Cesstibtantite—a geologic introduction to the inverse pyrochlores

Summary The crystal structure of cesstibtantite has been solved from diffractometer data collected on samples from Leshaia, Russia and the Tanco pegmatite, Manitoba. Cesstibtantite from the Leshaia pegmatite (type locality) hasa 10.515(2) Å, space groupFd3m, composition Cs_0.31(Sb_0.57Na_0.31Pb_0.02Bi_0.01)_O.91(Ta_1.88Nb_0.12)₂(O_5.69[OH, F]_0.31)₆(OH, F)_0.69, Z 8; its structure was refined toR 3.8,wR 4.3% using 96 observed (F > 3[F]) reflections (MoK). Cesstibtantite from the Tanco pegmatite hasa 10.496(1) Å, space groupFd3m, composition (Cs_0.22K_0.01)_0.23(Na_0.45Sb_0.39Pb_0.14· Ca_0.06Bi_0.02)_1.06(Ta_1.95Nb_0.05)₂(O_5.78[OH,F]_0.22)₆(OH,F)_0.55,Z 8; its structure was refined toR 3.9w R 3.7% using 104 observed reflections. Cesstibtantite differs from the normal pyrochlores in that it contains significant amounts of very large cations such as Cs. As these cations are too large (^VIII[r] > 1.60 Å) for the conventional [8]-coordinated A site, they occupy the [18]-coordinated site, which normally contains monovalent anions. Natural cesstibtantite samples are non-ideal in that both Cs and monovalent anions occur at the site; thus cesstibtantite is intermediate to thenormal pyrochlores (with only monovalent anions at the site) and theinverse pyrochlores (with only large cations at the site).

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Cesstibtantit—eine geologische Einfiihrung in die inversen Pyrochlore

Zusammenfassung Die Kristallstruktur von Cesstibtantit wurde auf der Basis von Diffraktometerdaten von Proben von Leshaia, Russland and dem Tanco Pegmatit, Manitoba, gelöst. Cesstibtantit aus dem Leshaia Pegmatit (Typlokalität) hat a 10.515(2) Å, RaumgruppeFd3m, die Zusammensetzung CS_0.31(Sb_0.57Na_0.31Pb_0.02Bi_0.01)_0.91(Ta_1.88Nb_0.12)₂· (O_5.69OH, F_0.31)₆(OH, F)_0.69 Z 8; die Struktur wurde aufR 3.8,wR 4.3% verfeinert unter Benützung von 96 beobachteten Reflexen. Cesstibtantit vom Tanco Pegmatit hat a 10.496(1) Å, RaumgruppeFd3m, die Zusammensetzung (Cs_0.22K_0.01)_0.23(Na_0.45· Sb_0.39Pb_0.14Ca_0.06Bi_0.02)_1.06(Ta_1.95Nb_0.05)₂(O_5.78OH,F_0.22)₆(OH,F)_0.55,Z 8; seine Struktur wurde aufR 3.9wR 3.7% auf der Basis von 104 beobachteten Rettexen verfeinert. Cesstibtantit unterscheidet sich von normalen Pyrochloren insofern, als er signifikante Mengen von sehr großen Kationen, wie z.B. Cs enthält. Da these Kationen zu groß sind (^VIII r 1.60 Å) für eine konventionelle [8]-koordinierteA Stelle, nehmen she die [18]-koordinierten Positionen ein, welche normalerweise monovalente Anionen enthalten. Natürliche Cesstibtantitproben sind nicht ideal insofern als sowohl Cs als auch monovalente Anionen in der Position vorkommen. Somit ist Cesstibtantit intermediär zu den normalen Pyrochloren (mit nur monovalenten Anionen auf der Position) and den inversen Pyrochloren (mit ausschließlichen großen Kationen an der Position).

     

Late Quaternary palaeolimnology of a tropical marl lake: Wallywash Great Pond,Jamaica

Wallywash Great Pond (17° 57 N, 77° 48 W, 7 m a.s.l.) is the largest perennial lake in Jamaica. It occupies a fault trough within the karstic White Limestone. The Great Pond is a hardwater lake with a pH of 8.2–8.6 and an alkalinity of 3.6–3.9 meq 1^–1. Its chemistry is strongly influenced by the spring discharge from the limestone. The lake water is subject to degassing, evaporation and bicarbonate assimilation by submerged plants and algae, resulting in marl precipitation. A 9.23 m core (WGP2), taken from a water depth of 2.8 m, was analysed for magnetic susceptibility, loss-on-ignition, carbonate content, mole % MgCO₃ in calcite, and stable isotopes in the fine carbonate fraction. The chronology is based on ten¹⁴C and four U/Th dates. Four main sediment types alternate in the core: marl; organic, calcareous mud; organic mud or peat; and earthy, brown, calcareous mud. The marls represent periods of wet/warm climate during sea-level highstands and the organic deposits, shallower, swampy conditions. In contrast, the brown, calcareous muds were laid down when the lake was dry or ephemeral. The last interglacial (120 000- 106 000 yr BP) is represented by three distinct marl units. After a dry interval, stable, wet/warm conditions set in from 106 000 to 93 000 yr BP. A dry/cool climate prevailed between 93 000 and at least 9500 yr BP. Three subsequent cycles of alternating wet and dry conditions culminated in flooding of the basin by the Black River during the late Holocene. These recent events cannot be accurately dated by¹⁴C due to significant and temporally-variable inputs of dead carbon from the springs.     

Theoretical infrared spectra of some model polycyclic aromatic hydrocarbons: effect of ionization

In order to test the hypothesis of ionized polycyclic aromatic hydrocarbons (PAHs) as possible carriers of the UIR bands, we realized a computational exploration on selected PAHs of small dimension in order to identify which changes ionization would induce on their IR spectra. In this study we performed ab initio calculations of the spectra of neutral and positively ionized naphthalene, anthracene, and pyrene. The results are significantly important. The frequencies in the cations are slightly shifted with respect to the neutral species, but no general conclusion can be reached from the three molecules considered. By contrast, the relative intensities of most vibrations are strongly affected by ionization, leading to a much better agreement between the calculated CH/CC vibration intensity ratios and those deduced from observations.     

The inverse planetary problem: a numerical treatment

The so-called inverse planetary problem can be stated as follows: given the distances from the centre, masses, and radii of (say) three planets of a planetary system, find the optimum polytropic index, mass, and radius of their star, and also other quantities of interest, which depend either explicitly or implicitly on the foregoing ones (e.g., central and mean density, central and mean pressure, central and mean temperature, etc.). It is hereafter tacitly assumed that the system is opaque with respect to observations concerning periods of planetary otbits; hence, we cannot have any relevant estimates due to the well-known period laws. In the present paper, the inverse planetary problem is treated numerically on the basis of the so-called global polytropic model, developed recently by the first author.     

Quasi-periodical structures in the galaxy populations: mass and luminosity functions for the cluster galaxies

A search and estimation of the statistical significance of the quasi-periodical structures (QPS) has been carried out: for the luminosity function of the galaxies in a few rich clusters; for the integrated mass function of galaxies in the superclusters identified via = 21 cm observations. Some statistically significant QPS have been revealed. The periods for these structures are in agreement between each other, at this basis the hypothesis has been proposed that an effect of the QPS at the level of galaxies has some universal character. Frequency doubling has been discovered for these QPS.     

Circular polarization of solar bursts at 22 GHz

The circular polarization of complex solar bursts was measured at short microwaves (22 GHz, × 1.35 cm) with high sensitivity (0.03 s.f.u. r.m.s.) and high time resolution (5 ms). The polarization shows up as soon as an excess burst emission is measured. Two components are found in the time development of the degree of circular polarization: (1) a steady level, sometime changing smoothly with time; (2) superimposed faster polarization time structures, small compared to the basic steady degree of polarization, and often not clearly related to the burst flux time structures. The observed degrees may range from 10% to more than 85%.In memoriam (1942–1981).     

Carbonatites from Eastern Paraguay and genetic relationships with potassic magmatism: C, O, Sr and Nd Isotopes

Summary Geochemical characteristics were systematically determined for Early Cretaceous samples of carbonatitic rocks from Eastern Paraguay (Rio Apa, Amambay and Central Provinces). The data show that all the occurrences have an enriched isotopic signature and that the carbonatites have negligible or absent crustal signature. A petrogenetic model (parent liquids, fractional crystallization, hydrothermal interactions and weathering) is proposed as a function of incompatible trace element, stable (O-C) and radiogenic (Sr-Nd) isotope variations with the aim to test the significance of carbonatitic complexes as a marker of the metasomatized subcontinental lithospheric mantle. The results indicate that the carbonatites and primary carbonates from eastern Paraguay, and those from the north eastern Paraná Basin (SE Brazil), were affected by metasomatic events distinct in time and composition.

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Karbonatite aus Ost-Paraguay und ihre genetische Beziehung zu Kalium-Magmatismus: C O, Sr und Nd isotope

Zusammenfassung Die geochemischen Charakteristika von frühkretazischen Karbonatitproben aus Ostparaguay (Rio Alpa, Amambay und Zentrale Provinzen) wurden untersucht. Die Daten belegen, daß alle Vorkommen eine isotopische Anreicherungssignatur zeigen und daß ihnen eine entsprechende Krustensignatur fehlt. Ein Petrologisches Modell (Ausgangsschmelze, fraktionierte Kristallisation, hydrothermale Interaktion und Verwitterung) wird auf Grund der Verteilung der inkompatiblen Spurenelemente, der stabilen (C-O) und radiogenen (Sr-Nd) Isotope vorgeschlagen. Es versucht die Bedeutung der Karbonatitkomplexe als Markerhorizonte des metasomatischen subkontinentalen Mantels zu überprüfen. Die Ergebnisse zeigen, daß die Karbonatite und die primären Karbonate in Ostparaguay, und jene aus dem Paraná Becken SüdostBrasiliens durch zeitlich und zusammensetzungsmäßig unterschiedliche metasomatische Prozesse erfaßt wurden.

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With 10 Figures     

Environmental geology problems in the Tyrrhenian coastal area of Santa Marinella, province of Rome, central Italy

 The Tyrrhenian resort of S. Marinella (central Italy) is subjected to significant anthropogenic pressures during the summer vacation period, a common situation all along the Italian coast. Located 65 km NW of Rome on the southern slopes of the Tolfa Mountains, S. Marinella is built on a gently sloping, E–W trending belt which is cut by 14 N–S oriented ephemeral streams that discharge into the Tyrrhenian Sea. The low to medium permeability turbiditic sandstones which outcrop along this belt belong to the Late Cretaceous Pietraforte unit. Three environmental problems are addressed in this study. The first problem is related to the high water supply demand during the summer months which has forced local residents to dig a large number of wells. Extensive pumping from these wells has caused salt-water intrusion into the Pietraforte, thus compromising the domestic use of the groundwater. The second problem consists of the illegal dumping of urban solid waste, material that represents a hazard during significant rain events as well as a possible cause of groundwater contamination. The final issue addressed concerns the flooding potential of the 14 ephemeral streams that cross the inhabited area of S. Marinella, a risk which is highlighted by the disastrous flood which occurred on 2 October 1981 and during the period of the Roman Emperor Settimio Severo (205 A.D.). Some suggestions are proposed to mitigate and contain the effects of these problems. Received: 7 November 1995 / Accepted: 5 December 1996     

Cycle anatomy and variability in the storm-dominated type cincinnatian (Upper Ordovician): coming to grips with cycle delineation and genesis

Although parasequence and sequence are scale-independent terms, they are frequently applied only to specific scales of cycles. For example, meter-scale cycles are commonly assumed to be parasequences or PACs. In the Upper Ordovician Kope and Fairview Formations of northern Kentucky, we examined a succession of 50 meter-scale cycles that have been variously interpreted as deepening-upward, shallowing-upward, or showing no relationship with water depth. Our analysis shows that these cycles, characterized by shifts in storm-bed proximality, are highly variable in their thickness and internal construction. Most cycles are best considered high-frequency sequences, because deepening-upward intervals are common, and many cycles contain evidence of abrupt basinward shifts in facies as expected at sequence boundaries. A minority fit the parasequence model of shallowing-upward cycles bounded by flooding surfaces. Larger, 20 m scale cycles are defined by systematic thickening and thinning trends of meter-scale cycles. However, meter-scale cycles do not display any systematic trends in cycle anatomy as a function of position within the 20 m cycles or position within the Kope and Fairview Formations. The high cycle variability and the lack of systematic stratigraphic organization with respect to longer-term cyclicity reflect either the irregularity of relative sea-level changes, the poor recording of sea-level changes in this deep-water setting, or the generation of these cycles by climate-induced cyclicity in storm intensity. These three mechanisms would generate similar patterns at the outcrop scale, so it is not possible at the present to distinguish between them.