Simple covalent potential models of tetrahedral SiO2: Applications to α-quartz and coesite at pressure

We present a covalent potential model of tetrahedrally coordinated SiO₂. The interactions include covalent effects in the form of a Si-O bond-stretching potential, O-Si-O and Si-O-Si angle-bending potentials, and oxygen-oxygen repulsion. Calculated equations of state of α-quartz and coesite agree well with experiment (calculated densities within 1 percent of experiment up to 6 GPa). The calculated α-quartz-coesite transition pressure agrees with the experimental value of ≈2 GPa. Furthermore, the compression mechanisms predicted by the model (i.e. pressure induced changes in Si-O bond lengths and O-Si-O and Si-O-Si angles) are accurate.     

The dynamical evolution of massive black hole binaries - II. Self-consistent N-body integrations

We use a hybrid N-body program to study the evolution of massive black hole binaries in the centers of galaxies, mainly to understand the factors affecting the binary eccentricity, the response of the galaxy to the binary merger, and the effect of loss-cone depletion on the merger time. The scattering experiments from paper I (Quinlan, 1996)[NewA, 1, 35] showed that the merger time is not sensitive to the eccentricity growth unless a binary forms with at least a moderate eccentricity. We find here that the eccentricity can become large under some conditions if a binary forms in a galaxy with a flat core or with a radial bias in its velocity distribution, especially if the dynamical friction is enhanced by resonances as suggested by Rauch & Tremaine (1996)[NewA, 1, 149]. But the necessary conditions seem unlikely, and our prediction from paper I remains unchanged: in most cases the eccentricity will start and remain small. The ejection of stars caused by the hardening of a binary may explain why large elliptical galaxies have weaker density cusps than smaller galaxies. If so, the central velocity distributions in those galaxies should have strong tangential anisotropies. The wandering of a binary from the center of a galaxy counteracts the effects of loss-cone depletion and helps the binary merge.     

Validation of a region-wide model of landslide susceptibility in the Manawatu–Wanganui region of New Zealand

Since European settlement 160 years ago, much of the indigenous forest in New Zealand hill country has been cleared for pastoral agriculture, resulting in increased erosion and sedimentation. To prioritise soil conservation work in the Manawatu–Wanganui region, we developed a model of landslide susceptibility. It assigns high susceptibility to steep land not protected by woody vegetation and low susceptibility everywhere else, following the commonly used approach for identifying inappropriate land use. A major storm on 15–16 February 2004 that produced many landslides was used to validate the model. The model predicted hills at risk to landsliding with moderate accuracy: 58% of erosion scars in the February storm occurred on hillsides considered to be susceptible. The model concept of slope thresholds, above which the probability of landsliding is high and below which the probability is low, is not adequate because below 30° the probability of landsliding is approximately linearly related to slope. Thus, reforestation of steep slopes will need to be combined with improved vegetation management for soil conservation on moderate slopes to significantly reduce future landsliding.     

Providing the evidence: Geographic accessibility of maternity units in New Zealand

Abstract:  Public health planners should be providing evidence to the public that maternity units are fairly distributed. This research demonstrates how a Geographical Information System (GIS) can be used to provide information on travel time to the closest maternity unit from the 38 000 population census enumeration districts in New Zealand. The distribution of accessibility is mapped and regions and population groups that appear under-serviced are highlighted. We conclude by stating that GIS accessibility models provide important evidence for health policy and that the information generated from these models should be routinely produced for a wide range of health services and communicated to the public.     

Comparison of Different Gold Recovery Methods with Regard to Pollution Control and Efficiency

This case study performed at the largest gold mining village in the Philippines compares four methods for gold recovery: amalgamation, cyanide leaching, a shaking sluice, and Cleangold® sluices. The results show that a combination of manual panning or Cleangold® sluices followed by cyanidation is the best solution under present conditions, followed by cyanidation alone. Based on the knowledge obtained, remarks on suitable policy actions, which may be applied also in other gold fields, are made.     

Groundwater-dependent ecology of the shoreline of the subtropical Lake St Lucia estuary

The ecology of the St Lucia estuary in South Africa is of unique international importance. During droughts the estuary experiences high salinities, with values above that of seawater. Ion-poor groundwater flowing into the estuary from prominent sand aquifers along its eastern shoreline forms low-salinity habitats for salt-sensitive biota. During droughts, plants and animals can take refuge in the groundwater discharge zone until the condition in the estuary regains tolerable salinity. Simulations of the groundwater discharge indicate that the flow can persist during droughts over at least a decade, and be of great important for the resilience of the estuary. Anthropogenic activities have reduced the river inflow and made the St Lucia estuary more sensitive to droughts. The groundwater has thereby become increasingly important for the estuary’s ecology. Protection of the groundwater discharge along the shoreline itself and actions to increase the groundwater recharge are therefore important management tasks.     

Further studies on the Fennoscandian gravity field versus the Moho depth and land uplift

Whether post-glacial rebound or/and crustal variation contributes to the pattern of the Fennoscandian gravity field has been of great interest to geoscientists. Previous numerical studies are based on different Moho maps, different global Earth gravity models and different isostatic models of Pratt type, resulting in quite different conclusions.In this study, we use the improved Moho depth map of Korja et al. (1993), the OSU91A Earth gravity model and a refined modeling of the Moho depth contribution. We conclude that not more than about 40% and 30% of the Fennoscandian geoid and gravity depressions of the orders of -12m and -40 mGal might be caused by crustal thickening, leaving at least -6 m and -28 mGal to be adjusted in accordance with post-glacial rebound.     

An airborne tensor VLF system. From concept to realization1

Radio signals from very low frequency (VLF) transmitters distributed world-wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.     

The application of potentiometric techniques to study complexation reactions at the mineral/water interface

The most widely used technique to characterize the acid/base and complexing properties of hydrous particle surfaces is based on potentiometric titrations. Properly applied it provides accurate data over wide concentration ranges and, with help of automatic measuring and data collecting systems, it has become possible to collect large amounts of data within a reasonably short time period. However, collection and interpretation of experimental data is beset with difficulties. Some of these difficulties are identified and discussed with special reference to some goethite systems.