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PubMed1999

Properties of Goethites Prepared under Acidic and Basic Conditions in the Presence of Silicate.

Goethite in natural environments usually grows in the presence of dissolved silicate. To study silicate-associated goethite with specific properties, goethite was synthesized in an Fe(III) system at RT under acidic (OH/Fe = 2; pH 1.6-1.8) and basic (OH/Fe = 4; pH 12-13) conditions at Si concentrations between 10(-5) and 1 M. The goethites were characterized by transmission (TEM) and scanning (SEM) microscopy, X-ray diffraction (XRD), Mössbauer spectroscopy (MS), and chemical analyses. Despite large differences in size and morphology, all goethite crystals were dominantly bound by 110 and 021 faces. Nanosized crystals (ca. 20 nm) were formed at low pH, where the influence of Si was weak. In the basic system, where Si retarded crystallization, much larger (tens to hundreds of nanometers) crystals were formed whose shape varied from acicular and multidomainic at low Si (10(-5) M) to monodomainic, blocky crystals at high (10(-2) M) Si concentration that had reduced growth along [001] in favor of [100] and [010]. The sizes and shapes of the crystals are discussed in terms of nuclei and growth unit concentrations in the system. Part of the retained Si could be released by phosphate and NaOH (surface-Si), and part was only liberated into HCl congruently with Fe (up to 46 and 17 g kg-1 for the acid and basic goethites, respectively). Neither XRD nor MS were able to prove structural Si-for-Fe substitution, probably because no tetrahedral positions are available to accommodate Si in the goethite structure. It is assumed that this nonsurface Si fraction is located between the crystal domains. Copyright 1999 Academic Press.

Journal Article↗
PubMed2001

The nature of siliceous mosaics forming the first shell of the brachiopod Discinisca.

The juvenile shell of the brachiopod Discinisca consists of a mosaic of micrometer-sized siliceous tablets embedded in a chitinous substrate. The first-formed tablets are secreted on glycocalyx by a newly differentiated collective of outer epithelial cells. They are mainly rhombic but may also be ellipsoidal, discoidal, or deformed and sporadically overlap one another. On the surrounding juvenile shell, secreted by an incipient outer mantle lobe, the tablets are nearly all perfect rhombic plates in rhombic arrays. Their constant size, arrangement, and centripetal crystallization suggest intracellular assembly. The tablets, which are normally bilamellar, consist of discrete aggregates of crystalline spherules of silica in rhombic arrays within an organic matrix of fibrous protein and, presumably, a soluble polysaccharide(s). Mosaic secretion ceases at about the time when juveniles settle on the sea bed, which more or less coincides with the secretion of a ring of lamellae around the mosaic, induced by rapid advances and retractions of the outer mantle lobe prior to deposition of the organophosphatic mature shell. Energy dispersion X-ray analyses of pelagic and newly settled juveniles show that phosphatic secretion, even in the site of the first-formed outer epithelial collective, does not begin until all siliceous secretion has ceased.

Animals↗
PubMed1983

Siliceous deposits in human urinary calculi--an E. M. study.

Silica calculi in man are extremely rare; only 12 cases have been reported. In an electron microscopic study of human urinary stones, siliceous deposits were encountered in three out of 180 stones as a discrete component of stones. There were no histories of magnesium trisilicate administration in these cases. Morphology of the deposits was distinctive from any other components of the stones and bore some resemblance to animal silica calculi. Siliceous deposits in human urinary stones appear to be more common than generally recognized.

Electron Probe Microanalysis↗
PubMed2003

Speciation of dissolved silicates in natural waters containing alkaline and alkaline-earth ions. A case study--waters from arid lands (North West China).

The concentration of silica in water samples from the desert area of Xinjiang, N. W. China, has been measured by colorimetry with ammonium molybdate. The observed pattern of dependence of the concentration of silica on the concentration of sodium ion (Na(+)) in the water samples is consistent with the pattern obtained by experiments on in-vitro dissolution of silica gel in sodium chloride (NaCl) solution. This indicates that the dissolution of silica in the hydrologic system in this area depends on the concentration of Na(+). Calcium ion (Ca(2+)), which is known to play an important role on the dissolution of silica on the basis of in-vitro experiments, was observed to take little part in the dissolution of silica in actual natural water samples. This implies that the Ca(2+) is bound to the hydrogen carbonate anion or that the Ca(2+) content of natural water containing salts is very low, owing to precipitation. In these samples silicate-Na(+) was identified as the dissolution species of silica; it was also ascertained that Ca(2+) did not form complexes with silicate species. These observations resulted from direct identification of dissolved chemical species by use of FAB-MS (fast atom bombardment mass spectrometry). The research indicates that in water samples in this critically arid region the concentration of "dissolved" silica is basically determined by the concentration of Na(+), indicative of pure inorganic conditions in the desert area of Xinjiang, N.W. China.

Journal Article↗
PubMed1990

Hypothesis: is lung disease after silicate inhalation caused by oxidant generation?

Inhaled silicate dusts may cause lung disease through their surface coordination of iron with subsequent oxidant generation via the Fenton reaction. Pneumoconiosis, irritant bronchitis, focal emphysema, and carcinoma may be produced by oxidants either directly through lipid peroxidation and protein inactivation, or indirectly by oxidant-mediated release of cytokines such as platelet-derived growth factor. The increased incidence of tuberculosis observed among silicate workers could be explained by accumulation of iron complexed by dust particles in the lung and made available to dormant mycobacteria as a virulence factor.

Dust↗
PubMed2006

WDS versus silicon drift detector EDS: a case report for the comparison of quantitative chemical analyses of natural silicate minerals.

Electron probe microanalysis (EPMA) is an essential analytical approach to determine elemental concentrations of various solid specimens quantitatively in mineralogical, petrological and materials research. Either wavelength dispersive X-ray (WDS) or energy dispersive X-ray (EDS) spectrometric techniques can collect the characteristic X-rays generated from each element in the specimen by an incident electron beam in order to define chemical constituents. Although WDS has been the preferred technique because of its higher spectral resolution and ability to detect trace elements, new generation EDS systems with silicon drift detectors (SDD), equipped with thin windows and integrated digital processing electronics, are claimed to approach the WDS throughput. In this study, we compared the analytical capability of a SDD EDS system with respect to WDS equipped systems on natural silicate minerals. For this purpose, natural rock samples, in which the silicate minerals present had already been analysed by various WDS systems, were chosen to compare these results with the ones acquired with a SDD EDS system. SDD EDS yielded satisfactory results for major elements (Na, Mg, Al, Si, K, Ca, Ti, Mn and Fe) compared with the results of the same minerals obtained by various WDS systems.

Journal Article↗
PubMed2006

A haptophyte bearing siliceous scales: ultrastructure and phylogenetic position of Hyalolithus neolepis gen. et sp. nov. (Prymnesiophyceae, Haptophyta).

A haptophyte alga bearing hyaline but conspicuous scales was discovered in surface water samples of Shiribeshi Seamount, Japan. X-ray elemental analysis confirmed that silica was the major element in these scales. These scales were hat-shaped, ellipsoidal in top view, 4-6mum wide and 5-7mum long, perforated by several small pores, and were deposited on cells in several layers. Beneath the siliceous scale layers, organic scales were present, which are typical of haptophytes. The cells were non-motile despite having two short flagella hidden in the scale case. The haptonema, extended over the scale case, and one and a half times the cell length. The intracellular features were typical of haptophytes, including the peripheral endoplasmic reticulum (PER), Golgi cisternae with peculiar dilations, and the flagellar apparatus. The siliceous scales were produced in vesicles in the posterior region of the cell. Motile cells lacking silica scales were observed occasionally when cultures were maintained at lower temperatures and under oligotrophic conditions. This alga was described as Hyalolithus neolepis gen. et sp. nov. Phylogenetic analysis using the SSU rDNA and rbcL gene sequences indicated that Hyalolithus is a member of the Prymnesiales and falls in a clade including Prymnesium, Platychrysis, and Chrysochromulina polylepis. Based on these results, the evolutionary implications of the presence of silicified scales in haptophytes is discussed.

Animals↗
PubMedDate not supplied

33S solid state NMR of sulphur speciation in silicate glasses.

33S solid-state NMR is reported from some model crystalline sulphides, sulphates, sulphites and thiosulphates. This is the first report of (33)S NMR signals of the latter two species from a solid. Good quality spectra, which have distinct, well separated shift ranges can be identified for all these groupings, except for sulphites and hydrogen sulphites whose resonances are very broad. Nonetheless the presence of sulphites and hydrogen sulphites can be confirmed from their characteristic sharp, intense time domain echoes. (33)S MAS NMR is also applied to a range of dry and hydrous silicate glasses with approximately 1wt% 99at% (33)S-enriched sulphur. It is clear that in all these glasses sulphate is present, and in mixed cation systems there is some evidence of preferential association of sulphate with a specific cation. In a dry potassium silicate glass sample two resolved (33)S resonances are observed, a sulphate, and the second from thiosulphate. Hence solid state (33)S NMR is shown to be a feasible probe that can be applied to this problem that can readily distinguish different sulphur species.

Journal Article↗
PubMed2000

Long term survival of a patient with malignant pleural mesothelioma as a late complication of radiotherapy for Hodgkin's disease treated with 90yttrium-silicate.

A case of malignant pleural mesothelioma (PM) 24 years after thoracic radiotherapy for Hodgkin's disease is presented. As primary treatment and to relieve symptoms of dyspnea secondary to pleural effusion a thoracic drain was installed, followed by intracavitary radiation therapy with 90yttrium-silicate. Minor complaints of fever and a dry cough as a side-effect of this treatment were effectively treated with prednisone during 2 weeks. The patient remains in a good clinical condition now 6 years after diagnosis. Considering the few therapeutic options the use of 90yttrium-silicate intrapleural installation could be propagated as a safe and effective antitumour treatment for a selected group of patients with malignant PM.

Hodgkin Disease↗
PubMed2003

Electrochemically deposited sol-gel-derived silicate films as a viable alternative in thin-film design.

Sol-gel-derived silicate films were electrochemically deposited on conducting surfaces from a sol consisting of tetramethoxysilane (TMOS). In this method, a sufficiently negative potential is applied to the electrode surface to reduce oxygen to hydroxyl ions, which serves as the catalyst for the hydrolysis and condensation of TMOS. The electrodeposition process was followed by the electrochemical quartz crystal microbalance and cyclic voltammetry. The electrodeposited films were characterized for their surface morphology, porosity, and film thickness using atomic force microscopy, electrochemical probe techniques, surface area and pore size analysis, and profilometry. The electrodeposited films were found to have a completely different surface structure and to be significantly rougher relative to spin-coated films. This is likely due in part to the separation of the gelation and evaporation stages of film formation. The electrodeposited films were found to be permeable to simple redox molecules, such as ruthenium(III) hexaammine and ferrocene methanol. Film thickness can be easily varied from < 75 nm to > 15 microm by varying the electrode potential from -600 mV to more than -1000 mV, respectively. The electrodeposition process was further applied for the electroencapsulation of redox molecules and organic dyes within the silicate network. Cyclic voltammograms for the gel-entrapped ferrocene methanol (FcCH2OH) and ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) exhibited the characteristic redox behavior of the molecules. The electroencapsulation of organic dyes in their "native" form proved to be more difficult because these species typically contain reducible functionalities that change the structure of the dye.

Journal Article↗
PubMed2000

Silicate polymerization for the preparation of bed-retention frits in capillary electrochromatography

Bubble formation, which is associated with bed-retention frits, is a critical experimental problem in capillary electrochromatography systems. In this investigation, porous silica frits were prepared via spot-heating of a silicate solution, and the effects of several experimental parameters on their performance were studied. The optimal sodium silicate concentrations were 10.8% and 5.4% (w/v) for outlet and inlet frits, respectively. The heating times were 5-6 s for outlet frits and < 1 s for inlet frits. Under optimized conditions, outlet frits were 75 microns (+/- 12 microns) and the heat treatment did not make the capillary fragile at the frit location. Bubble formation was affected by frit length, density, and silanization of the frits with trimethylchlorosilane. Packed capillaries with optimized frits were used successfully in a commercial CE instrument over a normal working day without pressurization, at relatively high ionic strengths (10 mM), and over a wide range of acetonitrile compositions (20%-80%). Currents were also stable for > or = 3 h under very high current (27 microA) conditions. As part of this study, the efficiency and reproducibility of packed capillaries were also briefly evaluated.

Journal Article↗
PubMed2001

Monocyclic zwitterionic lambda(5)Si-silicates with an SiO(2)FC(2) framework: syntheses and structural characterization in the solid state and in solution.

Treatment of the acyclic zwitterionic pentacoordinate silicate F(3)MeSiCH(2)NMe(2)H with 1 molar equiv of Me(3)SiOC(6)H(4)OSiMe(3), Me(3)SiOCH(2)C(O)OSiMe(3), Me(3)SiOC(Ph)=NOSiMe(3), or Me(3)SiOC(O)C(O)OSiMe(3) (solvent CH(3)CN, room temperature) yielded the respective monocyclic zwitterionic pentacoordinate silicates (11a), (12a), (13a), and (14a), along with 2 molar equiv of Me(3)SiF. The derivatives 11b-14b with a 2,2,6,6-tetramethylpiperidinio substituent instead of the dimethylammonio group were prepared analogously, starting from F(3)MeSiCH(2)NR(2)H (NR(2)H = 2,2,6,6-tetramethylpiperidinio). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 11a.1.5CH(3)CN, 12a-14a, and 11b-14b are distorted trigonal bipyramids, the axial positions being occupied by the fluorine atom and one of the two oxygen atoms (12a/12b, carboxylate oxygen atom; 13a/13b, carbon-linked oxygen atom). These results are in agreement with the NMR data ((1)H, (13)C, (19)F, (29)Si) obtained for these compounds in solution. The chiral (C(1) symmetry) zwitterions 11a-14a and 11b-14b exist as pairs of (A)- and (C)-enantiomers in solution. VT (1)H NMR studies with 11b-14b in CH(3)CN in the temperature range 25-85 degrees C gave no indications for an enantiomerization process [(A)/(C)-enantiomerization] at the silicon atom.

Journal Article↗
PubMed2001

Synthesis and crystal structure of Ae(2)LiInGe(2) (Ae = Ca, Sr): new Zintl phases with a layered silicate-like network.

Two new Zintl phases Ae(2)LiInGe(2) (Ae = Ca 1; Sr 2) were obtained from stoichiometric reactions of the pure elements in sealed Nb tubing at 1000-1050 degrees C. The isomorphous polar intermetallic phases crystallize in the orthorhombic space group Pnma, with cell constants of a = 7.2512(7), b = 4.4380(5), and c = 16.902(1) A for compound 1, and a = 7.5033(8), b = 4.6194(5), and c = 17.473(2) A for compound 2. The crystal structure can be derived from the vertex-sharing of InGe(4/2) tetrahedral units that form "corrugated" sheets normal to the crystallographic c-axis. Calcium and lithium atoms act as "spacers" that effectively separate the anionic [InGe(2)](5-) layers. The layered anionic substructure is similar to those exhibited by layered metal oxides, sulfides, and silicates. The connectivity of the tetrahedral building unit, [InGe(4/2)](5-), is analogous and isoelectronic to the silicate [SiO(4/2)] unit.

Journal Article↗
PubMed2002

Reaction of silicate minerals to form tetramethoxysilane.

Several silicon dioxide sources were used as reagents in the base-mediated reaction with dimethyl carbonate (DMC) to make tetramethoxysilane (Q'). Several commercially available diatomaceous earth materials were investigated. High throughput screening was employed to explore over 200 silicate rocks and minerals as alternative silicon dioxide sources for formation of Q' from DMC and base. Amorphous silicon dioxide materials are effective reagents for the Q' forming reaction. Effective silicon dioxide sources in addition to the diatomaceous earth materials include opal and various synthetic silicates (Li, Co, and Ca).

Journal Article↗
PubMed2005

Behavior of tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]silicate in aqueous solution: analysis of a sol-gel process by small-angle neutron scattering.

The racemic hexacoordinate silicon(IV) complex tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]silicate (1) was synthesized by treatment of Si(OMe)4 with 2 molar equiv of citric acid and 2 molar equiv of N(n-Bu)3. The corresponding germanium analogue, tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]germanate (5; structurally characterized by single-crystal X-ray diffraction), was obtained analogously, starting from Ge(OMe)4. Upon dissolution in water, the lambda6Si-silicate dianion of 1 hydrolyzes spontaneously (formation of Si(OH)4 and citric acid), whereas the lambda6Ge-germanate dianion of 5 was found to be stable in water. Aqueous "solutions" of 1, with concentrations that are significantly higher than the saturation concentration of Si(OH)4, look absolutely clear over a period of several weeks; however, in reality, these solutions are sols with very small particles that slowly grow with time and finally form a gel that precipitates. This sol-gel process was monitored by small-angle neutron scattering (SANS). For reasons of comparison, an aqueous solution of the hydrolytically stable germanium compound 5 was also studied by the SANS technique.

Journal Article↗
PubMed2005

Local order in depolymerized silicate lattices.

In glycerol, near 200 degrees C, the silicate networks of mesoporous silicates and zeolites undergo a deep depolymerization process. In a few hours, depending on the initial concentration of the solid in glycerol and on the temperature, amorphous gels are obtained. In these gels, a fraction of the Si-O-Si bonds are transformed into Si-O-C. The constitutional aluminum remains bound to the silica network in the gel. The short range ordering is maintained to some extent: the size of the smallest structural unit in gels obtained from zeolites is in the range of the cubic nanometer, nm3.

Journal Article↗
PubMed2005

Characterization and ab initio XRPD structure determination of a novel silicate with Vierer single chains: the crystal structure of NaYSi2O6.

The crystal structure of a sodium yttrium silicate with composition NaYSi2O6 has been determined from laboratory X-ray powder diffraction data by simulated annealing, and has been subsequently refined with the Rietveld technique. The compound is monoclinic with space group P2(1)/c and unit cell parameters of a=5.40787(2) A, b=13.69784(5) A, c=7.58431(3) A, and beta=109.9140(3) degrees at 23.5 degrees C (Z=4). The structure was found to be a single-chain silicate with a chain periodicity of four. The two symmetry dependent [Si4O12] chains in the unit cell are parallel to c. A prominent feature is the strong folding of the crankshaft-like chains within the b,c-plane resulting in intrachain Si-Si-Si angles close to 90 degrees. The coordination of the Y3+ ions by O2- is 7-fold in the form of slightly irregular pentagonal bipyramids, with oxygen atoms from four different chains contributing to the coordination polyhedron. Na+ ions are irregularly coordinated by 10 oxygens from two neighboring chains. No disorder of Na+ and Y3+ between the two nontetrahedral cation sites could be observed. Furthermore, micro-Raman spectra have been obtained from the polycrystalline material.

Journal Article↗
PubMed1998

Silicon-29 NMR Studies of Tetraalkylammonium Silicate Solutions. 1. Equilibria, (29)Si Chemical Shifts, and (29)Si Relaxation.

The addition of tetraalkylammonium cations to aqueous silicate solutions enhances the abundance of symmetric, cagelike, polysilicate anions including the cubic octamer, Si(8)O(20)(8)(-). The equilibrium ratio of tetramethylammonium (TMA) cations to the octameric silicate anion is 8:1 for solutions with a concentration ratio [OH(-)]:[Si] >/= 1:1. Evidence indicates that organocations directly associate with cagelike polyanions to form a protective shell of hydrophobic hydration that impedes hydrolysis of the central anion.

Journal Article↗