Showing papers on "Silicate minerals published in 1993"

An automated leaching method for the determination of opal in sediments and particulate matter

Abstract: An automated leaching method for the analysis of biogenic silica (opal) in sediments and particulate matter is described. The opaline material is extracted with 1 M NaOH at 85°C in a stainless steel vessel under constant stirring, and the increase in dissolved silica is continuously monitored. For this purpose, a minor portion of the leaching solution is cycled to an autoanalyzer and analyzed for dissolved silicon by molybdate-blue spectrophotometry. The resulting absorbance versus time plot is then evaluated according to the extrapolation procedure of DeMaster (1981). The method has been tested on sponge spicules, radiolarian tests. Recent and Pliocene diatomaceous ooze samples, clay minerals and quartz, artificial sediment mixtures, and on various plankton, sediment trap and sediment samples. The results show that the relevant forms of biogenic opal in Quaternary sediments are quantitatively recovered. The time required for an analysis is dependent on the sample type, ranging from 10 to 20 min for plankton and sediment trap material and up to 40–60 min for Quaternary sediments. The silica co-extracted from silicate minerals is largely compensated for by the applied extrapolation technique. The remaining degree of uncertainty is on the order of 0.4 wt% SiO2 or less, depending on the clay mineral composition and content.

Leaching and reconstruction at the surfaces of dissolving chain-silicate minerals

Abstract: THE pathways by which silicate minerals transform to solutes, clays and amorphous solids are relevant to a wide range of natural, industrial and even medical concerns. For example, weathered layers on silicate may have a high sorptive capacity, affecting nutrient and contamination retention in soils; less obviously, such layers on inhaled silicate grains might affect their interaction with lung tissue. Here we report the observation, in dissolution experiments on a range of chain-silicate minerals, of the formation of a near-surface amorphous region enriched in silicon and hydrogen, and depleted in other metals. Raman spectroscopy and ion-beam elemental analysis show that portions of the polymeric silicate anion in this region spontaneously reconstruct to form a network that contains four-member silicate rings and areas of incipient crystallization. If hydrolysable metals interact with the silicate anion during this reconstruction, clays and amorphous products may form directly. This process complements traditional dissolution-precipitation pathways of mineral diagenesis1, as the silicon does not have to be present in solution before being incorporated into a growing secondary phase.

Temperature dependence of silicate weathering in nature: How strong a negative feedback on long-term accumulation of atmospheric CO2 and global greenhouse warming?

Abstract: Estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent Arrhenius activation energy of 18.4 kcal/mol (77.0 kJ/mol), much higher than most laboratory values. This finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought. This result provides a stronger negative feedback on long-term greenhouse warming than has been assumed in most models of global carbon cycling. The present estimate was determined from the ratio of feldspar weathering rates (determined by geochemical mass balance) in the southern Blue Ridge Mountains of North Carolina, United States. Temperature (a function of elevation) is the only factor that differs between the two catchments; parent rock type, aspect, hillslope hydrology, and vegetation type and successional stage are the same in both.

Optical Mineralogy : Principles And Practice

Abstract: Introduction to the microscopic study of minerals Silicate minerals The non-silicates Transmitted-light microscopy Reflected-light theory Appendices: Biaxial minerals Uniaxial positive minerals Uniaxial negative minerals Isotropic minerals 2 V and sign of biaxial minerals Properties of ore minerals Mineral identification chart Gangue minerals.

Borate Minerals and Related Authigenic Silicates in Northeastern Mediterranean Late Miocene Continental Basins

Abstract: A-ct Borate minerals, such as colemanite, ulexite and borax, have been reported in Tertiary continental basins of West Anatolia, Turkey, and Samos Island, Greece. Tuffaceous rocks interbedded with the borates are rich in authigenic silicate minerals such as zeolites, potassium feldspar and opal-CT, all of which have a genetic association with the borates. Chemical analyses of the tuffaceous rocks show variation in major elements due to the varying percentage of carbonaceous, detrital, and volcanic derived material. The volcanic activity in the vicinity of each particular lacustrine basin created ion-rich fluids from which a series of borates were precipitated under arid climate. Additionally, the We-alkaline character of these fluids was responsible for the diagenesis of the silicic glass of the tuffaceous rocks, altering them to silicates and silica polymorphs.

Relationships between silicate melts and carbonate-precipitating melts in CaO-MgO-SiO 2 -CO 2 -H 2 O at 2 kbar

Abstract: Phase fields intersected by three joins in the System CaO-MgO-SiO_2-CO_2-H_2O at 2 kbar were investigated experimentally to determine the melting relationships and the sequences of crystallization of liquids co-precipitating silicate minerals and carbonates. These joins connect SiO_2 to three mixtures of CaCO_3-MgCO_3-Mg(OH)_2 with compositions on the primary field for calcite, between the composition CaCO_3 and the low-temperature (650°C eutectic liquid co-precipitating calcite, dolomite and periclase. In the pseudo-quaternary tetrahedron calcite-magnesite-brucite-diopside, two of the significant reactions found are: (1) a eutectic at 650°C, calcite + dolomite + periclase + forsterite + vapor = liquid, and (2) a peritectic at 1038°C which is either calcite + akermanite + forsterite + vapor = monticellite + liquid calcite + monticellite + forsterite + vapor = akermanite + liquid. The eutectic liquid has high MgO/CaO and CO_2/H_2O and only 2–3% SiO_2 (estimated 15–20% MgCO_3, 35–40% CaCO_3, 40–45% Mg(OH)_2, and 5–6% Mg_2SiO_4). The composition joins intersect a thermal maximum for akermanite + forsterite + vapor = liquid, which separates high-temperature liquids precipitating silicates together with a little calcite, from low-temperature liquids precipitating carbonates with a few percent of forsterite; there is no direct path between the silicate and synthetic carbonatite liquids on these joins, but it is possible that fractionating liquid paths diverging from the joins may connect them. More complex relationships involving the precipitation of monticellite and akermanite are also outlined. Magnetite-magnesioferrite may replace periclase in natural magmatic systems. The results indicate that the assemblage calcite-dolomite-magnetite-forsterite represents the closing stages of crystallization of carbonatites, whereas assemblages such as calcite-magnetite-forsterite and dolomite-magnetite-forsterite span the whole range of carbonatite evolution in terms of temperature and composition, and provide the link between liquids precipitating silicates and those precipitating carbonates.

Chemical analysis of thin black layers on building stone

Abstract: Thin black layers often occur on building stone in polluted areas. Various techniques including scanning electron microscopy/energy dispersive X-radiography, electron spectroscopy for chemical analysis, Mossbauer spectroscopy and gas chromatography-mass spectrometry have been used to analyze these layers in samples from Sweden and some other European countries. The layers mainly consist of various iron oxides and iron oxide hydroxides, sulphates, soot and silicate minerals, with smaller amounts of metal, rubber and asphalt particles, and numerous organic constituents in low concentration. Air pollutants, and to some degree minerals from the interior of the stone, are the main source of the components which form the black layers.

In-situ analysis of nitrogen in minerals

Abstract: In-situ analysis of nitrogen content in silicate minerals represents an important tool for the understanding of the paragenetic relations in rocks. Three microscopic techniques are applied to the in-situ analysis of nitrogen trapped as ammonium ions in silicate minerals (muscovite, biotite, feldspar) from the metamorphic schists of the Jebilet series (Morocco) and Cevennes (France). 1) Spectra obtained with Fourier transform infrared microspectroscopy show vibrational v 1 , v 2 , v 3 bands and some overtone and combinations of the ammonium molecule. The quantitative analysis is disturbed by dichroism effects, and needs the use of natural or synthetic silicate standards. The ammonium ion of the analysed sheet minerals is not detected by Raman microspectroscopy. 2) The nuclear microprobe is applied on the same mineral preparation and nitrogen content is measured using the 14 N(d,α 0 ) 12 C and 14 N(d,p 0 ) 15 N nuclear reactions with a deuteron energy of 1.8 MeV. TiN is used for calibration; concentrations from 360 to 2200 ppm are obtained. 3) The ion microprobe has similar application fields as the nuclear microprobe, but is more sensitive to matrix effects. The isotopic composition cannot be obtained considering the sensitivity of these techniques to the low 15 N content of minerals (near 1 to 10 ppm). The detection limit lies between 10 and 100 ppm, and is similar to the detection limits of the bulk techniques

Elastic properties of minerals

Abstract: Investigations of the elastic properties of the main rock-forming minerals were begun by T.V. Ryzhova and K.S. Aleksandrov over 30 years ago on the initiative of B.P. Belikov. At the time, information on the elasticity of single crystals in general, and especially of minerals, was very scanty. In the surveys of that time there was information on the elasticity of 20 or 30 minerals. These, as a rule, did not include the main rock-forming minerals; silicates were represented only by garnets, quartz, topaz, tourmaline, zircon, beryl, and staurolite, which are often found in nature in the form of large and fairly high-quality crystals. Then and even much later it was still necessary to prove a supposition which now seems obvious: The elastic properties of rocks, and hence the velocities of elastic (seismic) waves in the earth`s crust, are primarily determined by the elastic characteristics of the minerals composing these rocks. Proof of this assertion, with rare exceptions of mono-mineralic rocks (marble, quartzite, etc.) cannot be obtained without information on the elasticities of a sufficiently large number of minerals, primarily framework, layer, and chain silicates which constitute the basis of most rocks. This also served as the starting point and mainmore » problem of the undertakings of Aleksandrov, Ryzhova, and Belikov - systematic investigations of the elastic properties of minerals and then of various rocks. 108 refs., 7 tabs.« less

Mineralogical constraints on magmatic and hydrothermal Sn-W-Ta-Nb mineralization at the Nong Sua aplite-pegmatite, Thailand

Abstract: The chemistry of silicate minerals provides some constraints on the origins of disseminated cassiterite and Ta-Nb-Ti oxide minerals, and vein-hosted cassiterite-wolframite mineralization at the Nong Sua aplite-pegmatite complex. Zoned tourmaline crystals in aplite and pegmatite have preserved their primary compositions. Similar compositions are observed in hydrothermal tourmaline, suggesting an overlap of P, T, fHF and cation activities between the magmatic and hydrothermal stages. All tourmaline types contain excess (Y-site) aluminum, which is typical of the pegmatite environment. A temperature dependence of Al substitution in tourmaline, proposed for pegmatites elsewhere, is shown to be implausible at Nong Sua, indicating that the alkali:aluminum ratio of the melt and fluid have a greater importance in controlling tourmaline composition than has previously been suggested. Muscovite compositions record a history of post-magmatic re-equilibration down to approximately 400°C. By comparing the fluorine contents of muscovite and tourmaline, which have a contrasting tendency to re-equilibrate, it is apparent that the fHF was relatively high at the magmatic and early hydrothermal stages, and was lower during late hydrothermal activity. Enrichment of Ca and Mg in the rims of magmatic and hydrothermal tourmaline indicates that there was infiltration or diffusion of these elements at the magmatic and hydrothermal stages. Such infiltration-diffusion may have also triggered cassiterite crystallization by increasing fO 2 . Textural and compositional data for garnet suggest that it crystallized from melt. Some garnet cores are defined by primary fluid inclusions. The garnet in these cores contains up to 1500 ppm SnO 2 and provides a link between the late magmatic and the early hydrothermal evolution of the pegmatite system. Columbite, ixiolite and niobian-tantalian-rutile are restricted to being aplite-pegmatite-hosted and commonly occur as inclusions in disseminated cassiterite, or in the cores of Sn-rich garnet. These oxide minerals are interpreted to have crystallized from melt

The influence of local sources on the mineral content of bulk deposition over an altitudinal gradient in the Filabres Range (SE Spain)

Abstract: Mineralogical characteristics of bulk deposition have been determined at seven sites along an altitudinal gradient in the southern face of Filabres Range from 1770 m altitude to sea level, over a period of 8 months, the rainiest in this semiarid area. X ray diffraction and scanning electron microscopy were used for identifying and semiquantifying minerals. The aim was twofold: to ascertain the contribution of mineral particles to both the high pH and alkalinity of bulk deposition, the highest in Europe, and to use mineralogy of bulk deposition as a possible fingerprinting technique to identify the source of insoluble particles. The pH, alkalinity, and electrical conductivity of rainwater increase down the altitudinal gradient as precipitation decreases, indicating a clear increase in alkaline particles, in agreement with the presence of calcite, aragonite, dolomite, and gypsum, which also explains high pH and alkalinity of the bulk deposition. Moreover, pH and conductivity of rainwater are somewhat more related to E-SE winds than to W-NW ones. In addition to carbonates and gypsum, seven silicate minerals were found. Some minerals, like feldspars, chlorite, and aragonite, are neither related to site nor to rain events. Some others, like quartz, high-crystalline illite, paragonite, and kaolinite, are only related to rain events. Finally, a few minerals, like low-crystalline illite, palygorskite, paragonite, and to some extent calcite and gypsum, are site related. Some minerals have been interpreted to come from local sources, e.g., high-crystalline illite, gypsum, and paragonite; others, like palygorskite and low-crystalline illite, smaller in size, may come from distant sources.

Flood Transport and Deposition of Tracer Heavy Minerals in a Gravel-Bed Meander Bend Channel

Abstract: Heavy-mineral transport and deposition in a gravel-bed river was studied by placing 40,000 kg of heavy minerals at the upstream end of a meander-bend channel prior to a flood. During a subsequent flood, redeposition of the tracer heavy minerals was limited to the meander-bend part of the channel. High concentrations of heavy minerals formed on the point-bar face and in particular downstream of the point-bar apex. Zones of flow separation in the lee of boulders and cobbles are favorable sites for heavy-mineral concentration. Low heavy-mineral concentrations were formed in the thalweg and on the outer bank of the meander bend despite high bed roughness there. At the downstream end of the meander bend a swamp effectively filtered out heavy minerals from suspension.

Oxygen and Carbon Isotope Variations of Carbonates and Silicates from the Carbonatite Massif Tiksheozero (Northern Karelia)

Abstract: Carbon and oxygen isotope compositions have been determined for carbonate minerals from different types of carbonatites (large carbonatite bodies, veins, eruptive breccia), as well as from different temperature classes of carbonatites (according to Samoilov [1]). It could be shown, that only high temperature carbonatites exhibit small variations of δ18O and δ13C falling in the range of “high temperature mantle carbonates”, whereas low temperature calcites and dolomites show wide variations in δ18O. These changes in both isotope and mineral compositions of low temperature carbonatites are interpreted as a result of repeated interaction with fluids. The oxygen isotope ratios in most of the eruptive breccia have been changed by secondary processes. The primary deep-seated isotope record has been preserved only in samples, in which carbonate occurs as fine-grained calcite groundmass. Preliminary results on oxygen isotope composition of different silicate minerals (amphibole, magnetite, biotite, albit...

Oxygen-Isotope, X-Ray-Diffraction and Scanning-Electron-Microscope Examinations of Authigenic-Layer-Silicate Minerals from Mississippian and Pennsylvanian Sandstones in the Michigan Basin

Abstract: ! Introduction ! Oxygen-isotope, X-ray-diffraction, and scanning-electron-microscope examinations 2 References cited 4

Preliminary study of lead isotopes in the carbonate-silica veins of Trench 14, Yucca Mountain, Nevada

Abstract: The sub-vertical carbonate-silica veins filling the Bow Ridge Fault, where exposed in Trench 14 on the east side of Yucca Mountain, carry a lead isotopic signature that can be explained in terms of local sources. Two isotopically distinguishable--silicate and carbonate--fractions of lead are recognized within the vein system as well as in overlying surficial calcrete deposits. The acid-insoluble silicate fraction is contributed largely from the decomposing Miocene volcanic tuff, which forms the wall rock of the fault zone and is a ubiquitous component of the overlying soil. Lead contained in the silicate fraction approaches in isotopic composition that of the Miocene volcanic rocks of Yucca Mountain, but diverges from it in some samples by being more enriched in uranogenic isotopes. The carbonate fraction of lead in both vein and calcrete samples resides dominantly in the HCl- and CH{sub 3}COOH-soluble calcite. HCl evidently also attacks and removes lead from silicate phases, but the milder CH{sub 3}COOH dissolution procedure oftentimes identifies a significantly more radiogenic lead in the calcite. Wind-blown particulate matter brought to the area from Paleozoic and Late Proterozoic limestones in surrounding mountains may be the ultimate source of the calcite. Isotopically more uniform samples suggest that locally the basaltic ash and other volcanic rock have contributed most of the lead to both fractions of the vein system. An important finding of this study is that the data does not require the more exotic mechanisms or origins that have been proposed for the veins. Instead, the remarkably similar lead isotopic properties of the veins to those of the soil calcretes support their interpretation as a surficial, pedogenic phenomenon.

Mineralogy of a Few Entisols

Abstract: Study on mineralogy of sand, silt and clay fractions of a few Entisols of West Bengal show that quartz,feldspar and mica are the major silicate minerals in sand and siltfractions. In the clay fractions, the dominant clay mineral is mica in association with minerals like kaolinite, smectite, vermiculite, chlorite, microcline coupled with considerable amount of amorphous mineral. Mica, feldspar and chlorite are considered as partly inherited from parent materials and weathering sequence of mica has been shown. Minerals are in vermiculite weathering stage calculated from weathering means.

The stability of oxyamphiboles: Existence of Ferric-bearing minerals under the reducing conditions on the surface of Venus

Abstract: An enigma of Venusian mineralogy is the suggestion that Fe(3+)-bearing minerals exist under the reducing conditions of the Venusian atmosphere. Analysis of the spectrophotometric data from the Venera 13 and 14 missions, combined with the laboratory reflectance spectral measurements of oxidized basalts at elevated temperatures, led to the suggestion that metastable hematite might exist on Venus. Heating experiments at 475 C when f(sub O2) approximately 10(exp -24) demonstrated that the hematite to magnetite conversion is rapid indicating metastable hematite is not present on Venus. In addition to hematite, several other ferric oxide and silicate minerals have been proposed to occur on Venus, including laihunite or ferrifayalite, Fe(3+)-bearing tephroite, oxyamphiboles, and oxybiotites. Heating experiments performed on these Fe(3+)-bearing minerals under temperature-f(sub O2) conditions existing on Venus suggest that only oxyamphiboles and oxybiotites may be stable on the surface of Venus.