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941.
Mark J. Nilges Yuanming Pan Rudolf I. Mashkovtsev 《Physics and Chemistry of Minerals》2009,36(2):61-73
The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative
analysis of a 29Si hyperfine structure (A ~12.6 MHz) and an 27Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The 29Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique
A axis close to a Si–O bond direction. The 27Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra,
is characterized by the unique A axis approximately along a twofold symmetry axis. These 29Si and 27Al hyperfine data, together with published theoretical results on peroxy radicals in SiO2 as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to
have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al3+ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al3+ ion corresponds closely to the diamagnetic precursor to the [AlO4]0 center.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
942.
S. Lafuerza N. Sultan M. Canals J. Frigola S. Berné G. Jouet M. Galavazi F. J. Sierro 《International Journal of Earth Sciences》2009,98(4):751-768
Data from in situ piezocone tests (CPTU) and laboratory analyses are utilized for the interpretation of the stress history
of Quaternary sedimentary sequences in the upper continental slope of the Gulf of Lion, northwestern Mediterranean Sea. A
CPTU based preconsolidation pressure profile referenced to the current effective stress indicates that the deposit is underconsolidated
from 12 meters below the seafloor (mbsf) down to at least 150 mbsf. Excess pore pressure below 12 mbsf is further supported
by results from oedometer and dissipation tests. Subseafloor pockmarks and indications of free gas in seismic reflection profiles
reveal four main overpressure sources (SI–SIV) with overpressure ratios >0.3 at subseafloor depths coinciding with levels
where the dominantly silty-clayey sediment contains increased proportions of sand. We relate the excess pore pressure related
to free gas due to gas exsolution processes and sea level variations driven by Pleistocene sea level changes. 相似文献
943.
A. M. Petrishchevsky 《Doklady Earth Sciences》2009,426(1):636-641
Based on rheological interpretation of formalized gravity models, earlier known deep-seated structures in the Earth’s crust and mantle of Transbaikalia have been detailed and new ones discovered. The structures are asymmetric and transverse relative to the Baikal rift zone. Their presence explains the peculiar features of the Baikal rift, including the one-way southeasterly direction of horizontal displacement of tectonic masses and northwestern migration of the Earth’s crust extension processes. The prolonged history (more than 250 Ma) of the Baikal rift zone and Transbaikalia mountainous country involved gravity or rotational detachments of rigid tectonic slabs from the craton and their sliding along intracrustal and subcrustal decollement zones into the above-dome area of the Transbaikalia asthenolith. 相似文献
944.
In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite
layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates.
P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures
did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite.
Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion
they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical
proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion
they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume
compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures. 相似文献
945.
Crack damage stress as a composite function of porosity and elastic matrix stiffness in dolomites and limestones 总被引:4,自引:0,他引:4
Twenty-five uniaxial compression tests were performed to determine stress at onset of dilation, referred to herein as “the crack damage stress,” in heterogeneous dolomites and limestones. A simplified model for crack damage stress (σcd) is developed here using porosity, elastic modulus, Poisson's ratio and three empirical coefficients. The model shows that when porosity decreases and elastic modulus increases, σcd rapidly increases and approaches its maximum value. On the other hand, when porosity increases and elastic modulus decreases, σcd rapidly decreases and approaches its minimum value. The proposed model is validated for six heterogeneous limestone and dolomite formations which are widely distributed in Israel. 相似文献
946.
Summary Lovozero, the largest of the world’s layered peralkaline intrusions, includes gigantic deposits of Nb + REE-loparite ore.
Loparite, (Na,Ce,Ca)2(Ti,Nb)2O6, became a cumulus phase after crystallisation of about 35% of the ‘Differentiated Complex’, and its compositional evolution
has been investigated through a 2.35 km section of the intrusion. The composition of the cumulus loparite changes systematically
upwards through the intrusion with an increase in Na, Sr, Nb and Th and decrease in REE and Ti. This main trend of loparite
evolution records differentiation of the peralkaline magma through crystallisation of 1600 m of the intrusion. The formation
of the loparite ores was the result of several factors including the chemical evolution of the highly alkaline magma and mechanical
accumulation of loparite at the base of a convecting unit. At later stages of evolution, when concentrations of alkalis and
volatiles reached very high levels, loparite reacted with the residual melt to form a variety of minerals including barytolamprophyllite,
lomonosovite, steenstrupine-(Ce), vuonnemite, nordite, nenadkevichite, REE, Sr-rich apatite, vitusite-(Ce), mosandrite, monazite-(Ce),
cerite and Ba, Si-rich belovite. The absence of loparite ore in the “Eudialyte complex” is likely to be a result of the wide
crystallisation field of lamprophyllite, which here became a cumulus phase.
Received November 6, 2000; revised version accepted January 18, 2001 相似文献
947.
948.
949.
Refined analytical expressions for the frequencies corresponding to the Chandler motion of the pole and the diurnal rotation of the deformable Earth are derived. Numerical estimates of the period and amplitude of the polar oscillations are presented. The trajectory of the Chandler polar motion derived via numerical modeling is in qualitative and quantitative agreement with experimental data from the International Earth Rotation Service (IERS). An evolutionary model describing slow variations in the Earth’s rotation parameters under the action of the dissipative moments of the tidal gravitational forces on time scales considerably longer than the precession period of the Earth’s axis is constructed. The axis of the Earth’s figure tends to approach the angular momentum vector of the proper rotation. 相似文献
950.