A new solid state diffusion model applied to inverse zoning and diffusion rims in minerals     

Friedemann Freund  Bruce V. King 《Physics and Chemistry of Minerals》1984,11(3):113-124

Any oxide and silicate mineral which is nominally anhydrous but crystallized in the presence of H₂O incorporates traces of H₂O in solid solution. In the case of MgO it can be shown that OH^? pairs convert into H₂+O ₂ ^2? . If the H₂ molecules are lost, the O ₂ ^2? remain in the lattice as excess oxygen stabilized by excess cation vacancies. When the O ₂ ^2? anions decay either thermally or by decompression unbound O^? states (positive holes) are generated which lead to surface charges and subsurface space charge layers. Calculated space charge profiles are presented. O^? concentrations as small as 10–20 ppm suffice to create electric surface fields of the order of 4·10⁷ V·m^?1. The diffusion mechanism which derives from these premises incorporates novel features: the cation diffusion is coupled to the counterdiffusion of unbound and vacancy-bound O^? states. The cation diffusion is predicted to be very fast because first, it is field-enhanced (electrochemically driven) and second, it is not rate-limited by the intrinsic cation vacancy concentration nor by the counter-diffusion of other cations. The model may apply to cases of inverse zoning and diffusion rim formation in minerals under certain P-T conditions.  相似文献   

A deuterium-hydrogen fractionation mechanism in magnesium oxide     

Friedemann Freund  Heinz Wengeler  Reinhard Martens 《Geochimica et cosmochimica acta》1982,46(10):1821-1829

Water dissolved in a nominally anhydrous oxide like MgO forms not only OH^? ions, as is usually assumed, but also molecular H₂ and peroxy anions O₂^2?. The specific lattice site where this reaction occurs has been identified for the model-type MgO structure: it is the fully OH^? compensated cation vacancy. In partially deuterated synthetic MgO, molecular D₂ forms preferentially to H₂, by a factor of the order of 4. Three methods have been used to study this very large D/H fractionation experimentally: mass spectrometry, electron spin resonance and infrared spectroscopy. All give consistent results and confirm that internal D/H fractionation occurs and is a function of the temperature. Theoretically the D/H fractionation is explained by the lower zeropoint energy of D₂ with respect to H₂, amplified by secondary processes through which the molecular hydrogen species become mobilized and eventually lost via diffusion. The possible consequences of such internal D/H fractionation processes are indicated for the interpretation of water and hydrogen release patterns, with varying D/H ratios, from terrestrial and extraterrestrial samples.  相似文献   

Excitation of whistler waves by reflected auroral electrons     

C.S. Wu  D. Dillenburg  L.F. Ziebell  H.P. Freund 《Planetary and Space Science》1983,31(5):499-507

Excitation of electron cyclotron waves and whistlers by reflected auroral electrons which possess a loss-cone distribution is investigated. Based on a given magnetic field and density model, the instability problem is studied over a broad region along the auroral field lines. This region covers altitudes ranging from one quarter of an Earth radius to five Earth radii. It is found that the growth rate is significant only in the region of low altitude, say below the source region of the auroral kilometric radiation. In the high altitude region the instability is insignificant either because of low refractive indices or because of small loss cone angles.  相似文献   

Earthquake forewarning — A multidisciplinary challenge from the ground up to space     

Friedemann Freund 《Acta Geophysica》2013,61(4):775-807

Most destructive earthquakes nucleate at between 5–7 km and about 35–40 km depth. Before earthquakes, rocks are subjected to increasing stress. Not every stress increase leads to rupture. To understand pre-earthquake phenomena we note that igneous and high-grade metamorphic rocks contain defects which, upon stressing, release defect electrons in the oxygen anion sublattice, known as positive holes. These charge carriers are highly mobile, able to flow out of stressed rocks into surrounding unstressed rocks. They form electric currents, which emit electromagnetic radiation, sometimes in pulses, sometimes sustained. The arrival of positive holes at the ground-air interface can lead to air ionization, often exclusively positive. Ionized air rising upward can lead to cloud condensation. The upward flow of positive ions can lead to instabilities in the mesosphere, to mesospheric lightning, to changes in the Total Electron Content (TEC) at the lower edge of the ionosphere, and electric field turbulences. Advances in deciphering the earthquake process can only be achieved in a broadly multidisciplinary spirit.  相似文献   

Solar Eclipses and the Surface Properties of Water     

Fuchs  Elmar C.  Oudakker  Gerrit  Justinek  Martin  Dyer  Nigel  Woisetschläger  Jakob  Godines  Kevin  Mäder  Matthias  Freund  Friedemann T. 《Earth, Moon, and Planets》2019,123(1-2):15-43

Earth, Moon, and Planets - During four solar eclipse events (two annular, one total and one partial) a correlation was observed between a change in water surface tension and the magnitude of the...  相似文献   

Hydrostatic pressure tests for the permeability - formation factor relation on crystalline rocks from the KTB drilling project     

Dieter Freund  Georg Nover 《Surveys in Geophysics》1995,16(1):47-62

Transport properties (permeability and electrical conductivity) have been measured at different hydrostatic pressure runs on 7 crystalline rocks (gneisses and amphibolites) sampled from the KTB drilling project. The decrease of permeability by pressure are compared with the pressure-dependent data of the electrical conductivity (formation factor) resulting from complex impedance measurements. According to the equivalent-channel model (ECM), there exists a linear relationship between these parameters by representing both properties on logarithmic scales. The results show that it is possible to extrapolate high-pressure permeability from low-pressure (< 60 MPa) permeability data by using the pressure-dependent electrical conductivity (up to 300 MPa).  相似文献   

Mechanism of the water and carbon dioxide solubility in oxides and silicates and the role of O−     

Friedemann Freund 《Contributions to Mineralogy and Petrology》1981,76(4):474-482

The dissolution of water does not stop at the OH^– stage but may proceed further towards H₂ plus O^– formation. The discovery of atomic carbon dissolved in minerals suggests that, if CO₂ enters oxides and silicates at high pressures and temperatures, not only [CO₃]^2– ions but also [CO ₄ ^. ]^4– complexes are formed via a charge transfer which produces O^– and essentially zero-valent, atomic carbon. Under P —T-conditions of the mantle, where the solubility for water and CO₂ is high, the silicate phases formed may therefore consist to a large volume fraction of O^– ions which are much smaller than O^2–ions and strongly cova-lently bonding. The implications for the crystal chemistry of high pressure phases, for the petrology of mantle rocks are outlined.  相似文献   

Carbon in solid solution in forsterite—a key to the untractable nature of reduced carbon in terrestrial and cosmogenic rocks     

F. Freund  H. Kathrein  H. Wengeler  R. Knobel  H.J. Reinen 《Geochimica et cosmochimica acta》1980,44(9):1319-1333

Carbon is an incompatible element in oxide and silicate lattices. Until now it has been believed to exist only in the form of CO^2?₃ ions, molecularily dissolved CO₂ or graphitic inclusions. Recently it has been shown that carbon can dissolve in refractory oxides, like MgO and CaO, in the form of carbon atoms.The experimental results obtained with carbonaceous MgO are reviewed and new results are presented which demonstrate that synthetic forsterite and natural olivines can also take up atomic carbon in solid solution. The incorporation of the carbon atoms is treated thermodynamically. Near the melting point they probably occupy cation vacancies, but with decreasing temperature they are progressively transferred on interstitial sites. On these sites they are very mobile and tend to segregate into the elastically relaxed subsurface zone, but exsolution to graphite is prevented by the strain fields surrounding each carbon atom.Upon heating, however, the atomic carbon may react with lattice oxygen to give CO₂ and with co-dissolved hydrogen to give a wide variety of hydrocarbons. The underlying reaction mechanisms, involving the formation and decay of O^? ions, are discussed in view of the so-called ‘carbonatic carbon’ and ‘reduced carbon’ in magmatic minerals and meteorites, in view of the diamond genesis and also in view of the reversible CO₂ solubility in silicate magmas at high pressures and temperatures.  相似文献   

Charge generation and propagation in igneous rocks   总被引：5，自引：0，他引：5  

Friedemann Freund   《Journal of Geodynamics》2002,33(4-5)

Various electrical phenomena have been reported prior to or concurrent with earthquakes such as resistivity changes, ground potentials, electromagnetic (EM), and luminous signals. Doubts have been raised as to whether some of these phenomena are real and indeed precursory. One of the reasons for uncertainty is that, despite decades of intense work, there is still no physically coherent model. Using low- to medium-velocity impacts to measure electrical signals with microsecond time resolution, it has now been observed that when dry gabbro and diorite cores are impacted at relatively low velocities, 100 m/s, highly mobile charge carriers are generated in a small volume near the impact point. They spread through the rocks, causing electric potentials exceeding +400 mV, EM, and light emission. As the charge cloud spreads, the rock becomes momentarily conductive. When a dry granite block is impacted at higher velocity, 1.5 km/s, the propagation of the P and S waves is registered through the transient piezoelectric response of quartz. After the sound waves have passed, the surface of the granite block becomes positively charged, suggesting the same charge carriers as observed during the low-velocity impact experiments, expanding from within the bulk. During the next 2–3 ms the surface potential oscillates, indicating pulses of electrons injected from ground and contact electrodes. The observations are consistent with positive holes, e.g. defect electrons in the O²⁻ sublattice, traveling via the O 2p-dominated valence band of the silicate minerals. Before activation, the positive holes lay dormant in the form of electrically inactive positive hole pairs (PHP), chemically equivalent to peroxy links, O₃X/^OO\XO₃, with X=Si⁴⁺, Al³⁺, etc. PHPs are introduced into the minerals by way of hydroxyl, O₃X–OH, which all nominally anhydrous minerals incorporate when crystallizing in H₂O-laden environments. The fact that positive holes can be activated by low-energy impacts, and their attendant sound waves, suggests that they can also be activated by microfracturing. Depending on where in the stressed rock volume the charge carriers are activated, they will form rapidly moving or fluctuating charge clouds that may account for earthquake-related electrical signals and EM emission. Wherever such charge clouds intersect the surface, high fields are expected, causing electric discharges and earthquake lights.  相似文献   

Methane in the southern North Sea: Sources, spatial distribution and budgets   总被引：3，自引：0，他引：3  

Maik Grunwald  Olaf Dellwig  Melanie Beck  Joachim W. Dippner  Jan A. Freund  Cora Kohlmeier  Bernhard Schnetger  Hans-Jürgen Brumsack 《Estuarine, Coastal and Shelf Science》2009,81(4):445-456

Measurements of methane (CH₄) so far have always shown supersaturation in the entire North Sea relative to the atmospheric partial pressure and the distribution of surface CH₄ reveals a distinct increase towards the shore. Since North Sea sediments presumably are an insignificant source for CH₄ the coastal contribution via rivers and tidal flats gains in importance.In this work, CH₄ data from the River Weser, the back barrier tidal flats of Spiekeroog Island (NW Germany), and the German Bight are presented. Results from the River Weser are compared to other rivers draining into the German Bight. Measurements in the tidal flat area of Spiekeroog Island highlight this ecosystem as an additional contributor to the overall CH₄ budget of the southern North Sea. A tidally driven CH₄ pattern is observed for the water column with maximum values during low tide. Tidal flat sediments turn out to be the dominating source because pore waters discharged during low tide are highly enriched in CH₄. In contrast, the freshwater contribution to the tidal flats by small coastal tributaries has almost no impact on water column CH₄ concentrations. The CH₄ level seems to be disturbed irregularly by wind forcing due to elevated degassing and prevention of advective flow when tidal flats remain covered by water.Based on our data, two model calculations were used to estimate the impact of tidal flats on the CH₄ budget in the German Bight. Our results demonstrate that the back barrier tidal flats of the east Frisian Wadden Sea contribute CH₄ in an order of magnitude between the Wash estuary and River Elbe and thus have to be considered in budget calculations.  相似文献