-CT, opal-A to metastable β-crystobalite and reordered opal-CT→quartz. The moganite in the samples indicates a potential volcanic silica source for the wood petrifactions. Silica phase transitions evident under SEM include opal-A→opal-CT, opal-A→cuboid quartz (pseudocubic) and opal-CT→quartz. Certain stages of silica transformation and transition processes seem to overlap. Such overlapping results when lepisphere blades transform to the high crystallinity of prismatic quartz through a solid-state mechanism. The results provide some insights into the petrified wood genesis in the region. Thermal inputs into the silicification process may have pre-dated final burial of woods in flood deposits.

Chert spheroids of the Monterey Formation, California (USA): early‐diagenetic structures of bedded siliceous deposits

AbstractChert spheroids are distinctive, early‐diagenetic features that occur in bedded siliceous deposits spanning the Phanerozoic. These features are distinct in structure and genesis from similar, concentrically banded ‘wood‐grain’ or ‘onion‐skin’ chert nodules from carbonate successions. In the Miocene Monterey Formation of California (USA), chert spheroids are irregular, concentrically banded nodules, which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite. During shortening, there is brittle fracture of diatomite around, and horizontally away from, the convex surface of strain‐resistant chert nodules. Unlike most older siliceous deposits, the Monterey Formation still preserves all stages of silica diagenesis, thus retaining textural, mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures. Chert spheroids found in opal‐A diatomite form individual nodules composed of alternating bands of impure opal‐CT chert and pure opal‐CT or chalcedony. With increased burial diagenesis, surrounding diatomite transforms to bedded porcelanite or chert, and spheroids no longer form discrete nodules, yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony. Petrographic observations show that the purer silica bands are composed of void‐filling cement that precipitated in curved dilational fractures, and do not reflect geochemical growth‐banding in the manner of Liesegang phenomena invoked to explain concentrically banded chert nodules in limestone. Chertification of bedded siliceous sediment can occur more shallowly (< 100 m) and rapidly (< 1 Myr) than the bulk silica phase transitions forming porcelanite or siliceous shale in the Monterey Formation and other hemipelagic/pelagic siliceous deposits. Early diagenesis is indicated by physical properties, deformational style and oxygen‐isotopic composition of chert spheroids. Early‐formed cherts formed by pore‐filling impregnation of the purest primary diatomaceous beds, along permeable fractures and in calcareous–siliceous strata.

Effect of Alkali Ions on the Amorphous to Crystalline Phase Transition of Silica

The effect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural effect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS diffractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identification of the fraction of the different silica polymorphs like quartz, tridymite, and cristobalite. SAXS measurements, using the classical method based on Porod's law, yielded the total surface area of the systems. The calculated areas were compared with the surface areas determined by the nitrogen adsorption technique using the analytical method of Brunauer–Emmett–Teller. The results are explained in terms of sizes of the alkali ions and cell volume of the different crystalline phases.

Sequences of silica phase transitions: effects of Na, Mg, K, Al, and Fe ions

Hydrothermal experiments on silica phase transitions in artificial solutions tested the effects of dissolved aluminum and iron on the evolution of solution composition and the mineral compositions of solid phases, Dissolved aluminum and iron come from detrital aluminosilicate phases, including their coatings. Aluminosilicate phases retard formation of opal-CT, a metastable silica phase, but they may promote transformation of opal-CT to quartz, the stable phase. The four starting solutions all had alkalinity greater than seawater, either 90 or 8 mM. The four solutions were dominated by different cations: one solution contained magnesium ion, one magnesium and sodium ions, one potassium ion, and one sodium ion. Compositions of the solutions changed within 4 weeks. Quartz formed under a higher initial alkalinity of ~90 mM, with or without magnesium. Minor phases, including opal-CT, clay minerals and melanophlogite, were also observed in some experiments. Opal-CT formed in the presence of magnesium and an initial alkalinity above 8 mM. Dissolved aluminum or iron reduced concentrations of dissolved silica, causing precipitation of an initially more ordered opal-CT in the presence of magnesium, and quartz in its absence. These effects influence the rates and sequences of silica diagenesis.

Generalized gradient theory for silica phase transitions.

Density functional theory based on the generalized gradient approximation to the exchange and correlation energy is shown to correct a qualitative error of the local density approximation in describing a high-pressure phase transition of ${\mathrm{SiO}}_{2}$. Advantages of an adaptive curvilinear coordinate method for such generalized gradient calculations are discussed.

Periodic Hartree-Fock investigation of the stishovite CaCI2-like phase transition of silica

Periodic Hartree-Fock calculations have been performed in order to investigate the stishovite-CaCl2 -like structure phase transition observed for silica above 100 GPa. The mechanism involves the B1g ([math]) mode which corresponds to the rotation of the SiO6 octahedra around their C2 axes. The B1g frequency presents negative pressure shifts. It is shown that the potential energy surface with respect to the relevant normal coordinate appears to be a symmetric single well at low pressures whereas at pressures higher than 80 GPa it becomes a symmetric double well. This explains why the CaCl2 -like structure is unquenchable. Moreover we predict for this silica modification an order-disorder phase transition at high temperatures.

Chemical factors influencing the rates and sequences of silica phase transitions: Effects of organic constituents

Dissolved organic matter affects the solution chemistry, hence silica recrystallization reactions, in laboratory experiments. In the presence of organic matter, the reaction rates of silica phase transitions (opal-A to opal-CT) are reduced, and the opal-CT phase produced is more “ordered” (lower d(101) spacing) than that in the absence of organic materials. The degree of this decrease in d(101) spacing depends on the solution properties of the organic materials used in the experiments. The lower the pH of a solution and the higher its organic alkalinity, the slower the formation rate of opal-CT and the lower the d(101) spacing of the opal-CT phase that forms.

Chemical and Sr isotopic variations during diagenesis of Miocene siliceous sediments of the Monterey Formation, California; discussion of the Sr isotopic data and its relevance to the timing of Monterey Formation fracturing

ABSTRACT A further evaluation of the Rb/Sr data from Monterey Formation diatomites, CT-cherts, and quartz cherts (Brueckner and Snyder 1985) confirms that a mixing relationship exists for the diatomites and porcelanites but not for the CT- and quartz cherts. The Rb/Sr system is believed to be responding to solution-reprecipitation reactions which accompany the silica-phase transitions. Sr isotopic compositions of calcites from tar-filled fractures in dilation breccias are identical to the initial Sr ratio of the quartz chert linear array (0.70909), suggesting a synchroneity of fracturing, petroleum migration, and quartz chert formation. The Rb/Sr data are used to calculate a mid-Pliocene age for the fracturing event.