Search results for "silica"
showing 10 items of 1092 documents
Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russi…
2020
Highlights • Parental melts of sulfide-bearing KM rocks have near primary MORB-like composition. • Crystallization of these S-saturated melts occurred in near-surface conditions. • Extensive fractionation and crustal assimilation are not the causes of S-saturation. • S content in melts can be restored by accounting for daughter sulfide globules. Abstract Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide …
Biogeochemical Cycle of Silica in an Apolyhaline Interdunal Holocene Lake (Chad, N'Guigmi Region, Niger)
1999
During the Holocene, the apolyhaline conditions in interdunal ephemeral lakes in the Lake Chad region led to various diagenetic processes, which resulted in: (a) clay authigenesis, (b) organic matter lithification (plants and cyanobacterial mats), (c) precipitation of sodium silicate (magadiite, kenyaite, zeolite), and (d) chert neoformation. Each step in this diagenetic process involves silica, which can be highly mobile under such conditions. Therefore, the paleoenvironmental variations can be investigated using the Si cycle.http://link.springer. de/link/service/journals/00114/bibs/9086010/90860475.htm</HEA
Cracking Activity and Hydrothermal Stability of MCM-41 and Its Comparison with Amorphous Silica-Alumina and a USY Zeolite
1996
It has been found that the cracking activity of MCM-41 for a reaction catalyzed by strong acids site, such asn-heptane cracking, is much lower than that of a USY zeolite, and similar to that of amorphous silica-alumina. The higher activity of USY is due to the presence of stronger Bronsted acid sites in the zeolite. In the case of gas oil cracking, the greater accessibility of the large molecules to acid sites in MCM-41 relative to USY makes the difference in activity between these two catalysts much smaller than for the pure hydrocarbon. In the calcined state MCM-41 is more active and gives more gasoline and less gases and coke than amorphous silica-alumina. However, when the catalysts wer…
Fast high performance liquid chromatography analysis in lipidomics: Separation of radiolabelled fatty acids and phosphatidylcholine molecular species…
2006
Abstract HPLC procedures using conventional C 18 columns are usually used to separate simple and complex lipid mixtures but these methods of separation remain often laborious and very slow. Here, monolithic columns were successfully applied to separate lipids – radiolabelled fatty acid mixtures and individual phosphatidylcholine (PC) molecular species. For that, isocratic elution was performed using two Chromolith™ Performance RP-18e columns connected in series. Detection was achieved by online measurement of radioactivity for radiolabelled fatty acids and by UV absorbance at 205 nm for PC molecular species. The performances of such silica rods were compared to conventional reverse-phase si…
Polymer support synthesis
1989
Non-porous silica gel microbeads of diameter 1.5 microns have been investigated as supports for oligonucleotide synthesis. In the preparation of oligothymidylates of chain length up to 150 bases, with 5'-di-p-anisylphenylmethyl-3'-phosphoramidite as an intermediate, the average yields per chain elongation were up to 99%. Lower overall yields were observed in the case of a support which developed a strong tendency towards aggregation after the build up of an oligonucleotide coating.
Poly(silicate)-metabolizing silicatein in siliceous spicules and silicasomes of demosponges comprises dual enzymatic activities (silica polymerase an…
2007
Siliceous sponges can synthesize poly(silicate) for their spicules enzymatically using silicatein. We found that silicatein exists in silica-filled cell organelles (silicasomes) that transport the enzyme to the spicules. We show for the first time that recombinant silicatein acts as a silica polymerase and also as a silica esterase. The enzymatic polymerization/polycondensation of silicic acid follows a distinct course. In addition, we show that silicatein cleaves the ester-like bond in bis(p-aminophenoxy)-dimethylsilane. Enzymatic parameters for silica esterase activity are given. The reaction is completely blocked by sodium hexafluorosilicate and E-64. We consider that the dual function o…
Mesoporous silica-based materials for use in electrochemical enzyme nanobiosensors
2012
Abstract Biology and medicine have seen great advancements in the development of enzyme nanobiosensors capable of characterizing and quantifying biomolecules. We provide an overview of mesoporous silica (MPS)-based enzyme nanobiosensors developed for biological and medical applications, and we describe significant advances in these technologies. We review progress in constructing high-performance electrochemical enzyme biosensors. We also discuss: • MCM-41 and SBA-15-modified and MPS composite electrodes for enzyme biosensing; • incorporation of biorecognition elements into MPS material electrodes for enzyme biosensing; and, • MCM-41 and SBA-15-supported electrocatalytic MPS-based electroch…
The effect of partial exfoliation of multilayer silicate filler particles on the elastic properties of a polymer composite
2018
Abstract A method for stepwise calculation the elastic constants of a three-phase composite, including the polymer matrix, the fully exfoliated silicate nanolayers and the platelike silicate particles in the form of multilayer intercalated stacks is considered. A theoretical analysis is carried out by using the Mori–Tanaka theory of an equivalent medium. The influence of degree of exfoliation (the ratio of exfoliated filler content to the total filler content) on the reinforcing efficiency is studied and discussed. A comparative analysis of experimental data of elastic modulus for a rigid polyurethane/montmorillonite composite and prediction results is carried out. It is shown, that with gr…
Biochemistry and cell biology of silica formation in sponges
2003
The main inorganic material forming the skeletal elements in Demospongiae as well as in Hexactinellida, the spicules, is amorphous silica. The spicules occur in the cytoplasm and the extracellular space and also in the nucleus (as silicate crystals) of some sponge cells; the function in the latter compartment is unknown. Recent evidence shows that the formation of spicules is mediated by the enzyme silicatein. The cDNA as well as the gene encoding this enzyme was cloned from Suberites domuncula. The recombinant silicatein catalyzes the syn- thesis of amorphous silicate using tetraethoxysilane as substrate. The enzyme is dependent on ferric iron. Silicatein also has proteolytic (cathepsin-li…