Showing papers on "Silicate minerals published in 1999"

Sorption of heavy metals from industrial waste water by low-cost mineral silicates

Abstract: The adsorption by different silicate minerals of some heavy metals, present in industrial waste water, has been studied. These adsorbents (mainly clay minerals) are readily available, inexpensive materials and offer a cost-effective alternative to conventional treatment of wastes from the metal finishing industry. The results show that some mineral species are suitable for the purification of such residual waters down to the limits prescribed by current legislation concerning industrial wastes. The Langmuir model was found to describe such adsorption processes best. Sepiolite (Orera, Spain) has an adsorption capacity of 8.26 mg g (super -1) Cd (super 2+) , the capacities depending on the metal adsorbed in the order: Cd (super 2+) >Cu (super 2+) >Zn (super 2+) >Ni (super 2+) . This mineral shows the highest sorption capacity relative to the other minerals studied. Factors in the reaction medium such as pH and ionic strength influenced the adsorption process.

Evidence for the Formation of Trioctahedral Clay upon Sorption of Co2+ on Quartz

Abstract: The sorption mechanism of Co on quartz at room temperature has been investigated by an in-depth analysis of published extended X-ray absorption fine structure (EXAFS) spectroscopy and solution chemistry data. In particular, the 3.5–5 A mid-range atomic environment of Co has been determined with unprecedented precision by combining ad initio FEFF7.02 calculations and results obtained by polarized EXAFS on the mid-distance structure of sheet silicate minerals. The local atomic environment around sorbed Co atoms is identical to that of Co in trioctahedral clays and substantially different from that in the cobalt hydroxide Co(OH)2(s). Neoformation of a trioctahedral clay is consistent with calculated thermodynamic solubilities, which indicate that 2:1 and 1:1 Co-rich hydrous silicates, similar to kerolite and chrysotile, are less soluble than Co(OH)2(s). Consequently, precipitation of Co-rich clay is favored over that of Co(OH)2(s) at pH values below 9 and for a dissolved Si concentration equal to quartz solubility. New experimental data show that dissolved Si concentrations can approach, and even exceed, that of quartz solubility during the short times of sorption experiments. Based on the available data, it is not possible to conclude unequivocally if the Co layer silicate grew epitaxially on the quartz surface, topotactically in a surface amorphous layer, or independently of the quartz framework structure. The structural and chemical interpretation is supported by recent published studies in which sorption of a hydrolyzable cation leads to the neoformation of a mixed layer phase formed from the sorbate species and the sorbent metal. This surface-induced precipitation mechanism is a general phenomenon that may account for the formation of secondary clays as coatings on silicates.

Coordination and local structure of magnesium in silicate minerals and glasses; Mg K-edge XANES study

Abstract: We present Mg K-edge XANES spectra of selected Mg-bearing oxide and silicate minerals with different coordination states of Mg. The Mg K-edge peak shifts to higher energy with increasing coordination, from talMg in spinel, to tslMg in grandidierite, to t61Mg in diopside and many other silicates, and to I8lMg in pyrope. The correlation between the energy of the Mg K-edge and Mg-O bond distance of the model minerals is also established; it can be used to estimate the average Mg-O bond distance in disordered systems. The curve fitting of Mg K-edge XANES spectra may be used to distinguish the coordination of Mg, and to determine the relative proportion of different coordination sites, as demonstrated in yoderite, which contains both t5lMg and t6lMg. The structural role of Mg in CaMgSi2O6 @i) NaAlSi3Os (Ab) glasses was studied using Mg K-edge XANES spectra. The Mg-O bond distance in these glasses is estimated to be 2.00 + 0.04 A. Thus, Mg in these glasses may be five-coordinated with oxygen, or Mg may have multiple structural sites, t41Mg, t5lMg and t6lMg. Structutally, the Di-Ab glasses may possess a medium-range order, and have dramatically different multiple scattering (MS) paths from those of crystalline model minerals.

Surface Area of Primary Silicate Minerals

Abstract: The rates of many heterogeneous reactions are dependent upon the mineral-water interfacial area. Examples include release of nutrients from primary minerals, rate of growth of authigenic minerals, adsorption and desorption of metal and organic contaminants on soil and sediment grains, neutralization of acid deposition by weathering reactions, oxidation and reduction of mmetal-containing phases and solutes, clumping of colloids or bacteria by electrostatic attraction, and photocatalytic degradation of organic pollutants at metal oxide surfaces (Davis et al., 1993; White and Brantley, 1995). Several workers have also shown that the surface area is a key parameter in predicting weathering rates using geochemical models (e.g. PROFILE) and soil chemistry under the influence of acid rain (Jonsson et al., 1995; Hodson et al., 1996, 1997a). Along with the permeability, the surface area is one of the most difficult physical parameters to quantify in extrapolating from the laboratory to the soil plot to the watershed (White and Peterson, 1990). Most models of solute transport in aquifers and in soils simply ignore the mineral-water surface area term by combining it with the kinetic rate constant into one fitting parameter, despite the fact that the specific surface area may vary over several orders of magnitude — 102–106 cm2/g as functions of grain size, mineralogy, oxide coating, weathering history, or biological effects, or as a combination of these factors. Despite the importance of the mineral surface area in many areas of geochemistry, little systematic effort has been expended to understand or predict this term for primary silicates (more work has been completed on the surface area of clays and simple oxides).

Synthesis and Structural Characterization of Microporous Framework Zirconium Silicates

Abstract: The synthesis and structural characterization of synthetic analogues of the microporous zirconium silicate minerals petarasite (AV-3), gaidonnayite (AV-4), and umbite (AM-2) are reported. AM-2 materials have been synthesized with different levels of titanium substitution (Zr/Ti molar ratios of ∞,1.9, 0.5, and 0) indicating the existence of a continuous solid solution. All materials have been characterized by several techniques, viz., SEM, powder XRD, single- and triple-quantum 23Na and 29Si MAS NMR, water adsorption measurements, and TGA analysis. As a common structural feature, all these solids possess corner-sharing [ZrO6] octahedra and [SiO4] tetrahedra forming a three-dimensional framework. Unlike framework microporous titanosilicates where Ti−O−Ti−O chains are usually present, petarasite, gaidonnayite, umbite, and their synthetic analogues display structures where the [ZrO6] octahedra are isolated from each other by [SiO4] tetrahedra and, hence, Zr−O−Zr−O chains do not occur. All these materials are ...

Microanalysis of Fe 3+ /ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M 2,3 edge

Abstract: The Fe M2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe3(Al1–xFex)2[SiO4]3 and andradite-skiagite (Fe1–xCax)3Fe2[SiO4]3), pyroxenes (acmite-hedenbergite (Ca1–xNax)(Fe2+1−xFe3+x)Si2O6), and spinels (magnetite-hercynite Fe(Al1–xFex)2O4) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe3+/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe3+/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M2,3 edge is observed. Since the partial cross sections of the Fe M2,3 edges are some orders of magnitude higher than those of the Fe L2,3 edges, the Fe M2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe3+/ΣFe ratio over distances on the order of 100 nm.

Microscopic properties and processes in minerals

Abstract: Preface. 1. Crystal structures at extremes of pressure and temperature R. Miletich, R.J. Angel. 2. Equations of state J.-P. Poirier. 3. Introduction to Raman spectroscopy at extreme pressure and temperature conditions Ph. Gillet. 4. Thermodynamic properties of minerals at high pressures and temperatures from vibrational spectroscopic data Ph. Gillet, et al. 5. Introduction to the application of neutron spectroscopy in the Earth Sciences B. Winkler. 6. Synchrotron radiation and solid state science C.R.A. Catlow, G. Sankar. 7. In situ structural and kinetic powder diffraction studies of aluminosilicates G. Artioli. 8. Mineral synthesis by flux growth methods H. Skogby. 9. Bonding and electronic structure of minerals R.E. Cohen. 10. Crystal chemistry of metal sulfide minerals K. Wright, D.J. Vaughan. 11. Introduction to the theory of lattice dynamics M.T. Dove. 12. Disorder and defects in solids C.R.A. Catlow. 13. The modelling of bulk diffusion in solids L. Vocadlo, G.D. Price. 14. Classical computer simulations A.N. Cormack. 15. Ab initio LCAO periodic simulations of crystal properties. Phase transitions, magnetic and electronic behaviour, X-ray diffraction M. Catti. 16. Atomic structure of disordered materials M.T. Dove, D.A. Keen. 17. Principles of solid state NMR spectroscopy and applications to determining local order in minerals A. Putnis, V. Vinograd. 18. Introduction to the theory of displacive phase transitions M.T. Dove. 19. Order/disorder phenomena in minerals: ordering phase transitions and solid solutions M.T. Dove. 20. Phase transitions and Equations of State R. Miletich, R.J. Angel. 21. IR spectroscopy and hydrogen bonding in minerals A. Beran, E. Libowitzky. 22. Water in nominally anhydrous minerals H. Skogby. 23. Contribution of IR spectroscopy to the problem of water in the Earth's mantle A. Beran. 24. Solubility and diffusion of 'water' in silicate minerals D. Kohlstedt, S.J. Mackwell. 25. The theory and simulation of the melting of minerals L. Vocadlo, G.D. Price. Molecular dynamics computer simulations of silicate glasses A.N. Cormack. 27. Mineral surface science: principles, applications and case studies A. Putnis, et al. 28. Atomistic simulation of mineral surfaces and interfaces N.H. de Leeuw, et al.

Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization

Abstract: Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyrite, and pentlandite) from a suite of samples from the platiniferous J-M Reef of the Stillwater Complex were measured to elucidate the temporal and genetic relationship between sulfides and host silicate minerals. Results indicate that sulfides and coexisting plagioclases are generally not in isotopic equilibrium, that both sulfides and feldspars record highly radiogenic initial ratios at 2.7 Ga, and that a component of “post-emplacement” radiogenic Pb has mixed with common Pb in the sulfides. A model involving introduction of radiogenic Pb carried by fluids derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally derived during an extensive hydrothermal episode, associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility that some fractions of the radiogenic Pb may have been derived from primary minerals altered during the low-grade metamorphism cannot be discounted. The amount of radiogenic lead added is variable and in some cases negligible. There is a good correlation between the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples that show little or no alteration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary minerals (serpentine, talc, actinolite, chlorite and zoisite) associated with greenschist facies metamorphism. Some of the radiogenic Pb in the sulfides can be removed by progressive stepwise leaching. However, in most samples recrystallization of sulfides during metamorphism has mixed common Pb and radiogenic Pb throughout the crystal structure such that, in these samples, stepwise leaching does not recover initial Pb isotopic ratios. Plagioclases are much more resistant to low temperature recrystallization and in almost all cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over a wide temperature range enhances their utility in understanding not only the processes involved in their formation at the time of magmatic emplacement but also postmagmatic processes which were important in the redistribution and enrichment of platinum group elements (PGE) within the ore zone.

High‐temperature dynamics in cristobalite (SiO2) and Carnegieite (NaAlSiO4): A Raman Spectroscopy Study

Abstract: β-cristobalite (SiO2) and β-carnegieite (NaAlSiO4) have been investigated by Raman spectroscopy up to their melting points at 1999 and 1799 K, respectively. In both cases, the major spectral changes are observed at the α-β transitions, near 540 and 970 K, especially at low frequencies where the modes, involving mainly intertetrahedral motion, merge to a single, broad envelope. These observations and comparisons made between the Raman spectra of the crystalline and liquid phases are consistent with the rigid-unit model recently proposed for describing the dynamics of SiO4 and AlO4 tetrahedra in framework silicates. From a structural standpoint, an important consequence is that the atomic mobility associated with premelting sets in at the low temperatures of these transitions, not in the vicinity of the melting point as for other silicate minerals.

U-Th-Pb dating of the Brossasco ultrahigh-pressure metagranite, Dora-Maira massif, western Alps

Abstract: The Brossasco metagranite is part of the coherent ultrahigh-pressure metamorphie terrane in the Dora-Maira massif and, although it has reached Alpine peak metamorphic conditions of about 725°C/30 kbar, still locally preserves an undeformed magmatic texture. In order to obtain new chronological constraints on this key area of the western Alps, we studied the behaviour of the U-Th-Pb system in zircons and monazites from such an undeformed facies. Only zircon fractions containing very small needle-shaped inc\usions are concordant at 304 ± 3 Ma. This result is interpreted as the emplacement age of the granite. All other zircons show an old inherited memory and the more U-rich of these crystals were affected by an episodic Pb loss with a poorly defined lower intercept at 60 ± 45 Ma. Both TIMS (thermo-ionisation mass spectrometry) and electron-microprobe analysis of monazites show that this last event has also disturbed their U-Pb and Th-Pb isotopic systems, without resetting them completely. Comparison with cofacial rock-types which have been completely reworked under these extreme P-T conditions shows that deformation and mass transfer as weil as the nature of the protoliths and the characteristics of their zircons have a much more pronounced influence on the behaviour of isotopic systems than the temperature and/or press ure parameters. This evidence is an obvious limitation to the use of the blocking-temperature concept. Key-words: metagranite, Dora-Maira massif, ultrahigh-pressure metamorphism, U-Pb zircon, monazite, elec­ tron-microprobe dating.

Quantifying the Effects of Metasomatism in Mantle Xenoliths: Constraints from Secondary Chemistry and Mineralogy in Udachnaya Eclogites, Yakutia

Abstract: In mantle xenoliths, metasomatism is recorded by compositional variations within and between minerals, and by the introduction of secondary minerals. However, metasomatism has not been quantitatively evaluated as a process with respect to the fluid composition involved. Diamondiferous eclogites from the Udachnaya kimberlite provide a unique suite of samples that allow a semiquantitative estimation of metasomatic fluid composition. The basis of our analysis involves comparison of reconstructed whole-rock compositions with measured whole-rock analyses. Primary minerals in these samples are relatively homogeneous, and permit the use of modal analyses and mineral chemistry for reconstruction of “pristine” whole-rock compositions. The metasomatic overprint, which is similiar in all samples studied, has produced depletions in SiO2, Na2O, and FeO and enrichments in TiO2, K2O, MgO, and LREE. Secondary minerals from the samples are interpreted as the direct result of metasomatism (i.e., typical metasomatic mineral...

Microscopic strain, local structural heterogeneity and the energetics of silicate solid solutions

Abstract: Evidence for the existence and scale of local microscopic strains in silicate minerals is reviewed. From the compositional limit of the plateau for the monoclinic ⇌ triclinic phase transition in alkali feldspars, the microscopic strain fields around impurity atoms in framework silicates can be ∼20–40 A in diameter. This dimension fails within the characteristic length scale of phonons in these materials, and phonon spectra should therefore be particularly sensitive to the extent and magnitude of local elastic strain effects. The most prominent feature is expected to be broadening of absorption peaks, which can now be followed quantitatively in complex spectra by autocorrelation analysis. Such strain fields contribute elastic energy to the enthalpy of a crystal and might be expected to be largely responsible for enthalpy changes accompanying cation ordering processes and solid solution formation. Comparisons of line broadening data from powder IR absorption spectra with calorimetric determinations...

Irradiated interplanetary dust particles as a possible solution for the deuterium/hydrogen paradox of Earth's oceans

Abstract: Determining the source of Earth's oceans is a longstanding problem in planetary science. Possible sources of water include water ice or water of hydration of silicate minerals in the original material from which the bulk Earth accreted and water brought in by late-arriving planetesimals during the heavy bombardment period (4.5-3.8 Gyr ago) [Chyba, 1989, 1991]. Comets are an attractive source of water because their origin in the outer solar system is consistent with the long timescale for heavy bombardment. However, the high deuterium/hydrogen (D/H) ratio of the three comets that have been studied, Halley, Hyakutake, and Hale-Bopp, indicates that Earth must have had a source with a low-D/H ratio as well. Here we suggest that solar wind-implanted hydrogen on interplanetary dust particles (IDPs) provided the necessary low-D/H component of Earth's water inventory.

Mineral micro- and nano-variability revealed by combined micromorphology and in situ submicroscopy

Abstract: A combination of micromorphology and different in situ sub microscopical techniques, performed on thin sections of soil, saprolite, and rock samples, was used to assess possible relationships between micro- and nano-variability and macro- and micro-environmental conditions of mineral transformation during weathering. Four examples, presented in this study illustrate variability in morphology, chemistry and mineralogy of secondary minerals present in pseudomorphs after primary silicate minerals in volcanic soils present in the humid tropics. The study was based on micromorphological characterization of secondary (clay) minerals, with complementary submicroscopical analysis such as scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDXRA), performed on uncovered thin sections, step scan X-ray diffraction (SSXRD) and transmission electron diffraction (TEM–EDS) carried out either on disturbed powder samples or undisturbed microsamples, obtained by micro-drilling in thin sections. These techniques highlight a very complex mineralogy in pseudomorphs after primary minerals, including secondary minerals that generally are assessed to be formed under contrasting physicochemical conditions (gibbsite and 2:1 phyllosilicates in a pseudomorph after plagioclase; halloysite, kaolinite, smectite and aluminum and iron oxi-hydroxides in pseudomorphs after pyroxenes). The study by TEM of undisturbed microfabrics of secondary minerals in a clay pseudomorph after pyroxene revealed typical distribution patterns, with mineralogical compositions varying over some microns. The combination of techniques gave an improved insight on the complexity and variability of the nature composition and distribution of secondary minerals formed during weathering. It indicates the influence of sequential weathering processes on mineral micro- and nano-variability.

COMPARATIVE STUDY OF TWO PEGMATITIC FIELDS FROM MINAS GERAIS, BRAZIL, USING THE Rb AND Cs CONTENTS OF MICAS AND FELDSPARS

Abstract: The distribution of alkali elements and F was investigated in minerals of pegmatites and their contact aureole of metasomatism, in two groups of pegmatites from the State of Minas Gerais, southeast Brazil: the Transamazonian-cycle age (1,930 Ma) Volta Grande pegmatites near Sao Joao del Rei and the Brasiliano-cycle age (525 Ma) pegmatites of Itinga. The Volta Grande pegmatites are albite and spodumene bearing and their contact aureole of metasomatism, in the amphibolite host rock, are characterized by the presence of holmquistite and zinnwaldite. The Itinga pegmatites correspond to complex lepidolite and pet alite-bearing pegmatites with tourmaline, zinnwaldite and muscovite in their contact aureole developed in quartz-biotite-muscovite-schists. The K-bearing minerals of the Volta Grande pegmatite are exceptionally rich in Rb. Using the experimentally determined partition coefficients between minerals and fluids, the atomic ratio K/Rb of the pegmatitic fluid may be evaluated. This ratio shows homogenous values, around 4. Similar values are found in the metasomatic aureole, at the contact, which increase as a function of the distance from the contact, due to the crystallization of trioctahedral micas that deplete the fluid in Rb. The K/Rb ratios of the fluid could represent the ratio of the initial pegmatitic magma before differentiation. Rb content in the Itinga pegmatites is much lower and strongly differs from border to core. The presence of K-bearing minerals in the wall rocks contributes for the increasing the K/Rb ratios of the neoformed minerals at the contact zone. It is then more difficult to evaluate the K/Rb ratio of the initial magma. Considering the composition of the minerals at the border of each pegmatite, it varies from 25 at Urubu to 50 at Jose de Linto and Xuxa localities. The Itinga pegmatites are clearly poorer in Rb than the Volta Grande pegmatites. Geochemically, a relationship between the Rb and F contents in the micas is clearly demonstrated in each group of pegmatites. The situation is more complex for Cs because crystallization of all minerals concentrates Cs in the fluid. Urubu pegmatite is the only one where pollucite was found in the core. Cs content in K-bearing minerals is higher in the lepidolite of the core and in zinnwaldite of the aureole. Cs2O in the Volta Grande pegmatite can be as high as 2%, at 1 meter from the contact, as metasomatic fluids become enriched in Cs while the metasomatism proceeds. This element could be used as guide for exploration of the alkali pegmatites.

In situ complex impedance spectroscopy of mantle minerals measured at 20 GPa and 1400°C

Abstract: Electrical conductivities of mantle silicate minerals (Mg0·9Fe0·1)2SiO4 olivine, wadsleyite and ringwoodite were determined at pressures up to 20 GPa and temperatures up to 1400°C using complex impedance spectroscopy in a high pressure multianvil apparatus. All samples were polycrystalline, synthesized in separate high pressure experiments prior to the electrical measurements. Olivine conductivities up to 10 GPa are very close to values determined at ambient pressure under controlled oxygen fugacities in previous studies indicating a very small pressure dependence. The conductivities of wadsleyite at 15 GPa and ringwoodite at 20 GPa are similar, and both about 100 times greater than for olivine. When compared to conductivity models of Earth's mantle, these results suggest that the steep increase in conductivity near the transition zone is mainly due to the olivine to wadsleyite phase transformation at 410 km depth, with only minor changes in conductivity occurring over the wadsleyite to ringwoodi...

Chemical evolution of leaked high-level liquid wastes in Hanford soils

Abstract: A number of Hanford tanks have leaked high level radioactive wastes (HLW) into the surrounding unconsolidated sediments. The disequilibrium between atmospheric C0{sub 2} or silica-rich soils and the highly caustic (pH > 13) fluids is a driving force for numerous reactions. Hazardous dissolved components such as {sup 133}Cs, {sup 79}Se, {sup 99}Tc may be adsorbed or sequestered by alteration phases, or released in the vadose zone for further transport by surface water. Additionally, it is likely that precipitation and alteration reactions will change the soil permeability and consequently the fluid flow path in the sediments. In order to ascertain the location and mobility/immobility of the radionuclides from leaked solutions within the vadose zone, the authors are currently studying the chemical reactions between: (1) tank simulant solutions and Hanford soil fill minerals; and (2) tank simulant solutions and C0{sub 2}. The authors are investigating soil-solution reactions at: (1) elevated temperatures (60--200 C) to simulate reactions which occur immediately adjacent a radiogenically heated tank; and (2) ambient temperature (25 C) to simulate reactions which take place further from the tanks. The authors studies show that reactions at elevated temperature result in dissolution of silicate minerals and precipitation of zeolitic phases. At 25 C, silicate dissolution is not significant except where smectite clays are involved. However, at this temperature CO{sub 2} uptake by the solution results in precipitation of Al(OH){sub 3} (bayerite). In these studies, radionuclide analogues (Cs, Se and Re--for Tc) were partially removed from the test solutions both during high-temperature fluid-soil interactions and during room temperature bayerite precipitation. Altered soils would permanently retain a fraction of the Cs but essentially all of the Se and Re would be released once the plume was past and normal groundwater came in contact with the contaminated soil. Bayerite, however, will retain significant amounts of all three radionuclides.

Some investigations of sea sands from the Black Sea coastline

Abstract: Sands from the coastline of Bourgas Bay, Black Sea, were studied by means of Mossbauer spectroscopy, X-ray diffraction, infrared spectroscopy and gamma-spectrometry. Samples of the residue after magnetic separation contained quartz, carbonates, biotite, brookite and silicate minerals. Radionuclides, such as uranium, thorium, potassium, etc., were detected in all sand samples.

The distribution and behaviour of re and os amongst mantle minerals and the consequences of metasomatism and melting on mantle lithologies

Abstract: The fractionation of Re and Os during basalt genesis is one of the key processes governing the present-day distribution of these elements between the earth’s crust and mantle. However, the cause of this fractionation remains poorly understood, in a large part because there is little information on the distribution and behaviour of Re and Os in mantle minerals. This study presents rhenium-osmium (Re-Os) isotope and elemental abundances for primary mantle minerals, metasomatic phases, and a range of mantle rock types from xenoliths in recent voclanics in northern Tanzania. These results provide quantitative constraints on the distribution of Re and Os amongst mantle minerals, enable an assessment of the effects of metasomatism, and an understanding of the behaviour of Re and Os during melting and the consequences for the chemistry of the mantle residue. Re and Os abundances for sulphide and coexisiting silicates in garnet lherzolites from Lashaine confirm that sulphide dominates the Os budget (e.g. Hart and Ravizza, 1996), but also show that Re is almost exclusively sited in silicate phases. Silicate minerals from two different samples yield 187Re-188Os ages of 32±12 and 15±8 Myr, respectively. Comparison with 147Sm-143Nd, 87Rb-87Sr, and 232Th-208Pb ages for the same silicate minerals suggests differential closure between these isotope systems, and a closure temperature of about 700°C for the Re-Os system. However, sulphide inclusions preserve significantly less radiogenic Os isotope compositions, suggesting that they have been shielded from reaction or diffusion by their silicate hosts. Diffusion rates in sulphides are likely to be rapid at mantle temperatures (e.g. Birchenall, 1974), however, for sulphide inclusions diffusion through the silicate host is likely to be impaired by the high partition coefficient between sulphide and silicate pahses (Hofmann and Hart, 1978). The Os isotope composition of the sulphide (187Os/188Os = 0.10432±13) suggests a minimum age of 3.4 Gyr, indicating that the subcontinental lithospheric mantle in Tanzania, has a similar longevity to the Kaapvaal craton (Pearson et al., 1995). Whole-rock samples from Labait, Olmani and Lashaine generally have Os isotope compositions that are less radiogenic than estimates for the present-day upper mantle, suggesting Re loss occurred some time ago. Samples containing metasomatic phlogopite possess high Re concentrations and Re/Os ratios, but that they also possess relatively unradiogenic Os isotope compositions suggests that the metasomatism occurred relatively recently, consistent with data for metasomatic minerals. Some dunites possess both high Re/Os ratios and radiogenic Os isotope compositions. These samples differ from those affected by modal metasomatism in having exceptionally low Os concentrations (containing as little as 10 ppt Os). These rocks are extremely refractory and are considered to have experienced extensive melt loss (e.g. Rudnick et al, 1992). The Re-Os data are consistent with complete removal of sulphide during melting, leaving a radiogenic silicate residue with an extremely low Os concentration. These results suggest that at least some of the radiogenic Os component in the mantle, sampled by oceanic basalts, may arise from mantle processes themselves (prior melting or metasomatism) rather than from recycling (e.g. Hauri and Hart, 1993), or contribution from the lower mantle or core (Widom and Shirey, 1996).

Vitamin c-containing ceramic and its production

Abstract: PROBLEM TO BE SOLVED: To obtain a neutralizing, decomposing and removing material continuing long-term effect with respect to chlorine, a harmful organochlorine compd. such as a chlorine compd., dioxins or the like, environmental hormone, a virus or the like. SOLUTION: Vitamine C, a vitamine C-containing plant and alkali metal or alkaline earth metal minerals, silicate minerals and other minerals are combined to obtain a neutralizing material comprising vitamine C-containing ceramics. This neutralizing material neutralizes and decomposes bromine, chlorine, a chlorine compd., other harmful organochlorine compd., environmental hormone or a virus almost instantaneously to keep its effect for a long period of time.

Analyse de la porosité dans un profil d'encroûtement carbonaté sur les basaltes du Trias du Moyen Atlas (Maroc)

Abstract: A carbonate crust profile has developed from Triassic basalt flows in the Moroccan Middle Atlas. The study of pore distribution of these supergene formations shows a good agreement between calculated (from density measurements) and directly measured values of porosity (mercury porosimetry). Weathering of basalt de,yelopS'clayey layers (saprolite and soil) displaying increases in porosity (macroporosity essentially). 'In .the Quaternary, these very porous and permeable layers permitted easy meteoric water tirculation and a profile calcitization by per descemum processes. Carbonate precipitation occurs by epigenesis replacement of silicate minerals. In the friable saprolite, carbonate replaces directly the basaltic primajr. minerals, while this replacement operates on the weathering products (smectites) and residual primary minerals in the coherent saprolite, indicating the very fast progression of the calcitization front. At the weathering front, within the coherent saprolite, both smectitic weathering and carbonatation compete as functions of time and fluid chemical composition. (O 1999 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.)

Transformation of Iron-Containing Minerals in Kaolin During Growth of a Mixed Bacterial Culture Derived from Kaolin

Abstract: Kaolins are clay rocks composed mainly of kaolinites (not less than 50%), the rest being other silicates. They also contain iron and titanium minerals as impurities. Iron in kaolins occurs in the form of goethite, hematite, hydrogoethite, magnetite, and pyrite, or as iron oxide films on the surface of the silicate minerals, or it may be included in their crystalline lattices (Avgustinik, 1975).