Friedemann Freund

Time-resolved study of charge generation and propagation in igneous rocks

Electrical resistivity changes, ground potentials, electromagnetic (EM), and luminous signals preceding or accompanying earthquakes have been reported many times, in addition to ground uplift and tilt and other parameters. However, no concept exists that would tie these diverse phenomena together into a physically coherent model. Using low-to medium-velocity impacts to measure electrical signals with microsecond time resolution, it is observed that when 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 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. The positive holes propagate as charge clouds rather than as classical EM waves. Before activation, they lay dormant in 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 in the crust by microfractures during the dilatancy phase. Depending on where in the stressed rock volume the charge carriers are activated, they will form rapidly moving or fluctuating charge clouds that can 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.

Charge generation and propagation in igneous rocks

Abstract 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 O 2− 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, O 3 X/ OO \XO 3 , with X=Si 4+ , Al 3+ , etc. PHPs are introduced into the minerals by way of hydroxyl, O 3 X–OH, which all nominally anhydrous minerals incorporate when crystallizing in H 2 O-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.

Ground Water Chemistry Changes before Major Earthquakes and Possible Effects on Animals

Prior to major earthquakes many changes in the environment have been documented. Though often subtle and fleeting, these changes are noticeable at the land surface, in water, in the air, and in the ionosphere. Key to understanding these diverse pre-earthquake phenomena has been the discovery that, when tectonic stresses build up in the Earth’s crust, highly mobile electronic charge carriers are activated. These charge carriers are defect electrons on the oxygen anion sublattice of silicate minerals, known as positive holes, chemically equivalent to O− in a matrix of O2−. They are remarkable inasmuch as they can flow out of the stressed rock volume and spread into the surrounding unstressed rocks. Travelling fast and far the positive holes cause a range of follow-on reactions when they arrive at the Earth’s surface, where they cause air ionization, injecting massive amounts of primarily positive air ions into the lower atmosphere. When they arrive at the rock-water interface, they act as •O radicals, oxidizing water to hydrogen peroxide. Other reactions at the rock-water interface include the oxidation or partial oxidation of dissolved organic compounds, leading to changes of their fluorescence spectra. Some compounds thus formed may be irritants or toxins to certain species of animals. Common toads, Bufo bufo, were observed to exhibit a highly unusual behavior prior to a M6.3 earthquake that hit L’Aquila, Italy, on April 06, 2009: a few days before the seismic event the toads suddenly disappeared from their breeding site in a small lake about 75 km from the epicenter and did not return until after the aftershock series. In this paper we discuss potential changes in groundwater chemistry prior to seismic events and their possible effects on animals.

Critical review of electrical conductivity measurements and charge distribution analysis of magnesium oxide

The electrical conductivity σ of MgO single crystals shows a sharp increase at 500–800°C, in particular of σsurface, generally attributed to surface contamination. Charge Distribution Analysis (CDA), a new technique providing information on fundamental properties that was previously unavailable, allows for the determination of surface charges, their sign and associated internal electric field. Data on 99.99% purity, arc-fusion grown MgO crystals show that mobile charge carriers start to appear in the bulk of the MgO crystals between 200 and 400°C when σ (measured by conventional techniques) is in the 10−14 to 10−16 Ω−1cm−1 range. Above 500°C, as σ increases to 10−6 to 10−7 Ω−1cm−1, more charges appear giving rise to a strong positive surface charge supported by a strong internal field. This indicates that charges are generated in the bulk and diffuse to the surface by an internally controlled process. On the basis of their positive sign they are identified as holes (defect electrons). Because of the low cation content of these very pure MgO crystals, these holes cannot be associated with transition metal impurities. Instead, they are associated with the O2- sublattice, e.g. consist of O− states or positive holes. This conclusion is supported by magnetic susceptibility data showing the appearance of 1000 ± 500 ppm paramagnetic species between 200–500°C. The magnetic data are consistent with strongly coupled, diamagnetic O− pairs below 200–500°C, chemically equivalent to peroxy anions, O2−2 and probably associated with cation vacancies in the MgO matrix. The formation of O2−2 in arc-fusion grown MgO crystals is very unexpected because of the highly reducing growth conditions. Their presence implies an internal redox reaction involving dissolved “water” by which OH− pairs convert to O2−2 plus H2 molecules. This redox conversion is supported by mass spectroscopic measurements of the H2 release from highly OH−- doped, finely divided MgO and by wet-chemical analysis of its oxidant concentration.

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

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.

Hydrogen in rocks: an energy source for deep microbial communities

To survive in deep subsurface environments, lithotrophic microbial communities require a sustainable energy source such as hydrogen. Though H2 can be produced when water reacts with fresh mineral surfaces and oxidizes ferrous iron, this reaction is unreliable since it depends upon the exposure of fresh rock surfaces via the episodic opening of cracks and fissures. A more reliable and potentially more voluminous H2 source exists in nominally anhydrous minerals of igneous and metamorphic rocks. Our experimental results indicate that H2 molecules can be derived from small amounts of H2O dissolved in minerals in the form of hydroxyl, OH- or O3Si-OH, whenever such minerals crystallized in an H2O-laden environment. Two types of experiments were conducted. Single crystal fracture experiments indicated that hydroxyl pairs undergo an in situ redox conversion to H2 molecules plus peroxy links, O3Si/OO\SiO3. While the peroxy links become part of the mineral structure, the H2 molecules diffused out of the freshly fractured mineral surfaces. If such a mechanism occurred in natural settings, the entire rock column would become a volume source of H2. Crushing experiments to facilitate the outdiffusion of H2 were conducted with common crustal igneous rocks such as granite, andesite, and labradorite. At least 70 nmol of H2/g diffused out of coarsely crushed andesite, equivalent at standard pressure and temperature to 5,000 cm3 of H2/m3 of rock. In the water-saturated, biologically relevant upper portion of the rock column, the diffusion of H2 out of the minerals will be buffered by H2 saturation of the intergranular water film.

Solute carbon and carbon segregation in magnesium oxide single crystals — a secondary ion mass spectrometry study

If carbon is to be analyzed by secondary ion mass spectroscopy (SIMS) in an oxide such as MgO, one has to know how the carbon is incorporated in the oxide host structure, before a successful experiment can be planned. If the carbon impurities derive from dissolved CO2 component which form a solid solution while the crystal grew from a melt in equilibrium with CO2, upon cooling, the solid solution becomes supersaturated with respect to the volatile CO2 component. This creates a thermodynamic driving force for exsolution leading to carbon segregation towards the surface. At the surface rapid degassing occurs in vacuum, enhanced by ion bombardment and electron irradiation. Using freshly cleaved synthetic MgO single crystals it can be shown by SIMS (i) that contamination during short exposure to air and during evacuation remains slight, (ii) that rapid surface/subsurface segregation of solute carbon seems to compete with rapid degassing so that, while no extended segregation profile builds up, the carbon concentration in the bulk beneath the surface decreases to a constant level, (iii) that electron irradiation speeds up degassing, (iv) that heating speeds up carbon diffusion, hence its segregation from the bulk, and (v) that Ar+ ion sputtering for the purpose of removing possible contaminants reduces the driving force for carbon surface segregation to the point that no segregation profile can be observed. By placing freshly cleaved MgO crystals under isotopically 99 percent pure 13CO2 for various periods of time subsequent SIMS analysis reveals extended 12C profiles, probably about 1 μm wide, which can only have formed by 12C segregation from the bulk. These results confirm earlier reports that solute carbon exists as mobile impurity in synthetic MgO and natural olivine, probably due to dissolved CO2 component.

Positive holes in magnesium oxide - Correlation between magnetic, electric, and dielectric anomalies

Magnetic susceptibility measurements of high purity MgO single crystals (<50-wt. ppm transition metals) by means of a vibrating-sample magnetometer shows an anomaly at 800 K. At the same temperature the electric conductivity increases anomalously, the static dielectric constant epsilon increases from 9 to approximately 150, a pronounced positive surface charge appears, and Fe2+ in the MgO matrix oxidizes to Fe3+. The data are consistent with O2(2-) (peroxy) defects, representing self-trapped, spin-paired positive holes at Mg2+ vacancy sites. Diamagnetic at low temperatures, the holes start to decouple their spins > 600 K, probably forming at first V0 centers (two O- at an Mg2+ vacancy), then V- centers (single O- at an Mg2+ vacancy), and releasing mobile O- states. These O- represent itinerant charge carriers on acceptor levels near the O 2p-dominated valence band and conduct by O- /O2- valency fluctuations. The O- concentration is of the order of 8 X 10(19) cm-3.

Hydrogen and carbon in solid solution in oxides and silicates

The dissolution of H2O and CO2 in structurally dense, nominally anhydrous and non-carbonate oxide matrices such as MgO and CaO is reviewed. H2O and CO2 are treated as gaseous oxide components which enter into solid solution with the refractory oxide hosts. They form anion complexes associated with cation vacancy sites. Evidence is presented that OH− pairs which derive from the dissolution of H2O are subject to a charge transfer (CT) conversion into peroxy moieties and molecular hydrogen, O22−... H2. Because the O22−moiety is small (O−-O− distance ≈ 1.5 Å) high pressure probably favors the CT conversion. Mass spectroscopic studies show that molecular H2 may be lost from the solid which retains excess oxygen in the form of O22−, leading to the release of atomic O. The dissociation of O22−moieties into a vacancy-bound O− state and an unbound O− state can be followed by measuring the internal redox reactions involving transition metal impurities, the transient paramagnetism of the O− and their effect on the d.c. conductivity. Evidence is presented that CO2 molecules dissolve dissociatively in the structurally dense oxide matrix, as if they were first to dissociate into CO+O and then to form separate solute moieties CO22−and O22−, both associated with cation vacancy sites. In the CO22−moiety (C-O− distance 1.2–1.3 Å, OCO angle ≈ 130°) the C atom probably sits off center. The transition of the C atom into interstitial sites is accompanied by dissociation of the CO22−moiety into CO− and O−. This transition can be followed by infrared spectroscopy, using OH− as local probes. Further support derives from magnetic susceptibility, thermal expansion, low frequency dielectric loss and low temperature deformation measurements. The recently observed emission of O and Mg atoms besides a variety of molecules such as CO, CO2, CH4, HCN and other hydrocarbons during impact fracture of MgO single crystals is presented and discussed in the light of the other experimental data.

Conversion of dissolved “water” into molecular hydrogen and peroxy linkages

Abstract Peroxy linkages in silica glass represent particular structural and electronic defects which can probably be introduced either by dissolving excess oxygen (interstitial oxygen) or by charge transfer (CT) conversion of SiOH pairs into SiOOSi + H 2 , followed by outdiffusion of the H 2 . The latter step is reversible. Studies on crystalline oxides and silicates have helped to understand the CT reaction. They indicate that the peroxy entities may decay thermally by releasing mobile positive holes which, migrating via the oxygen sublattice, become trapped at the surface and cause release of oxygen. Thus the surface becomes positively charged. This in turn confines further holes to the bulk. Alternatively and maybe preferentially in silica, the peroxy linkage may decay by the diffusion of an oxygen atom without generation of a mobile charge carrier.