Formation Of Saponite Research Articles

Sepiolite and Other Authigenic Mg-Clay Minerals Formation in Different Palustrine Environments (Madrid Basin, Spain)

Lithofacies belonging to mud-flat and palustrine deposits (lake margin) in the Miocene of the Madrid Basin (Spain) have been studied. Four lithofacies corresponding to mud flat (1 and 3) and palustrine (2 and 4) deposits have been differentiated. Units 1 and 3 consist mainly of mudstones and carbonates (calcretes and diolocretes). The clay fraction is dominated by trioctahedral smectite (up to 79%) with illite and kaolinite as minor components. The d(060) spacing value shows reflections at 1.52 and 1.50 Å indicating also the presence of dioctahedral phyllosilicates. Unit 2 consists predominantly of lutites (claystones), locally with carbonate and chert nodules. The clay fraction is dominated by sepiolite (up to 96%) with variable contents of smectite and subordinate illite. The d(060) spacing value shows reflections at 1.51 and 1.52 Å indicating trioctahedral clay minerals. Unit 4 consists mostly of carbonates (limestones) with mudstone and lutite inserts. The clay fraction shows different contents of sepiolite, palygorskite and dioctahedral smectite. The analysis of a selection of trace elements (Cr, Co, Th, La, Sc) has allowed us to determine the characteristics of the source area as dioritic, somewhat different from those of the nearby materials from the Batallones sector. Sepiolite shows FWHM values ranging between 0.68 and 1.10 (2θ), indicating “low crystallinity sepiolite”. Differences in the conditions of formation of magnesian smectite and palygorskite have been observed in the mud-flat and palustrine deposits. The formation of sepiolite mainly by neoformation in palustrine deposits with different hydrochemistry is remarkable, leading to differences in fibre size and crystallinity of the fibrous clay mineral. Authigenic transformation processes from previous Al-rich phases would be responsible for the formation of saponite and palygorskite in mud flat and palustrine environments, with different pH conditions.

Mechanism and Control of Saponite Synthesis from a Self-Assembling Nanocrystalline Precursor.

Saponite is a clay mineral of the smectite group that finds applications in the chemical industry as a catalyst or catalyst precursor as well as in nanocomposites used for structural or catalytic applications. Saponite of controlled composition, crystallinity, particle size, and morphology would be highly beneficial to industry; however, such materials are not found in a sufficiently pure form in nature. Synthetic methods to produce saponite with specific properties are currently lacking as the understanding of the mechanisms controlling its formation, crystalline properties and particle morphology, is limited. Understanding the saponite formation mechanism is crucial for the development of a highly tuned and controlled synthesis leading to materials with specific properties. Here, we report a new chemical reaction mechanism explaining the nucleation and kinetics of saponite growth at different pHs, at 95-100 °C, and under the influence of pH-modifying additives explored via a combination of X-ray scattering methods and infrared spectroscopy. Our results show that the main factor affecting the nucleation and growth kinetics of saponite is the pH, which has a particularly significant impact on the rate of initial nucleation. Non-uniform reactivity of the aluminosilicate gel also significantly affects saponite growth kinetics and causes a change in the rate-determining step as seen in graphical abstract. The most crystalline saponite is obtained when the nucleation is suppressed by a low initial pH (<7), but the reaction is performed at a higher pH of about 9. The stacking of the saponite sheets can be further improved by a separate postsynthesis treatment with an alkali (NaOH) solution. A simple, ambient pressure method for synthesizing a highly crystalline saponite is proposed that could be easily upscaled for industrial purposes.

Authigenic Mg-clay assemblages in the Barra Velha Formation (Upper Cretaceous) from Santos Basin (Brazil): The role of syngenetic and diagenetic process

The compositional and textural characterization of authigenic Mg-clays from the Barra Velha Formation (Early Cretaceous) samples (well P1) was performed. The Mg-clay mineral assemblage is comprised by Mg-smectite (stevensite and saponite) and kerolite. Based on clay fraction XRD data, was possible to establish four assemblages rich in kerolite (A and B) and Mg-smectites (C and CB). Saponite and stevensite were identified on the basis of its behavior in XRD patterns after thermal treatment and corroborated by EDS chemical analysis under SEM examination. The vertical distribution of Mg-clay minerals shows the predominance of assemblages rich in Mg-smectite (C and CB) in its basal older zone, while assemblages rich in kerolite (A) or showing mixed layering of kerolite-stevensite (B) occur preferentially in the younger upper zone of the sedimentary succession. The predominance of stevensite in the lower part of the stratigraphic succession would indicate evaporative conditions (higher salinity and pH), while the upwards intercalations with saponite suggest detrital inputs that would favor the dilution of the lacustrine system giving rise to hydrochemical conditions of high pH but lower salinity favorable for the precipitation of kerolite and later kerolite-stevensite mixed layering. Kerolite-rich samples (>90%) presents ordering (“crystallinity”) higher than that observed in other worldwide deposits presenting FWHM values ranging between 0.52 and 1.23 (2θ).The formation of kerolite and stevensite is interpreted as authigenic by neoformation, with transformation between them by mixed layering. The formation of saponite would be authigenic by transformation of previous aluminum-rich phases, but without ruling out the possible presence of inherited saponite.There are petrographic evidences of diagenesis affecting both the Mg-clays minerals and carbonates. Especially Mg-clays show replacement by silica (quartz, chalcedony) and carbonates (dolomite, magnesite). Homogenization of the Mg-clay textures into laminar texture appears to have occurred during the diagenetic compaction process.

Experimental chondrite–water reactions under reducing and low-temperature hydrothermal conditions: Implications for incipient aqueous alteration in planetesimals

The presence of hydrous minerals in carbonaceous chondrites has been considered important evidence for the former presence of liquid water in parent asteroids. However, the evolution of water–rock reactions in hydrous asteroids remains not well constrained. Here, we conduct water–rock type experiments and chemical equilibria calculations under low-temperature hydrothermal and reducing conditions to investigate the alteration process and secondary mineral assemblages of chondritic rock in the earliest alteration stage. Using synthetic chondrite (mixtures of olivine (forsterite95), orthopyroxene (enstatite95), silicate glass, troilite and Femetal) as a starting material, our experiments were conducted at temperatures of 25 °C–80 °C for time periods between 1 and 460 days at a water-to-rock mass ratio of 10. A combination of X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses revealed that the primary secondary phases consisted of pyrrhotite, an amorphous SiO2-rich phase and saponite at 80 °C, while the secondary phase consisted of an amorphous SiO2-rich phase and saponite at 25 °C. At both temperatures, the SiO2-rich phases and saponite densely covered the surface of the primary phases. The Fe/Mg ratios of saponite and amorphous SiO2-rich phases showed clear difference between 80 °C and 25 °C. Saponite that was formed at 80 °C was richer in Fe than the initial silicate phases, and the highest Fe/Mg ratios were obtained in the saponite encrusting the troilite and Femetal. These results suggest that the Fe distributed from the troilite and Femetal induced the formation of Fe-rich saponite. Some of the secondary minerals observed from our alteration experiments were consistent with those expected by chemical equilibria calculations. However, the formation of serpentine, the dominant secondary mineral expected from chemical equilibria calculations, was not observed in our experiments up to 460 days, probably because the preferential dissolution of SiO2-rich silicate glass in the earliest stage of alteration induced the formation of saponite rather than serpentine. The secondary mineral assemblage and its morphological characteristics, as observed by our alteration experiments, showed similarities with carbonaceous chondrites such as CM2 and CO3 chondrites. This alteration might be explained by water–rock reactions at low temperatures and by the short time alteration. These findings better constrain the temperatures and timescales of aqueous alterations in hydrous asteroids, as well as the role of water in the early solar system bodies.

Formation of saponite by hydrothermal alteration of metal oxides: Implication for the rarity of hydrotalcite

AbstractConversion of hydrotalcite (Ht) to saponite was observed by hydrothermal alkaline alteration of metal oxides. The conversion was through a pathway of hydration-dissolution-precipitation. It involved several critical steps, including the construction of Ht from metal oxides, dissolution of Al3+ from Ht, condensation of metasilicate anions with Ht, and finally crystallization of saponite. The condensation was favored by relatively low Mg/Al ratios of Ht, along with high concentrations of Al3+ and silicate oligomers in the environment, resulting in highly crystalline saponite. The latter conversion was greatly accelerated by the isomorphous substitution of Al3+ for Si4+ in silicate oligomers. The substitution generated the extra negative charge and led to the aforementioned condensation with Ht surface, thereby promoting the formation of saponite TOT layers. During the process, CO2 is an indispensable component. Initially intercalated as CO32− to form Ht, CO2 was subsequently eliminated from the solid phase, and saponite formed when the layer charge was reversed. Thus, this study presents a novel formation mechanism of saponite from metal oxides via hydrotalcite and contributes to a better understanding of the crystallization, chemical stability, and transformation of Ht to saponite. The results are also relevant to evaluating metal availability and carbon cycling on the surface of the Earth.

Magnesium isotopic composition of altered oceanic crust and the global Mg cycle

To investigate the behavior of Mg isotopes during low-temperature alteration of oceanic crust and to further understand its role in the global Mg cycle, we measured the Mg isotopic compositions (25Mg/24Mg and 26Mg/24Mg) of a set of samples of altered oceanic crust (AOC) recovered from the Ocean Drilling Program Hole 801C, the reference site for old crust (∼170 Ma) subducting in the Pacific. The measured δ26Mg values range from −1.70‰ to 0.21‰, deviating from that of pristine oceanic basalts (−0.25 ± 0.07‰). Composite samples of volcanoclastic breccia that have experienced relatively intense alteration have larger variation in δ26Mg values (−1.01‰ to 0.15‰) than composite samples of massive basaltic flows (−0.53‰ to −0.04‰), indicating significant Mg isotope fractionation during low-temperature alteration of the oceanic crust. Moreover, the upper off-axis basement has on average lower δ26Mg values (−1.70‰ to −0.04‰) than the lower on-axis basement (−0.16‰ to 0.21‰). These findings, combined with the co-variations between MgO content and FeO∗/CaO ratio and between δ26Mg and FeO∗/CaO ratio, suggest that formation of Mg-bearing minerals (i.e., saponite and calcite) during low-temperature alteration of the oceanic crust accounts for the highly variable δ26Mg of AOC. Early formation of saponite under anoxic condition preferentially takes up heavy Mg isotopes and accounts for Mg enrichment and relatively high δ26Mg in the on-axis basement. Subsequent precipitation of carbonates results in the dilution of Mg and relatively low δ26Mg in the off-axis basement. In addition, accumulation of carbonate-rich interflow sediments in the upper basement may contribute further to the low δ26Mg. A weighted average δ26Mg value of 0.00 ± 0.09‰ is estimated for the AOC at Site 801, implying that low-temperature alteration of oceanic crust drives the ocean to a lighter Mg isotopic composition, and thus requires additional carbonate precipitation to maintain a steady-state Mg isotopic composition of seawater. A mass balance calculation suggests that the Mg output flux due to low-temperature alteration of the oceanic crust equals ∼12% of the annual Mg riverine input, indicating that AOC is a significant sink of Mg in seawater. Our study further highlights that recycling of AOC with highly variable δ26Mg along with overlying marine sediments into the mantle through subduction may generate Mg isotopic heterogeneity in the mantle at small scales.

Hydrothermal transformation of mixed metal oxides and silicate anions to phyllosilicate under highly alkaline conditions

Mixed metal oxides (MMO) were transformed to phyllosilicate in the presence of silicate anions under hydrothermal conditions. The product was shown to be saponite with a trioctahedral layer structure by characterization with XRD, FTIR, 27Al and 29Si MAS NMR, HRTEM and TG-DTG. The formation of saponite was through a rehydration-dissolution-precipitation pathway, involving the critical steps such as the reconstruction of MMO to hydrotalcite (Ht), exsolution of Al3+ from Ht, condensation of metasilicate anions with Ht, and finally crystallization of saponite. Isomorphous substitution of Al3+ for Si4+ in silicate oligomers is the key for the above conversion, which generated the negative charge necessary for the condensation between the silicate oligomers and Ht surface and the formation of 2:1 saponite TOT layers. High pH value (over 14.0) favored the condensation of silicate oligomers and led to crystallization of saponite. Thus, we report a novel method for the crystallization of saponite.

Changes in mineral assemblages during serpentine carbonation

The carbonation of serpentine (lizardite [Mg3Si2O5(OH)4]) was investigated at a fixed temperature of 290°C under a 30-bar CO2 atmosphere in aqueous solutions of different alkalinities to understand the pH-dependence of the process. To activate the carbonation, the serpentine was heat-treated at 650°C beforehand for 2h. At pH6, the main reaction products were magnesite (MgCO3) and well-crystallized saponite [NaxMg3(Si4Al4−x)O10(OH)2]. The carbonation rate of the Mg ions was limited at acidic pH, and thermogravimetry (TG) analysis of the solid product showed that only 13% carbonation rate of serpentine to magnesite was achieved. The partial dissolution of SiOSi and the resultant Mg-rich solution provided a favorable environment for the formation of saponite. In weakly alkaline and neutral solutions, the rate of carbonation was dramatically enhanced well-faceted rhombohedral magnesite crystals of approximately 1μm in size with a round shape appeared together with a small amount of “curled-flaky type” saponite. The conversion of silicate to carbonate was found to be approximately 45%. As the pH of the solution increased further, the dissociation of carbonic acid accelerated and the free Mg2+ cations then reacted with the bicarbonate ions to form magnesium carbonate. Thus, active conversion to magnesite resulted in poor formation of saponite because of the low concentration of Mg ions in the solution. At pH2, well-crystallized saponite appeared, but magnesite was not observed. Under strongly alkaline conditions, saponite may form by a mechanism wherein the undissolved Mg layer of serpentine is available to act as a substrate for saponite precipitation. The transformation of serpentine to saponite and magnesite was influenced mostly by temperature, alkalinity, and mineral composition.

Glauconitic laminated crusts from hydrothermal alteration of Jurassic pillow-lavas (Betic Cordillera, S Spain): a microbial influence case

The green crusts infilling the spaces among pillow-lava bodies from the Median Subbetic (Betic Cordillera, S Spain) are the subject of this textural, mineralogical and geochemical study. The exceptional laminated morphology and mineral composition made it possible to establish the genetic conditions and different phases of the infilling, while also interpreting the possible influence of microbial activity in the origin and growth of the laminated crusts. Two types of crusts were discerned: green laminated crusts and black crusts. The green crusts are mostly composed by glauconite and celadonite, and a minority by smectites, whereas black crust and lens-shaped nappes are saponitic. The record of filaments and coccoid-shaped forms at different scales from the glauconitic crusts indicates the potential implication of chemoorganotrophic microbes in the precipitation of the glauconite and the development of laminated textures under low-temperature hydrothermal conditions. In an early stage, alteration of pillow-lava surfaces with influence of hydrothermal reducing fluids results in black films of saponite and calcite filling the void spaces. During a cooling phase, green laminated crusts composed by glauconite and celadonite grow under oxic conditions due to the circulation and diffusion of oxygenated sea-water along inter-pillow spaces and the chemoorganotrophic microbial activity. A new stage of saponite formation with calcite occurs under higher T and confined-reducing conditions resulting from deposition of marine sediments, and finally calcite and quartz crystallise as the latest product closing the remaining space (or producing geodes) among the pillow-lava bodies.

The Fe-Mg-saponite solid solution series – a hydrothermal synthesis study

AbstractThe boundary conditions of saponite formation are generally considered to be well known, but significant gaps in our knowledge persist in respect to the influence of solution chemistry, temperature, and reaction time on the mineralogy, structure, stability, and chemical composition of laboratory-grown ferrous saponite. In the present study, ferrous saponite and Mgsaponite were synthesized in Teflon-lined, stainless steel autoclaves at 60, 120 and 180°C, alkaline pH, reducing conditions, and initial solutions with molar Si:Fe:Mg ratios of 4:0:2, 4:1:1, 4:1.5:0.5, 4:1.75:0.25, and 4:1.82:0.18. The experimental solutions were prepared by dissolution of sodium orthosilicate (Na4SiO4), iron(II)sulfate (FeSO4·6H2O) and magnesium chloride salts (MgCl2·6H2O with ≤ 0.005 mass% of K and Ca) in 50 mL ultrapure water that contained 0.05% sodium dithionite as the reducing agent. The precipitates obtained at two, five and seven days of reaction time were investigated by X-ray diffraction techniques, transmission electron microscopy analysis, infra-red spectroscopy, and thermo-analytical methods.The precipitates were composed mainly of trioctahedral ferrous saponite, with small admixtures of co-precipitated brucite, opal-CT, and 2-line ferrihydrite, and nontronite as the probable alteration product of ferrous saponite. The compositions of the obtained ferrous saponites were highly variable, (Na0.44−0.59K0.00−0.05Ca0.00−0.02) (Fe2+0.37−2.41Mg0.24−2.44Fe3+0.00−0.28)Σ2.65−2.85[(Fe3+0.00−0.37Si3.63−4.00)O10](OH)2, but show similarities with naturally occurring trioctahedral Fe and Mg end members, except for the Al content. This suggests that a complete solid solution may exist in the Fe-Mg-saponite series.A conceptual reaction sequence for the formation of ferrous saponite is developed based on the experimental solution and solid compositions. Initially, at pH ≥ 10.4, brucite-type octahedral template sheets are formed, where dissolved Si-O tetrahedra are condensed. Subsequent reorganization of the octahedra and tetrahedra via multiple dissolution-precipitation processes finally results in the formation of saponite structures, together with brucite and partly amorphous silica. The extent of Fe2+incorporation in the octahedral template sheets via isomorphic substitution is suggested to stabilize the saponite structure, explaining (i) the abundance of saponite enriched inVIFe2+at elevated Fe supply and (ii) the effect of structural Fe on controlling the net formation rates of ferrous saponite.

Mg-rich clay mineral formation associated with marine shallow-water hydrothermal activity in an arc volcanic caldera setting

Abstract Shallow-water hydrothermal activity, represented by venting of hydrothermal fluid around 200 °C, occurs in the Wakamiko submarine crater at 200 m water depth in Kagoshima Bay, southwest Japan. The crater is the center of large eruptions that formed a volcanic caldera, which is semi-submerged at present. The crater is covered with thick volcanic sediments of felsic composition. We studied the distribution and chemical composition of hydrothermal clay minerals that are abundant in the sediment collected by piston coring. We also conducted chemical analysis of pore fluids squeezed from the sediment to understand hydrothermal interactions that resulted in formation of these clay minerals. The PC-2 core (340 cm in length) collected in the vicinity of a high-temperature fluid venting site was characterized by abundant Mg-saponite that is limited to a layer between 270 and 300 centimeters below the seafloor (cmbsf) and montmorillonite throughout the core below 55 cmbsf. Vertical profiles of pore fluid chemistry suggest that saponite formation is related to the interface between the seawater and the hydrothermal component in the sediment layer. Formation temperatures of the montmorillonite were estimated to be 118–163 °C, based on oxygen isotope thermometry. Formation of the montmorillonite is attributed to hydrothermal interaction between seawater-dominant pore fluid and volcanic glass. The formation temperature of the saponite was estimated to be ~ 164 °C, based on oxygen isotope thermometry. Formation of the saponite is attributed to hydrothermal interaction between seawater-dominant pore fluid and the montmorillonite, which had been formed at a prior stage. The PC-1 core (240 cm in length) collected from a relatively low-temperature fluid shimmering site was characterized by the occurrence of kerolite in the lower section (210–240 cmbsf). Vertical profiles of pore fluid chemistry suggest that the kerolite formation occurred at the interface between seawater and the hydrothermal component of the sediment layer. Formation temperature of the kerolite was estimated to be about ~ 211 °C, based on oxygen isotope thermometry. Formation of the kerolite is attributed to precipitation from a fluid that was a mixture of a hydrothermal component and seawater. This study revealed the occurrence of Mg-rich clay minerals, saponite and kerolite, beneath a submarine hydrothermal field that developed within sediment of felsic composition. During hydrothermal interactions that formed these clay minerals, seawater penetrated into the sediment and was an important Mg source. Formation of Mg-rich clay minerals, saponite and kerolite, are controlled by pore fluid chemistry, which varies from a seawater-dominant to hydrothermal-dominant component. Exclusive formation of Mg-rich clay minerals at different sites could be explained by different water–rock ratios of the hydrothermal interaction — saponite formation at low water-rock ratio and kerolite precipitation at high water–rock ratio. Occurrence of Mg-rich clay minerals provides clues to the hydrological structure in sediment-covered hydrothermal systems in an arc volcanic caldera setting.

A new look at saponite formation and its implications for early animal records in the Ediacaran of South China

Acanthomorphic acritarch fossils, including some interpreted to be the fossils of the earliest animal embryos, first appear in the lower Doushantuo Formation of the Yangtze Gorges area (YGA). Further, the complete paleontological and geochemical record for the YGA has played a central role in defining the global biological and geochemical backdrop that presaged and witnessed the dawn of diverse animal life. Despite the importance of the YGA in our understanding of Neoproterozoic Earth history, basic aspects about its depositional history remain debated. Foremost among the controversies, extensively studied sections in the YGA were recently tied to deposition in an alkaline lake, casting new but contentious light on the environments of early animal evolution and the broader significance of geochemical records from the YGA. Arguments for a lacustrine setting hinged on the presence of trioctahedral clays (saponite-corrensite). However, this clay type commonly forms in other environments, including the weathering profiles of mafic and ultramafic volcanics. Using a coupled geochemical and sedimentological approach, we argue that the trioctahedral clays in the lower Doushantuo of the YGA are better explained as weathering products from a regional mafic-to-ultramafic hinterland delivered by rivers to a shelf or lagoon in the Yangtze Gorges Basin. These novel provenance relationships for YGA sediments and associated clays are consistent with a marine setting for the early animal records and must factor in our current understanding of the broader geochemical fabric of the Doushantuo Formation.

Geology and mineralogy of a sepiolite-palygorskite occurrence from SW Eskişehir (Turkey)

AbstractThe present occurrence is the only deposit among those found in Eskişehir province containing both sepiolite and palygorskite in economic amounts. To describe the geology, mineralogy and genetic relationships, two representative measured sections were examined by field work and XRD, XRF and SEM analyses. Sepiolite and palygorskite occur in a ∼40 m thick part of Pliocene sequence where saponite is also found. These minerals appear in ascending order as: palygorskite, saponite, saponite + palygorskite, sepiolite and sepiolite + palygorskite, frequently in association with dolomite. Chemical precipitation from the alkaline lake environment under semiarid or arid conditions was the main process resulting in sepiolite, palygorskite and saponite formation. Of these, saponite and palygorskite were formed when the lake water was rich-in Al and Fe. Changes in pH may have controlled whether saponite or palygorskite could be formed. Palygorskite was also developed by transformation from saponite during diagenesis.

Methylene-functionalized saponite: A new type of organoclay with CH 2 groups substituting for bridging oxygen centers in the tetrahedral sheet

The synthesis of saponite in the presence of bis(triethoxysilyl)methane (BTESM) as an organosilicon reagent results in the replacement of up to 33.3% of the oxygen atoms in the tetrahedral sheet by bridging methylene groups. The methylene-functionalized saponites represent a new form of covalently bonded organoclay that truly is isomorphic with purely inorganic saponite made under equivalent reaction conditions from sodium silicate as the silicon source. The isoelectronic and isolobal relationship between methylene and bridging oxygen centers is essential for methylene saponite formation. Bridging ethylene groups are not incorporated into the Kagome net of the basal surfaces due to a mismatch in bridging group size. The textural properties of the methylene saponites are similar to those for purely inorganic magnesium saponite made under equivalent synthetic conditions in the absence of BTESM. Layer stacking disorders afford large surface areas (∼ 550 to 650 m 2/g), making the methylene saponites attractive candidates for use as adsorbents and functional fillers for polymer composites.

Mineralogy and geochemistry of clay samples from active hydrothermal vents off the north coast of Iceland

Active hydrothermal vent sites were sampled during 1997 in a series of submersible dives at the active Grimsey (GHF) and Kolbeinsey (KHF) hydrothermal fields off the north coast of Iceland. This study focuses on secondary clay minerals which were formed in two different settings. The GHF is characterized by the presence of clay minerals precipitated within active chimneys. By contrast, the KHF is characterized by the presence of secondary clay minerals, which are the products of hydrothermal alteration of lava fragments. Based on XRD, electron microprobe and ICP-MS analyses, the dominant clay mineral in both hydrothermal fields is saponite (Mg-rich smectite). Chlorite and chlorite–smectite mixed-layer minerals also occur at the KHF. The Mg-rich nature of saponite from the GHF chimneys suggests intense Mg metasomatism in the mixing zone where hydrothermal fluids interact with seawater at temperatures of 250 °C. Saponite formation resulted in the additional uptake of Cu, Zn, and Pb. Enrichment in Ba is evident in the almost pure saponite from the KHF. Based on oxygen isotope data, the saponite formation at the KHF occurred at 148 °C, which is close to the maximum measured fluid temperature of 131 °C in this field.

Synthesis and characterization of saponite clays

A new procedure has been developed for a fast preparation of saponite clays under the non-hydrothermal synthesis conditions of 90 degrees Celsius and 1 atmosphere. Saponites were synthesised from a stoichiometric mixture containing Si/Al3+ gel, M2+-nitrate (M2+ = Mg, Zn, Ni, Co, or Cu), urea and water within a few hours. The synthesis products were characterised with XRD, IR, TEM, XRF, N2-physisorption, Al-EXAFS, and 27Al- and 29Si-MASNMR Incorporation of Mg, Zn, Co, Ni, or a combination of these cations in the octahedral sheet, as well as controlling the Si/Al ratio in the tetrahedral sheet in the range between 5.67 and 39.0 could easily be established. Pure Cu-saponite could not be synthesised due to the preferred formation of chrysocolla, but a combination of Mg2+ and Cu2+ resulted in saponite formation. The chemical composition strongly influences the textural properties of the saponites. It is possible to prepare saponite samples of a specific surface area and pore volume of 100 to 750 m2/g and 0.15 to 1.05 ml/g, respectively. The lateral size and the amount of stacking of the saponite platelets could be influenced by the composition, synthesis duration and amount of urea. The new synthesis procedure provides an easy way to prepare high quantities of saponites with far-reaching control on the texture as well as the composition.

TEM study of the alteration of interstitial glass in MORB by inorganic processes

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations on a suite of mid-ocean ridge basalts (MORB) reveal abundant interstitial glass, which differs in occurrence and composition from basaltic glass at pillow rims. Interstitial glass consists dominantly of SiO 2 and Al 2O 3 and is depleted in other cations, representing a highly differentiated residual material. Low-temperature alteration by seawater solutions results in the formation of saponite (Mg-rich variety of smectite) replacing the interstitial glass; the Si/Al ratio of this saponite reflects that of the source glass. Chemical changes during this transition differ from those resulting from alteration of basaltic or rhyolitic glass. Alteration of interstitial glass results in uptake of Mg and Fe and loss of Si and Al, but there is little or no leaching of alkalis. This reflects a strong control on chemical reactions by solution composition (high Na and Mg concentrations of seawater fluids). In contrast to basaltic glass at pillow rims, our observations do not reveal microbial activities in the alteration of interstitial glass.

Alteration mineralogy of Cretaceous basalt from ODP Site 1001, Leg 165 (Caribbean Sea)

AbstractSecondary clay minerals observed in the two uppermost basalt lava flows at ODP Site 1001, in the Caribbean Sea, drilled from the large igneous province of Cretaceous age, result from low-temperature alteration processes. Alteration mainly proceeds by circulation and diffusion of sea water. Six different types of clay mineral assemblage were recognized. Initial alteration with oxygenated sea water involves Fe and K fixation, creating visible oxidation halos parallel to the sides of cracks and fissures. A saponite/ beidellite mixture, interstratified smectite-glauconite, interstratified glauconite-nontronite and Fe oxyhydroxides are obtained depending on the distance from fluid conduits. The presence of beidellite may be due to enhanced Al mobilization resulting from high fluid flux. These early minerals are cross-cut by thin veins of pure celadonite or glauconite with further vesicle infill. Late-stage alteration is typified by the formation of saponite and takes place under closed reducing conditions resulting from deposition of the sedimentary overburden.

Petrological investigations of low temperature hydrothermal alteration of the upper crust, Juan de Fuca Ridge, ODP Leg 168

Abstract Ten sites were drilled across the eastern flank of the Juan de Fuca Ridge during ODP Leg 168 to investigate the nature of ridge-flank hydrothermal circulation and crustal evolution in a region of varying sediment-basement topography. The sites were divided into: (i) the Hydrothermal Transition (HT) transect, a zone of gradually thickening sedimentary cover, extending from the ridge-flank boundary for a distance of 20 km to the east; (ii) the Buried Basement (BB) transect, which consists of uniformly, flat lying basement, capped by a regionally continuous thick sedimentary sequence; and (iii) the Rough Basement (RB) transect, represented by a region of basement highs and troughs, located ∼100 km east of the ridge axis, and capped by a variable thickness of sediment. Across the flank, a progressive sequence of alteration was recognized, produced as the hydrothermal system evolved from open oxidative conditions involving a slightly modified seawater derived fluid, to a more restricted/closed, non-oxidative environment in which more strongly modified seawater derived fluids interacted with the basement. Alteration commenced with the formation of chlorite at relatively high temperatures in non-oxidative to oxidative conditions, followed by the first wide spread stage of low temperature oxidative alteration with the formation of iron oxyhydroxides and celadonite. Progressive burial, along with continued alteration of the basement, restricted fluid circulation resulting in the formation of saponite ± pyrite over a range of temperatures under non-oxidative conditions. The final stage of alteration is represented by carbonate, which formed under non-oxidative conditions by interaction of strongly modified seawater and the crust.

Aqueous alteration and brecciation in Bells, an unusual, saponite-bearing, CM chondrite

The petrological and mineralogical characteristics of the unusual CM2 chondrite, Bells, have been investigated in detail by scanning electron microscopy (SEM), electron microprobe analysis (EPMA), and transmission electron microscopy (TEM). Bells is a highly brecciated chondrite which contains few intact chondrules, a very low abundance of refractory inclusions, and is notable in having an unusually high abundance of magnetite, which is disseminated throughout the fine-grained matrix. Fragmental olivines and pyroxenes are common and, based on compositional data, appear to have been derived from chondrules as a result of extensive brecciation. The fine-grained mineralogy of matrix in Bells differs considerably from other CM chondrites and has closer affinities to matrix in CI chondrites. The dominant phases are fine-grained saponite interlayered with serpentine, and phases such as tochilinite and cronstedtite, which are typical of CM chondrite matrices, are entirely absent. Pentlandite, pyrrhotite, magnetite, anhydrite, calcite, and rare Ti-oxides also occur as accessory phases.Based on its oxygen and noble gas isotopic compositions (Zadnik, 1985; Rowe et al., 1994), Bells can be considered to be a CM2 chondrite, although its bulk composition shows some departures from the typical range exhibited by this group. However, these variations in bulk chemistry are entirely consistent with the observed mineralogy of Bells. The unusual fine-grained mineralogy of Bells matrix can be reasonably attributed to the combined effects of aqueous alteration and advanced brecciation in a parent body environment. Extensive brecciation has assisted aqueous alteration by reducing chondrules and mineral grains into progressively smaller grains with high surface areas, which are more susceptible to dissolution reactions involving aqueous fluids. This has resulted in the preferential dissolution of Fe-rich chondrule olivines, which are now completely absent in Bells although present in other CM chondrites. The formation of saponite in Bells probably resulted from the dissolution of relatively silica-rich phases, such as pyroxene and olivine, that were derived from chondrules. The result of such dissolution reactions would be to increase the activity of silica in the fluid phase, at least on a localized scale, stabilizing saponite in preference to serpentine. An increase in aSiO2 would also have destabilized preexisting cronstedtite which may have reacted to form magnetite and Mg-Fe serpentine under conditions of constant ƒO2 .