Computation of surface energies in an electrostatic point charge model: II. Application to zircon (ZrSiO4)

The attachment energies, the slice energies and the specific surface energies can be calculated in an electrostatic point charge model using the formula derived by Madelung for the potential introduced by an infinite row of equally spaced point charges. Power series are given for the Hankel function iH ₍₀₎ ⁽¹⁾ (iy) and (x)=d ln x!/dx. The logarithmic expression in the Madelung formula converges rapidly when applying a power series, which combines equally charged cations and anions. Besides the specific surface energy ( ^hkl), the slice energy (E _s ^hkl ) and the attachment energy (E _a ^hkl ) can be considered as special categories of surface energies as they depend on surface configurations as well. The specific surface energy is the energy per unit area of surface needed to split the crystal parallel to a face (hkl). The attachment energy (E _a) is the energy released per mole, when a new slice of thickness d _hkl crystallizes on an already existing crystal face (hkl). The growth rate of the crystal face (hkl) is a function of its attachment energy. The slice energy (E _s) is the energy released per mole, when a new slice d _hkl is formed from the vapour neglecting the influence of edge energies. The lattice energy (E _c) which is the energy released per mole of a crystal crystallizing from the vapour, is given by the following relation: E _c=E _a+E _s.     

Synthesis and stability of bastnaesites in a part of the system (Ce,La)-F-H-C-O

Summary Bastnaesites of Ce and La and their OH-analogs were synthesized and their stability relations were determined atPf = 1 kbar andT = 400 to 900°C in a part of the system (Ce,La)-F-H-C-0. The initial fluid compositions were such that and HF/(HF + H₂O) ratios were 0 to 0.172. XRD and IR studies indicate that bastnaesites equilibrated in initial fluids low in HF are all F-enriched. The hydroxylbastnaesite-(La) is stable up to 810°C and the fluorbastnaesite-(La) is stable up to 860°C. Their condensed breakdown products are La₂O₂CO₃ and LaOF, respectively. The stability of Ce bastnaesites is slightly dependent. The hydroxylbastnaesite-(Ce) is stable up to 660°C at the defined by the IQF buffer and up to 640°C by the MH buffer. The fluorbastnaesite-(Ce) is stable up to 800°C at the defined by the IQF and up to 760°C by the MH buffer. The condensed breakdown product for the hydroxyl end-member is simply CeO₂ but for the fluorine one is a combination of CeO₂, CeF₃, and CeOF. Factors, such as OH vs F, , and bulk composition, that affect the stability of individual species are discussed. Petrogenic implications resulting from the present study include that bastnaesites can be stable from hydrothermal to magmatic conditions, that F-enriched species can form in an environment relatively low in F content, and that OH-species are rare and occur only in low-temperature environments essentially devoid of F.

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Synthese und Stabilität von Bastndsil in einem Teil des Systems (Ce,La)-F-H-C-O

Zusammenfassung Ce- und La-Bastnäsite, sowie deren OH-Analoga wurden synthetisiert und ihre Stabilitätsbeziehunger beiP _f = 1 kbar undT = 400 bis 900°C wurden im System (Ce,La)F-H-C-O bestimmt. Die anfänglichen Flüssigkeitszusammensetzungen waren so, daß und die HF/(HF + H₂O)-Verhältnisse 0–0.172 waren. Röntgenpulver- und Ultrarot-Untersuchungen zeigten, daß Bastnäsite, die mit anfänglich HF-armen Flüssigkeiten equilibriert wurden, alle an F angereichert sind. Hydroxilbastndsit-(La) ist bis 810°C und Fluorbastnäsit-(La) bis 860°C stabil. Ihre festen Zersetzungsprodukte sind La₂O₂O₃, bzw. LaOF. Die Stabilität der Ce-Bastnäsite hängt etwas von ab. Hydroxilbastnäsit-(Ce) ist bei des Eisen-Quarz-Fayalit-Puffers bis 660°C stabil und mit Magnetit-Hämatit-Puffer bis 640°C. Das feste Zerfallsprodukt ist für das Hydroxil-Glied nur CeO₂, für das Fluor-Glied eine Mischung aus CeO₂, CeF₃ und CeOF. Faktoren, welche die Stabilität der einzelnen Spezies beeinflussen, werden diskutiert, wie das Verhältnis OH zu F, und die Gesamtzusammensetzung. Petrogenetische Folgerungen aus der vorliegenden Studie schließen ein, daß Bastnäsite von hydrothermalen bis zu magmatischen Bedingungen stabil sein können, daß sich an F angereicherte Glieder in relativ F-armer Umgebung bilden können, und daß OH-Glieder selten sind und nur unter Bildungsbedingungen niedriger Temperatur und weitgehender Abwesenheit von F auftreten.

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With 8 Figures     

Fluid-rock interaction in the Mo-bearing Nebelstein greisen complex,Bohemian Massif (Austria)

Summary In the Nebelstein area, molybdenite-bearing greisens occur together with peraluminous leucogranites. In the compositional change of the granites to the greisens, there is an almost complete loss of Na, combined with a decrease in Ca, Mg, Sr, and Ti concentrations. The progressive alteration is reflected by lower homogenization temperatures and increasing salinity in aqueous fluid inclusions. The fluid regime prior to greisenization was water-dominated with low salt contents, while the early stage of the greisen development was characterized by a mixed fluid containing carbon dioxide and water. This was succeeded by a moderate saline aqueous fluid which caused the mineralization by exchange of metal ions for Na⁺(Ca²⁺, K⁺). A negative correlation between salt content in fluid inclusions and Na₂O concentrations in the bulk rocks supports this model. Mass balance calculations for this interaction yield a minimum fluid-rock ratio of approximately 2 : I. Greisenization took place at a minimum pressure of 180 MPa (1.8 kb) and in a temperature range between 200 and < 400 °C.

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Fluid-Gestein-Wechselwirkung in dem Molybdänit führenden Greisenkomplex Nebelstein, Böhmische Masse (Österreich)

Zusammenfassung Die Molybdänglanz führenden Greisengesteine des Nebelsteins sind an peraluminöse Granite gebunden. Bei der Alteration der Granite ist für den Übergang Biotitgranit zu Greisen eine weitestgehende Verarmung an Na zu beobachten, gleichzeitig nehmen auch die Gehalte an Ca, Mg, Sr und Ti ab. Die fortschreitende Greisenbildung dokumentiert sich in den wäßrigen Flüssigkeitseinschlüssen durch steigende Salinität bei sinkenden Homogenisierungstemperaturen. Die fluide Phase war vor der Greisenbildung H₂O dominiert und niedrig salinar. Der Beginn der Alterationsprozesse ist durch CO₂ und H₂O hältige Fluide gekennzeichnet. Danach folgt ein Anstieg der Salinität, der auf den Austausch von Metallchloridlösungen gegen Na⁺, K^- und Ca^2- zurückgeführt wird. Dies läßt sich durch eine negative Korrelation der Salinität in den Flüssigkeitseinschlüssen mit dem Na-Gehalt der Gesteine belegen. Daraus wurde die Volumsbeziehung der den Granit durchströmenden fluiden Phase relativ zum Gestein mit mindestens 2 : 1 abgeleitet. Die Mineralisation fand bei einem Minimaldruck von 1,8 kb in einem Temperaturbereich von 200 - < 400 °C statt.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids and Mineral Deposits, ETH Zürich, March21–23,1991.     

The gawler ranges,South Australia: an unusual volcanic massif

The volcanic residuals of the Gawler Ranges together form an extensive massif that in its gross morphology differs markedly from most exposures of silicic volcanic rocks. The upland developed in two stages, the first involving differential fracture‐controlled subsurface weathering, the second the stripping of the regolith. As a result, an irregular weathering front was exposed, with domical projections prominent. These bornhardts are etch forms, and they are of considerable antiquity.

The differential weathering of the rock mass reflects the exploitation of various fracture systems by shallow groundwaters. Orthogonal fracture systems at various scales, sheet fractures and columnar joints control the morphology of the bornhardts in gross and in detail.

The exploitation of the structural base, which was established in the Middle Protero‐zoic, probably took place throughout the Late Proterozoic and the Palaeozoic, though only minor remnants of the Proterozoic land surface remain. The major landscape features developed during the Mesozoic. The weathering which initiated the bornhardts occurred in the Jurassic or earlier Mesozoic, and the landforms were exposed in Late Cretaceous to Early Tertiary times.

Though structural forms dominate the present landscape, some major and some minor landforms are best explained in terms of climatic changes of the later Cainozoic. The palaeodrainage system, established under humid conditions by the Early Tertiary, was alluviated during the Cainozoic arid phases, and salinas were formed. The sand dunes of the region also reflect this aridity.     

Population and geography in India

The field of population geography was first introduced during the 1960s in India and advanced under the direction of Gosal at the Punjab University. Teaching and research in population geography were introduced by Chandigarh at Punjab University, which today is the main center of research activity. Population geography in India has followed the main tenets of geography in general and is based on spatial perspectives. Deficits are apparent in the paucity of research on socioeconomic implications of spatial distributions, but there is infrastructural feedback to support theory development. Theoretical advances moving from theory to fact or from empirical fact to theory are limited. Comprehensive training in methodology and quantitative techniques is needed for further development of population theory: multivariate analysis, factor analysis, principal component analysis, model building, hypothesis testing, and theory formulation. Methodological sophistication will also help in understanding and interpreting the diverse and complex Indian demographic situation. The analysis of population geography in the Indian spatial, cultural, political, and historical context may be applied to other less developed countries of similar sociocultural background. The Indian Census has contributed over the 100 years of its existence reliable and efficiently produced data on a wide variety of measures at assorted scales down to the village level. Field work among geographers has not achieved a level of development commensurate with population censuses. Recent doctoral research has focused on qualitative studies of local situations. Research topics range from the distribution and structure of population, mortality, fertility, and migration to peripheral issues of social segregation. Popular topics include urbanization, labor force, sex composition, literacy, and population growth. Distribution of population and density studies have amounted to only 2 in 30 years. Population texts are in abundance. The Punjab University is the headquarters of the Association of Population Geographers, which publishes the journal "Population Geography." Many advances are still needed in a timely fashion.     

The Mees CCD imaging spectrograph

The Mees CCD (MCCD) instrument is an imaging spectroscopy device which uses the 25 cm coronagraph telescope and the 3.0 m Coudé spectrograph at Mees Solar Observatory (MSO) on Haleakala, Maui. The instrument works with resolving power up to R 200 000 with significant throughput from 3934 Å (Caii K) to 10 000 Å. A fast guiding active mirror stabilizes the image during observations. A rapidly writing magnetic tape storage system allows observations to be recorded at 256 kbytes s^–1. Currently, the MCCD is used for imaging spectroscopy of solar flares at 6563 Å (H), and velocity measurements of umbral oscillations; future plans include emission line studies of active region coronae, and photospheric studies of solar oscillations.     

A new Bolivia-USSR astronomical observatory and its astrometric activities

In 1982 a Bolivia-USSR astronomical observatory began its activities near Tarija, Bolivia. Observations of 200 000 faint stars, 2822 bright stars (m6) and Halley's comet have been made with the expedition's astrograph.     

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area,Venus: Results from Arecibo image data

New radar images obtained from the Arecibo Observatory (resolution 1.5–4.0 km) for portions of the southern hemisphere of Venus show that: the upland of Phoebe Regio contains the southern extension of Devana Chasma, a rift zone extending 4200 km south from Theia Mons and interpreted as a zone of extension; Alpha Regio, the only large region of tessera within the imaged area, is similar to tessera mapped elsewhere on the planet and covers a smaller percentage of the surface than that observed in the northern high latitudes; the upland made of Ushas, Innini and Hathor Montes consists of three distinct volcanic constructs; Themis Regio is mapped as an ovoid chain of radar-bright arcuate single and double ring structures, edifices and bright lineaments. This area is interpreted as a region of mantle upwelling and on the basis of apparent split and separated features, a zone of localized faulting and extension. Linear zones of deformation in Lavinia Planitia are characterized by lineament belts that are often locally elevated, are similar to ridge belts mapped in the northern high latitudes and are interpreted to be characterized mainly by compression; radar-bright lava complexes within Lavinia Planitia are unique to this part of the planet and are interpreted to represent areas of eruption of high volumes of extremely fluid lava; the upland of Lada Terra is bound to the north by a linear deformation zone interpreted as extensional, is characterized by large ovoids and coronae, is interpreted to be associated with an area of mantle upwelling, and is in contrast to the northern high latitude highland of Ishtar Terra. Regions of plains in the southern hemisphere cover about 78%; of the mapped area and are interpreted to be volcanic in origin. Located within the area imaged (10–78 S) are 52 craters interpreted to be of impact origin ranging from 8 to 157 km in diameter. On the basis of an overall crater density of 0.94 craters/10⁶ km², it is determined that the age of this part of the Venus surface is similar to the 0.3 to 1.0 billion year age calculated for the equatorial region and northern high latitudes. The geologic characteristics of the portion of the Venus southern hemisphere imaged by Arecibo are generally similar to those mapped elsewhere on the planet. This part of the planet is characterized by widespread volcanic plains, large volcanic edifices, and zones of linear belt deformation. The southern hemisphere of Venus differs from northern high latitudes in that tessera makes up only a small percentage of the surface area and the ovoid chain in Themis Regio is unique to this part of the planet. On the basis of the analysis presented here, the southern hemisphere of Venus is interpreted to be characterized by regions of mantle upwelling on a variety of scales (ovoids, region made up of Ushas, Innini and Hathor Montes), upwelling and extension (Themis Regio) and localized compression (lineament belts in Lavinia Planitia).     

Shear-induced instability and arch filament eruption: A magnetohydrodynamic (MHD) numerical simulation

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a Dynamic Shearing Instability, with a growth rate of about 8{ovV} _A a, where {ovV} _A is the average Alfvén speed and a ^–1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfvén speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs).To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.     

Earthquake hazard of the northern parts of the Bohemian Massif and Western Carpathians

The new procedure of earthquake hazard evaluation developed by Kijko and Sellevoll is tested and applied for the border region of Czechoslovakia and Poland. The new method differs from the conventional approach. It incorporates the uncertainty of earthquake magnitudes, and accepts mixed data containing only large historical events and recent, complete catalogues. Seismic hazard has been calculated for nine regions determined in the border area. In the investigated area, data of historical catalogues are uncertain or, in many cases, the epicentral intensities are unknown. Thus, a number of assumptions have to be adopted in data preparation of catalogues since the year 1200. The calculated values of parameters b in the Gutenberg-Richter frequency-intensity relation as well as the return periods, seem to be reasonable and are generally confirmed by the results obtained from catalogues for the last 80–130 years.