Nucleation Of Quartz Research Articles

Quantification of Feldspar and Quartz Nucleation Delay in a Hydrous Peraluminous Granitic Melt

A modified model based on classical nucleation theory was applied to a natural hydrous peraluminous pegmatite composition and tested against crystallization experiments in order to further investigate the quantification of nucleation delay in felsic melts. Crystallization experiments were performed in a piston-cylinder apparatus at 630 MPa and temperatures between 650 and 1000 °C for durations ranging from 0.3 to 211 h. Experimental run products were investigated by scanning electron microscopy paired with energy dispersive spectroscopy analyses of both crystalline and quenched liquid phases, the results of which were compared to an established theoretical nucleation delay model from the literature. The experiments showed good agreement (within a factor of 5) with the model for quartz, while it showed moderate agreement (within a factor of 10) with the model for sodic feldspar. Other crystals also nucleated, demonstrating abundant features of disequilibrium. Our research further demonstrates the potential of the model to predict nucleation delay, showing promising results for the quantification of the nucleation delay of quartz and feldspar in natural felsic melts, thus adding to previously published studies on hydrous, metaluminous, felsic melts and dry basaltic melts.

Electrophoretic Deposited Quartz Powder-Assisted Growth of Multicrystalline Silicon

Ingot multicrystalline silicon (Mc-Si) needs to be improved in quality and reduced in cost compared with Czochralski monocrystalline silicon. A uniform and dense quartz nucleation layer was obtained by the electrophoretic deposition of quartz powder on the surface of the silicon wafer. The deposited silicon wafer was annealed at 600 °C for 1 h, and one side of the silicon wafer with the quartz layer was glued to the crucible. During the growth of Mc-Si crystal, the dense quartz powder can play a nucleation role. The results show that the average lifetime of the minority carriers a of quartz-assisted silicon ingot is 7.4 μs. The overall dislocation density of an electrophoretic deposition quartz-assisted silica ingot is low, and the defect density in the middle of the silica ingot is 1.5%, which is significantly lower than that of spray quartz (3.1%) and silicon particle (4.2%). Moreover, electrophoretic deposited quartz-assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Occurrence and origin of chlorite and associated impact on tight sandstone reservoir quality: A case study of the Xujiahe Formation (NE Sichuan Basin, China)

Chlorite is beneficial to the preservation of primary pores, and its formation mechanism in marine sandstone has been widely reported. However, few studies have focused on the genetic mechanism of various types of chlorite in lacustrine deltaic sandstone. Therefore, the occurrence, diagenetic time and origin of chlorite in Xujiahe tight sandstone were investigated using microscopic images and mineralogical chemical analysis, and three stages of chlorite were identified, namely, grain-coating (Ch1), pore-lining (Ch2), and pore-filling (Ch3) chlorite, which belong to the Fe chlorite, Al chlorite and transitional types, respectively. The variations in morphology and chemical composition reflect differences in the sources and formation mechanisms. Chlorite sandstones are mainly distributed in the underwater distributary channel, which has strong hydrodynamic conditions. The formation of Ch1 is controlled by the sedimentary environment and flocculation. Salinity differences resulted in a larger amount of flocculation deposits in the estuaries during the syngenetic period. In the shallow burial stage, the clay coating underwent in situ transformation under thermal impacts and formed the Ch1. Xujiahe sandstones experienced complex diagenetic processes, which include early compaction, three stages of chlorite cementation, two stages of quartz precipitation, dissolution and two stages of carbonate cementation. Organic acid dissolution of volcanic rock fragments, phyllite, and biotite likely supplied sufficient Fe 2+ for the growth of Ch2 in a relatively closed diagenetic system. As the pore water became weakly alkaline, diagenetic fluid migration originating from the clay mineral transformation in mudstone mainly provided Fe 2+ and Mg 2+ for Ch3 growth in sandstones. Chlorite is considered to have “composite” impacts on deep tight sandstones, with grain-coating and pore-lining chlorites protecting the primary pores by restricting the silica supply and occupying potential quartz nucleation sites; however, grain-coating chlorite caused throat obstruction or even closure under strong mechanical compaction, which reduced the permeability of the reservoir. • Three different types of chlorite have been found in the Xujiahe tight sandstones. • It has been clear that the formation time of chlorite and the paragenetic sequence of authigenic minerals. • The origin and formation mechanism of authigenic chlorite in three types have been investigated. • We considerd that authigenic chlorite has a “composite” impact on deep tight sandstone: preserve primary porosity but reduce the permeability.

Growth of multicrystalline silicon assisted by electrophoretic deposited quartz

Cast multicrystalline silicon (mc-Si) requires high quality at low cost, while fully-melted seed-assisted growth has a reasonable utilization rate but urges better initial grain control. An electrophoretic deposited quartz nucleation layer is introduced to obtain a uniform and oriented nucleation layer. Carrier recombination characterization, including photoluminescence and carrier lifetime mapping, shows that the density of recombination-active structural defects is reduced due to better grain distribution. The average conversion efficiency values of silicon particle, sprayed quartz, and deposited quartz assisted wafers are 20.52%, 20.48%, and 20.68%, respectively Moreover, deposited quartz assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Myrmekite and strain weakening in granitoid mylonites

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accommodated by a combination of fracturing, dissolution–precipitation, and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by the formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we use electron backscattered diffraction to (i) investigate the microstructure of a granodiorite mylonite, developed at ∼ 450 °C during cooling of the Rieserferner pluton (Eastern Alps); and (ii) assess the microstructural processes and the weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of plagioclase in pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain-size reduction and anti-clustered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain-size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws, calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite) during deformation at 450 °C, show that grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain-size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the felsic middle crust.

Nucleation of quartz under ambient conditions

Quartz nucleation normally requires harsh conditions; for instance, hydrothermal processes at 200–300 °C and 15–100 bar, where the high growth rate precludes selective formation of nanometer-sized quartz. Under ambient conditions, even quartz growth is considered to occur exceedingly slowly, requiring yearly timescales. Here we show that nanoquartz of size 1–5 nm can be nucleated from microemulsions under ambient conditions within 2 days. The nanoquartz particles are grown and annealed under mild hydrothermal conditions of 175 °C and autogenic pressure. This enables nanoquartz of size <10 nm and controllable structure to be obtained exclusively. Surface defects play a key role in determining the catalytic properties of the nanoquartz. Consequently, a similar two-stage microemulsion-hydrothermal synthesis strategy could provide significant benefits in optimizing nanocrystal catalysts.

Mechanical compaction in chlorite-coated sandstone reservoirs – Examples from Middle – Late Triassic channels in the southwestern Barents Sea

Abstract The relationship between diagenetic evolution and reservoir quality in large-scale Middle – Late Triassic aged channel systems (up to 20 km wide) in the southwestern Barents Sea is investigated through core plug data, XRD, SEM- and modal analyses. The studied channel systems are likely sourced from the southeastern Uralide mountain range and are characterized by chemically unstable clastic sediment and well-developed, porosity-preserving chlorite coatings. Chlorite coatings occupy potential quartz nucleation sites on the framework grain surfaces and likely prevent significant chemical compaction in deeply buried sandstones. Porosity-reduction is believed to follow mechanical compaction trends of similar sandstone compositions. Modelling and prediction of porosity preservation in Middle - Late Triassic channel sandstones in the study area is therefore possible, if temperature histories and sandstone compositions are well constrained. The tidally influenced channel and fluvial-dominated channels in this study show significant variation in reservoir quality. These differences are found to be linked to amount of allogenic matrix and grain size, which significantly reduces the permeability in the tidally influenced channel. If seismic distinction between different channel types is impossible, the distribution of permeability is considered unpredictable. Chlorite coatings in the investigated channels are interpreted to be diagenetic overprints of a precursor clay phase, which appears to have a strong link to the Uralian provenance. Coating precursor emplacement likely occurs prior to significant burial, but the exact physical conditions enabling this process remain elusive without systematic laboratory and analogue studies.

POROSITY PRESERVATION DUE TO AUTHIGENIC CHLORITE COATINGS IN DEEPLY BURIED UPPER TRIASSIC XUJIAHE FORMATION SANDSTONES, SICHUAN BASIN, WESTERN CHINA

Non‐marine sandstones in the Upper Triassic Xujiahe Formation form an important reservoir unit in the Sichuan Basin, western China. Widespread authigenic chlorite cement is present in the second and fourth members of the formation, referred to as the Xu II and IV members. This study, based on data from 12 wells from three gasfields, focusses on chlorite development and evolution and attempts to evaluate its influence on reservoir quality. Data included results of X‐ray diffraction (XRD) and electron microprobe analyses (EMPA), scanning electron microscopy (SEM), and stable carbon and oxygen isotope analyses. The results showed that chlorite coatings preserve primary pore spaces in the sandstones by inhibiting the nucleation of quartz cements on grain surfaces. Iron for chlorite growth came from the dissolution of Fe‐rich magmatic rocks derived from the palaeo‐Longmenshan fold‐belt, from dissolved and colloidal iron which had been precipitated in the Xujiahe Formation in a fluvio‐deltaic environment, and from compacting mudstones in other pars of the formation.The chlorite initially precipitated from pore waters during eodiagenesis (&lt; 70 °C; &lt; 2 km burial depth). During mesodiagenesis (&gt;70 °C, &gt; 2 km), the chlorite was modified due to exposure to Mg‐rich fluids derived from the underlying carbonate‐dominated Leikoupo Formation. Thus the Mg content of chlorite in the Xu II member increases with depth. However this phenomenon was not observed in the Xu IV member.The effects of authigenic chlorite on the sandstones' petrophysical properties were evaluated from correlations between chlorite content and porosity. Optimum thicknesses for chlorite coatings range from 5 to 10 microns. Thicker or thinner coatings appear to be less capable of preserving primary porosity and permeability.

Early diagenetic quartz formation at a deep iron oxidation front in the Eastern Equatorial Pacific – A modern analogue for banded iron/chert formations?

The mechanisms of early diagenetic quartz formation under low-temperature conditions are still poorly understood. In this study we investigated lithified cherts consisting of microcrystalline quartz recovered near the base of a 420m thick Miocene–Holocene sequence of nannofossil and diatom ooze at a drill site in the Eastern Equatorial Pacific (Ocean Drilling Program Site 1226). Precipitation seems still ongoing based on a sharp depletion in dissolved silica at the depth of the cherts. Also, palaeo-temperatures reconstructed from δ18O values in the cherts are in the range of adjacent porewater temperatures. Opal-A dissolution appears to control silica concentration throughout the sequence, while the solution remains oversaturated with respect to quartz. However, at the depth of the sharp depletion in dissolved silica, quartz is still saturated while the more soluble silica phases are strongly undersaturated. Hence, precipitation of quartz was initiated by an auxiliary process.A process, previously observed to assist in the nucleation of quartz is the adsorption of silica on freshly precipitated iron oxides. Indeed, a deep iron oxidation front is present at 400m below seafloor, which is caused by upward diffusing nitrate from an oxic seawater aquifer in the underlying oceanic crust. Sequential iron extraction showed a higher content of the adsorbed iron hydroxide fraction in the chert than in the adjacent nannofossil and diatom ooze. X-ray absorption near-edge structure (XANES) spectroscopy revealed that iron in the cherts predominantly occurs in illite and amorphous iron oxide, whereas iron in the nannofossil and diatom ooze occurs mainly in smectite. Mössbauer spectroscopy also indicated the presence of illite that is to 97% oxidized.Two possible mechanisms may be operative during early diagenetic chert formation at iron oxidation fronts: (1) silica precipitation is catalysed by adsorption to freshly precipitated iron oxide surfaces, and (2) porewater silica concentration is locally decreased below opal-A and opal-CT saturation allowing for precipitation of the thermodynamically more stable phase: quartz. This mechanism of chert formation at the iron oxidation front in suboxic zones may explain why early-diagenetic microcrystalline chert only occurs sporadically in modern marine sediments. It may also serve as a modern analogue for the deposition of much more abundant banded iron/chert formations at the time of the great oxidation event around 2.4Ga BP, which was probably the largest iron oxidation front in Earth's history.

How clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones: Observations and experiments

Observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones. Samples of deeply buried, porous sandstones with different types of clay coats were split in two, coats removed from one of each pair of splits, and grain surfaces inspected with scanning electron microscopy. Quartz grains in a fluvial-deltaic sandstone buried to 115C had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats, though well-developed overgrowths occurred on nearby, naturally uncoated grains. However, in similar sandstones buried to 164C, quartz-grain surfaces exposed by chlorite-coat removal were covered with small (5 m), mainly anhedral, syntaxial quartz overgrowths. Similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215C. We conclude that clay coats may retard quartz nucleation at moderate temperatures, but at high temperatures, many coats permit quartz nucleation and preserve porosity by limiting cement growth. To investigate cement growth-limitation mechanisms, samples with coats removed were subjected to quartz-cementing conditions in a hydrothermal reactor. During experiments, the naturally occurring small overgrowths on clay-cleaned grains coalesced and grew, suggesting that clay particles in coats inhibit cement growth by forming barriers to early-overgrowth coalescence. Although the fraction of grain-surface coverage is the primary control on cement inhibition by coats, cement growth–interference textures vary with coat type, providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness. In deeply buried sandstones, quartz cement can fill significant microporosity within diagenetic chlorite coats, potentially affecting mechanical and petrophysical rock properties.

Mineralogical and textural variation of silica minerals in hydrothermal flow-through experiments: Implications for quartz vein formation

We conducted hydrothermal flow-through experiments at 430 °C and 31 MPa to investigate the mechanism of silica precipitation on granite under crustal conditions. Two experiments were performed using different input solutions: a single-component Si solution, and a multi-component solution with minor Al, Na, and K. The degree of supersaturation with respect to quartz, Ω = CSi/CSi,Qtz,eq, where CSi and CSi,Qtz,eq indicate Si concentration in solutions and the solubility of quartz within water, respectively, decreased from 3–3.5 to <1.1 along the flow path. A variety of silica minerals formed during the experiments (opal-A, opal-C, chalcedony, and quartz), and their occurrences and modal abundances changed in response to Ω and the presence of additives in the solution. For near-equilibrium solutions (Ω ∼1.2), silica minerals were deposited on other surfaces in addition to quartz. In the single-component experiment, the dominant silica minerals changed in the order of opal-A → opal-C → quartz with decreasing Si concentration along the flow path. In contrast, in the multi-component experiment, quartz and minor chalcedony formed throughout the entire reaction vessel. This finding indicates that impurities (Na, K, and Al) in the solutions inhibited the precipitation of opal and enhanced the direct nucleation of quartz. The systematic appearance of metastable silica minerals were examined by nucleation processes and macroscopic precipitation kinetics. Our experimental results indicate that different precipitation mechanisms yield contrasting textures, which in turn suggests that vein textures can be used as indicators of solution chemistry within the fracture.

Microfabrics of quartz formed from coesite (Dora-Maira Massif, Western Alps)

The microfabrics of quartz formed from coesite during exhumation of ultrahigh pressure (UHP) metamorphic rocks of the Dora Maira Massif (western Alps) are investigated using polarizing microscopy with universal (U)-stage, electron backscattered diffraction (EBSD), and cathodoluminescence (CL) microscopy. Three types of microstructures are distinguished: aggregates of small elongate quartz grains with radial orientation (palisades) replacing coesite single crystals included in other minerals (type I); platy quartz single crystals showing tapered margins and midribs observed by CL-microscopy occurring associated with type I microstructures in the same inclusions (type II); radial aggregates of large elongate quartz grains in the rock matrix, where coesite is not preserved (type III). The radial shape preferred orientation (SPO) in the type I and III palisade structures is not combined with a crystallographic preferred orientation (CPO). There is no systematic orientation relation between quartz grains in type I aggregates and relict coesite or the host mineral of the inclusion. This indicates that the coesite-quartz transformation started with a distinct random nucleation stage along the original interphase boundary between coesite and the host mineral. The elongate grain shape reflects the progress of the reaction front, which is not influenced by crystallographic orientation. The concentric discontinuities in grain shape frequently observed in type I quartz palisades are attributed to distinct events during transformation, presumably brittle failure of the host mineral. By analogy, coarse-grained quartz palisades (type III) in the rock matrix originated by random nucleation of quartz along the high angle grain boundaries of coesite. Each aggregate of radially oriented elongate grains (type I and III) impinging at a centerline represents the size and shape of a single original coesite grain, with a grain size of up to several millimeters in the matrix of pyrope quartzites (type III), and significantly smaller in metagranites. Type II platy quartz crystals with tapered margins are interpreted to have grown along cracks in coesite, which formed in several generations with distinct orientation during progressive transformation, indicating a very low nucleation probability of quartz for that stage.

Diagenetic modeling to assess the relative timing of quartz cementation and brittle grain processes during compaction

This study describes porosity reduction by brittle deformation and the application of Touchstone sandstone diagenesis modeling software to assess the relative timing and interactions between grain fracturing and cement formation during burial compaction. Two examples from a previous study of compactional fracturing are used: the Oligocene Frio Formation, Gulf of Mexico Basin, and the Cambrian Mount Simon Formation, Illinois Basin, United States. Grain fracturing during compaction creates intragranular fracture surfaces that are favorable sites for quartz nucleation compared to external grain surfaces that may bear coatings that inhibit the nucleation and growth of quartz cement. Thus, the progress of brittle fracture processes during diagenesis affects quartz cementation. In turn, modeling of the quartz cementation process can serve to place fracturing into its proper context in burial history. In the Mount Simon Formation, the extent of brittle deformation of quartz grains correlates with reconstructed effective stress at the onset of quartz cementation. For Frio Formation samples, however, the extent of brittle deformation does not correlate well with reconstructed effective stress obtained using a one-dimensional basin model that uses compaction disequilibrium as the dominant mechanism for overpressure generation. Judging from the observed degree of grain fracturing, significant fluid overpressures in the Frio may not have developed at the shallow depths indicated by our basin models. The degree of compactional fracturing in sandstones constitutes observable evidence that can be used to decipher the complexities of pressure history.

Silica enrichment, graphic granite and aquamarine growth: a new exploration guide

Granitic pegmatites are traditionally known to contain graphic, perthitic and myrmekitic intergrowths related to quartz and K- and Na- feldspars. They are further considered to characterise the pegmatite types distinguishing them from the granites and other related plutonic rock types. Graphic granite is accepted also as a synonym to granitic pegmatite. Systematic studies, by the author and colleagues, on the granitic pegmatite gem deposits have permitted the definition of two aquamarine gem provinces in ENE Brazil, one in the NeoProterozoic and the other in the Archaean sequences. Potash feldspars in the pegmatites in the former show perthitic intergrowths, whereas in the latter graphic intergrowth dominates with anomalously coarse centimetric quartz along the cleavages of K-feldspar. Several granitic pegmatites hosted in Archaean complex, in Lages Pintadas Aquamarine Province, Santa Cruz, RN State, present this texture-structure. Graphic intergrowth is attributed to the eutectic crystallization, succeeded by hydrothermal fluids with silica enrichment permitting the growth through diffusion and nucleation of quartz and along cleavages of potash feldspar. In the Archaean terrain, the abundance of recycled chert forming metapsammitic migmatites traversed by numerous quartz veins and coarse graphic granites, has contributed to the growth of beryl and also the aquamarines.

Cathodoluminescence and micro-structural evidence for crystallisation and deformation processes of granites in the Eastern Lachlan Fold Belt (SE Australia)

Trace elements (Al, K, Ti, Fe), growth and deformation pattern in quartz of the multiple deformed Carcoar, Barry and Sunset Hills granites were investi- gated by electron micro probe and cathodolumines- cence. Zoned quartz phenocrysts with high Ti concentrations (>70 ppm) that show blue cathodolu- minescence originated from the early stage of magma crystallisation. Multiple deformation of quartz causes the redistribution of Al and K in the quartz lattice, which results in the accumulation of these elements in submicroscopic inclusions (<0.5 lm) of muscovite-like composition. In contrast, structural Ti in quartz is mostly kept in the lattice. Common halos of defect-poor, secondary quartz around fluid inclusions result from re- equilibration of fluid inclusions because of differences between fluid pressure and lithostatic pressure, e.g. duringuplift (isothermal decompression) and/or a/b- transition of quartz. Duringhealing , defect-poor sec- ondary quartz grows at the cost of the host quartz and releases or replaces defect centres. The results of micro- structural investigations combined with Al-in-horn- blende thermobarometry allow the reconstruction of regional processes. Carcoar and Barry granodiorites and Sunset Hills granite were intruded in the Late Ordovi- cian-Early Silurian at depths of 4-8.6 and 10-12 km, respectively. In contrast to the continuous crystallisation of the granodiorite magmas, the magma of the Sunset Hills granite ascended in a stepwise fashion, causing multiple quartz nucleation. The two granodiorites were multiple, post-magmatically deformed, first, during Early Devonian under more brittle conditions at temperatures of 350-400 � C, whereas the Sunset Hills granite experienced more ductile deformation at tem- peratures of around 550 � C.

Textural evolution in the transition from subsolidus annealing to melting process, Velay Dome, French Massif Central

Petrographic analysis is a useful, but underused tool to aid in distinguishing between subsolidus and anatetic‐related textures in migmatites. This study focuses on assessing the relative contributions of these two processes in the development of migmatitic orthogneiss textures in the Velay Massif, French Massif Central. The results of this study show that subsolidus processes are more important in the development of migmatitic textures in the orthogneiss than anatectic leucosome development. Four textural stages are identified from the mylonitic non‐anatectic orthogneiss, annealed, migmatitic orthogneiss to diatexite. The monomineralic K‐feldspar and plagioclase–muscovite banding was transformed with increasing temperature to polymineralic plagioclase–quartz–muscovite and K‐feldspar–quartz–muscovite layers by the wetting of feldspar boundaries during heterogeneous nucleation of quartz from a fluid phase at high surface energy triple points. A further increase of temperature led to the growth of K‐feldspar probably related to production of small amounts of melt in plagioclase rich aggregates, controlled by muscovite abundance. Solid state annealing processes in conjunction with incipient anatexis resulted in the formation of apparent granitic‐like textures in plagioclase dominated aggregates. By contrast, in K‐feldspar dominated aggregates exclusively subsolidus processes prevail, leading to the development of coarse grained leucosome. With the onset of biotite dehydration melting the plagioclase‐dominated aggregates are destroyed by the melt whereas the K‐feldspar aggregates may be preserved.

Siliceous Sediments of the Guaymas Basin: The Effect of High Thermal Gradients on Diagenesis

Recent siliceous sediments of the Guaymas Basin, a rapid spreading center in the Gulf of California, are intruded by basaltic sills, which induced significant diagenetic and metamorphic changes in the sediments. The transformation from opal-A to opal-CT, opal-CT to quartz, and opal-A directly to quartz in these sediments, cored on DSDP Leg 64, can be used to infer the temperature history and order of emplacement of intrusives. At or near contacts with sills of 30 m or greater average thickness, opal-A inverts directly to authigenic quartz, but there is less quartz than would be expected from the amount of opal-A dissolution. Opal-CT forms in sediments sandwiched between adjacent sills. Based on high rates of quartz nucleation and growth at high temperatures (>150°C), and on considerations of convective solution transfer, opal-CT is thought to form only where temperatures were lower or at positions between sills, an environment which prevents rapid convective dissipation of silica in solution. Where temper...

Croissance naturelle de paracristaux de quartz dans une saumure sulfatée calcique à basse température

In brines with dissolved Ca-sulphates, the growth of quartz is attained by a close packed assemblage of nuclei. These are depicted as sharp spots of 250 Å in size by SEM mode imaging. These are either spherical nuclei or electron optical diffusion features which are induced by smaller particles. Nuclei give quartz fibers by a coalescence process. The average diameter of fibers is 1 000 Å. Their length is 5 to 10 nm. By successive coalescences, fibers give ribbons, bidimensional sheets and tridimensional quartz ultimately. In the quartz paracrystal resulting from repeated attachment of nuclei, the tridimensional continuity is attained by ageing of the first precipitate. At intermediate stages of this evolution, the quartz paracrystal is crossed by very numerous inframicroscopic channels, leaving a high internal porosity in each paracrystal. The primary structure, characterized by a close packed assemblage of nuclei, is very similar to the organization of spherical amorphous silica particles in precious opal. The ordered precipitation of both quartz nuclei in a paracristalline structure and amorphous silica particles in a precious opal are the products of the colloidal ordered precipitation due to the second potential well of the interaction energy between two particles. Quartz nucleation appears by the salting out of dissolved monomere silica. All this results from mixing of a brine and of a silica saturated solution. Quartz nucleation may be improved by impurities (Al⁺⁺⁺, hydrocarbures) in the growth medium.

Discussion sur les conditions et les mécanismes de formation du quartz à 25 °C en milieu ouvert. Analyse réactionnelle par voie cinétique

The occurrence of quartz in soils and pedological formations, booth as primary residual mineral and as a secondary neogenesis constituent, is not fully explained at the moment. The subject has been studied experimentally using cristobalite as a source of silica in conditions of water or solutions flux. From a kinetic analysis, three main aspects are shown and discussed : — The first one is related to the experimental formation of quartz. A domain of occurrence in relation to amorphous silica is defined and conditions of crystallization are precised. — In the second one, the problem of a possible competition between heterogeneous quartz nucleation and metal adsorption on cristobalite surface (mainly Al) is studied. — The problem is extended to the more general competition between adsorption phenomena and silicate neoformation processes.