Formation of hydroxyl radicals catalyzed by clay surfaces     

D. Gournis  M. A. Karakassides  D. Petridis 《Physics and Chemistry of Minerals》2002,29(2):155-158

 The formation of superoxide and hydroxyl radicals at the surface of smectite clays due to oxygen reduction is demonstrated by electron paramagnetic resonance spectroscopy. The yield of hydroxyl radicals is mainly a function of particle size of the clays and depends, to a lesser extent, on the clay lattice iron. Synthetic laponite clay with small platelet size (∼20 nm) and without lattice iron is leading in the formation of hydroxyl radicals followed by montmorillonite (∼200 nm). Fluorohectorite (∼2000 nm) was inactive to hydroxyl radical formation by oxygen reduction. Received: 20 January 2001 / Accepted: 7 August 2001  相似文献   

Incorporation of Cr<Superscript>3+</Superscript> in dickite: a spectroscopic study     

E. Balan  T. Allard  G. Morin  G. Calas 《Physics and Chemistry of Minerals》2002,29(4):273-279

 We have investigated a well-ordered sample of natural Cr-bearing dickite from Nowa Ruda (Lower Silesia, Poland) using electron paramagnetic resonance (EPR) at X- and Q-band frequencies (9.42 and 33.97 GHz, respectively) and optical diffuse reflectance spectroscopy. The observation of the spin-forbidden transitions at 15500 and 14690 cm⁻¹ allows us to unambiguously identify the major contribution of octahedrally coordinated Cr³⁺ ions in the optical spectrum. The X- and Q-band EPR spectra show two superposed Cr³⁺ signals. The corresponding fine-structure parameters were determined at room temperature and 145 K. These results suggest the substitution of Cr³⁺ for Al³⁺ in equal proportions in the two unequivalent octahedral sites of the dickite structure. In kaolin group minerals, the distortion around Cr³⁺ ions (λ≈ 0.2–0.4) in Al sites is significantly less rhombic than that observed around Fe³⁺ ions (λ≈ 0.6–0.8). Received: 29 June 2001 / Accepted: 22 October 2001  相似文献   

Low-temperature thermodynamic properties of disordered zeolites of the natrolite group     

I. E. Paukov  N. K. Moroz  Yu. A. Kovalevskaya  I. A. Belitsky 《Physics and Chemistry of Minerals》2002,29(4):300-306

 The heat capacity of paranatrolite and tetranatrolite with a disordered distribution of Al and Si atoms has been measured in the temperature range of 6–309 K using the adiabatic calorimetry technique. The composition of the samples is represented with the formula (Na_1.90K_0.22Ca_0.06)[Al_2.24Si_2.76O₁₀]·nH₂O, where n=3.10 for paranatrolite and n=2.31 for tetranatrolite. For both zeolites, thermodynamic functions (vibrational entropy, enthalpy, and free energy function) have been calculated. At T=298.15 K, the values of the heat capacity and entropy are 425.1 ± 0.8 and 419.1 ±0.8 J K⁻¹ mol⁻¹ for paranatrolite and 381.0 ± 0.7 and 383.2 ± 0.7 J K⁻¹ mol⁻¹ for tetranatrolite. Thermodynamic functions for tetranatrolite and paranatrolite with compositions corrected for the amount of extraframework cations and water molecules have also been calculated. The calculation for tetranatrolite with two water molecules and two extraframework cations per formula yields: C _p (298.15)=359.1 J K⁻¹ mol⁻¹, S(298.15) −S(0)=362.8 J K⁻¹ mol⁻¹. Comparing these values with the literature data for the (Al,Si)-ordered natrolite, we can conclude that the order in tetrahedral atoms does not affect the heat capacity. The analysis of derivatives dC/dT for natrolite, paranatrolite, and tetranatrolite has indicated that the water- cations subsystem within the highly hydrated zeolite may become unstable at temperatures above 200 K. Received: 30 July 2001 / Accepted: 15 November 2001  相似文献   

Electrons in feldspar II: a consideration of the influence of conduction band-tail states on luminescence processes     

N. R. J. Poolton  K. B. Ozanyan  J. Wallinga  A. S. Murray  L. Bøtter-Jensen 《Physics and Chemistry of Minerals》2002,29(3):217-225

 Most natural feldspars contain many charged impurities, and display a range of bond angles, distributed about the ideal. These effects can lead to complications in the structure of the conduction band, giving rise to a tail of energy states (below the high-mobility conduction band) through which electrons can travel, but with reduced mobility: transport through these states is expected to be thermally activated. The purpose of this article is twofold. Firstly, we consider what kind of lattice perturbations could give rise to both localized and extended conduction band-tail states. Secondly, we consider what influence the band tails have on the luminescence properties of feldspar, where electrons travel through the sample prior to recombination. The work highlights the dominant role that 0.04–0.05-eV phonons play in both the luminescence excitation and emission processes of these materials. It also has relevance in the dating of feldspar sediments at elevated temperatures. Received: 11 May 2001 / Accepted: 6 September 2001  相似文献   

Fluid-bearing alkaline carbonate melts as the medium for the formation of diamonds in the Earth''s mantle: an experimental study     

N. Pal''yanov  A. G. Sokol  M. Borzdov  A. F. Khokhryakov 《Lithos》2002,60(3-4):145-159

We have experimentally studied the formation of diamonds in alkaline carbonate–carbon and carbonate–fluid–carbon systems at 5.7–7.0 GPa and 1150–1700 °C, using a split-sphere multi-anvil apparatus (BARS). The starting carbonate and fluid-generating materials were placed into Pt and Au ampoules. The main specific feature of the studied systems is a long period of induction, which precedes the nucleation and growth of diamonds. The period of induction considerably increases with decreasing P and T, but decreases when adding a C–O–H fluid to the system. In the range of P and T corresponding to the formation of diamonds in nature, this period lasts for tens of hours. The reactivity of the studied systems with respect to the diamond nucleation and growth decreases in this sequence: Na₂CO₃–H₂C₂O₄·2H₂O–C>K₂CO₃–H₂C₂O₄·2H₂O–C>>Na₂CO₃–C>K₂CO₃–C. The diamond morphology is independent of P and T, and is mainly governed by the composition of the crystallization medium. The stable growth form is a cubo-octahedron in the Na₂CO₃ melt, and an octahedron in the K₂CO₃ melt. Regardless of the composition of the carbonate melt, only octahedral diamond crystals formed in the presence of the C–O–H fluid. The growth rates of diamond varied in the range from 1.7 μm/h at 1420 °C to 0.1–0.01 μm/h at 1150 °C, and were used to estimate, for the first time, the possible duration of the crystallization of natural diamonds. From the analysis of the experimental results and the petrological evidence for the formation of diamonds in nature, we suggest that fluid-bearing alkaline carbonate melts are, most likely, the medium for the nucleation and growth of diamonds in the Earth's upper mantle.  相似文献   

Environmental response to a large, explosive rhyolite eruption: sedimentology of post-1.8 ka pumice-rich Taupo volcaniclastics in the Hawke''s Bay region, New Zealand     

B. Segschneider  C. A. Landis  V. Manville  J. D. L. White  C. J. N. Wilson 《Sedimentary Geology》2002,150(3-4):275-299

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The role of an H<Subscript>2</Subscript>O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region,N California   总被引：2，自引：1，他引：2  

T. Grove  S. Parman  S. Bowring  R. Price  M. Baker 《Contributions to Mineralogy and Petrology》2002,142(4):375-396

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Confinement Effects and Energy Balance Analyses for Buckling Failure Under Eccentric Loading Conditions     

V. N. Kazakidis 《Rock Mechanics and Rock Engineering》2002,35(2):115-126

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Intercomparison of IRS-P4-MSMR derived geophysical products with DMSP-SSM/I and TRMM-TMI finished products     

A. K. Varma  R. M. Gairola  A. K. Mathur  B. S. Gohil  Vijay K. Agarwal 《Journal of Earth System Science》2002,111(3):247-256

In this paper, MSMR geophysical products like Integrated Water Vapour (IWV), Ocean Surface Wind Speed (OWS) and Cloud Liquid Water (CLW) in different grids of 50, 75 and 150 kms are compared with similar products available from other satellites like DMSP-SSM/I and TRMMTMI. MSMR derived IWV, OWS and CLW compare well with SSM/I and TMI finished products. Comparison of MSMR derived CLW with that derived from TMI and SSM/I is relatively in less agreement. This is possibly due to the use of 37 GHz in SSM/I and TMI that is highly sensitive to CLW, while 37 GHz channels are not available on MSMR. Monthly comparison of MSMR geophysical products with those from TMI is all carried out for climatological purpose. The monthly comparisons were much better compared to instantaneous comparisons. In this paper, details of the data analysis and comparison results are presented. The usefulness of the MSMR vis-à-vis other sensors is also discussed.  相似文献   

Fluid-mobile trace element constraints on the role of slab melting and implications for Archaean crustal growth models   总被引：11，自引：0，他引：11  

Balz S. Kamber  Anthony Ewart  Kenneth D. Collerson  Michael C. Bruce  Graeme D. McDonald 《Contributions to Mineralogy and Petrology》2002,144(1):38-56

We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluid-mobile trace element content, implying that they did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature.  相似文献