Microtextures and crystal chemistry in P21/c pigeonites

Summary ?A single-crystal X-ray investigation was performed on crystals of P2₁/c natural pigeonite with varying Ca and Fe^* ( = Fe²⁺ + Mn²⁺) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe^* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances. Received October 18, 2001; revised version accepted February 15, 2002     

Evolution of galaxies: One-zone model with a birth-rate stellar function depending on gas density and temperature

In this paper closed one-zone models with a birth-rate stellar function depending on gas density and temperature are built up, in order to make a zeroth order approximation of thermo-chemical evolution of galaxies. To get a more realistic picture, the models are allowed first to expand with the universe and then to contract according to three successive time-scales, in order to simulate violent relaxation, rapid contraction, and slow contraction processes.Star formation occurs according to a birth-rate (m, t) _g ⁿ T ^r m ^p ( _g , gas density;T, gas temperature; andm, star mass), which reproduces the usual Schmidt-type law in the limitr0. Further we suppose that synthesized metals and helium are instantaneously mixed when returned to gas component at star death, and that relative metal abundances are time-independent: ( any element heavier than He, ). An energy fraction of each typical supernova event is assumed to heat interstellar medium, which is also though always to maintain a steady state.Under these hypotheses it becomes possible to use the interstellar cooling function of Dalgarno and McCray (1972), which is related to a special cosmic abundance of elements, to express analytically an interstellar cooling function related to any abundance. At this stage, numerical computations are performed up to about 12 Gy and the results analyzed in order to reproduce some morphological features observed in the galaxy, namely: (i) the extreme scarcity of stars with nearly zero metal content; (ii) the metal content and mass fraction of halo stars; (iii) the metal content and mass fraction of disk stars; and (iv) the ratio of the present-day birth-rate to the past birth-rate averaged over the age of the disk. It is found that a whole class of theoretical models exist, with slope of the mass spectrum between –2.5 and –2.6, which range from a Schmidt's law limitn=1.3 andr=0 to the opposite limitn=0 andr=–1.2, and are able to fit more or less at the same extent the observations mentioned above. On the analysis of further computations, it is also shown that an universal birth-rate stellar function is in contradiction with neither a dynamical hystory starting from the initial expansion of proto-galaxies, nor a morphological sequence where the amount of angular momentum is the only free parameter for any given mass, at least for what concerns spiral types. Finally, possible refinements for future work are briefly outlined.     

Crystal-chemistry of high-pressure clinopyroxene from spinel lherzolite nodules Mts. Leura and Noorat Suites,Victoria, Australia

Summary The clinopyroxene suites from lherzolite nodules from Mts. Leura and Noorat (Victoria, Australia) have been investigated by X-ray diffraction and electron probe microanalyses (Dal Negro et al., 1984; Cundari et al., 1986).The evolution of the host nodule is shown by general depletion of AlIv, AVI, Ti⁴⁺, Fe²⁺ and enrichment of Si, Mgm,, Fe³⁺, Cr³⁺, Ca, while Na depletion occurs only in the clinopyroxenes from Mt. Noorat. Different mechanisms of cation substitution are thus involved in the two clinopyroxene suites, suggesting different total pressures of equilibration in the stability field of spinel.Modifications involving the M1 octahedron are mainly ascribed to variable amounts of trivalent cations, the volume of the M 1 site increasing with decreasing content of trivalent cations in each suite. The differences in M 1 site configuration between the Mt. Leura and Mt. Noorat clinopyroxene suites are ascribed to the different amounts of A1v1 and FeM,.The volume of the T tetrahedral site is generally related to A1Iv in each suite. An increase in T site volume from Mt. Leura to Mt. Noorat clinopyroxenes was found however, due to lengthening of the T-O_brg bond lengths, for similar AIIv contents. The volume of the M2 site, generally related to Ca content, was generally higher in the Noorat clinopyroxenes for similar Ca (and Ca + Na) contents, due to the longer M₂-O₃C₁ bond length, strongly related to Na content. Cell volume is linearly correlated to M 1 volume in each suite, but is definitely higher in the Noorat clinopyroxenes for similar M 1 volume. All the structural data show that the total pressure of equilibration in the Noorat clinopyroxenes was lower than that in the Leura clinopyroxenes, as suggested by chemical data.

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Die Kristallchemie von hochdruck-klinopyroxenen aus spinell-lherzolithknollen von Mt. Leura und Mt. Noorat, Victoria, Australien

Zusammenfassung Die Klinopyroxenparagenesen aus Lherzolitheinschlüssen von Mt. Leura und Mt. Noorat (Victoria, Australien) wurden mittels Röntgendiffraktometrie und Elektronenmikrosonde untersucht (Dal Negro et al., 1984; Cundari et al., 1986). Die Lherzolitheinschlüsse zeigen generell eine Abnahme von Al^IV, Al^VI, Ti⁴⁺, Fe _MI ²⁺ und eine Zunahme von Si, Mgm,, Fe³⁺, Cr³⁺ und Ca. Eine Abnahme von Na tritt nur in Klinopyroxenen von Mt. Noorat auf. Verschiedene Substitutions-Mechanismen der Kationen weisen auf verschiedene Equilibrationsdrucke im Stabilitätsbereich der Spinelle hin und sind daher in den beiden Klinopyroxenserien zu berücksichtigen. Modifikationen in der oktaedrischen M1-Position wurden vor allem dem wechselnden Anteil an dreiwertigen Kationen zugeschrieben, wobei in jeder der beiden Serien das Volumen der Ml-Position mit abnehmenden Gehalten dreiwertiger Kationen zunimmt. Die unterschiedliche Konfiguration der M1-Position der Klinopyroxene von Mt. Leura und Mt. Noorat wird mit unterschiedlichen Gehalten an Al^VI und Fe _MI ²⁺ , in Zusammenhang gebracht.Das Volumen der tetraedrischen T-Position ist normalerweise mit den Gehalten an All^IV in jeder Serie verknüpft. Ein Vergleich der Klinopyroxene von Mt. Leura und Mt. Noorat zeigte jedoch, daß bei gleichen Gehalten an Al^VI das Volumen der tetraedrischen Position infolge einer Aufwertung der T-O_brg.-Bindungen zunimmt. Das Volumen der M2-Position, üblicherweise mit dem Ca-Gehalt korreliert, ist in den Klinopyroxenen von Mt. Noorat bei ähnlichen Ca (und Ca f Na) Gehalten infolge größerer M2--O₃C₁-Abstände größer. Sie stehen also mit den Na-Gehalten in Verbindung.Das Volumen der Elementarzelle korreliert mit dem der M1-Position in jeder Serie; es ist aber in den Klinopyroxenen von Mt. Noorat deutlich höher. Die Ergebnisse der Strukturuntersuchungen zeigen-wie auch die chemischen Daten-, daß die genannten Equilibrationsdrucke für die Klinopyroxene von Mt. Noorat niedriger waren als für die von Mt. Leura.

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With 2 Figures     

Fundamental physics and absolute positioning metrology with the MAGIA lunar orbiter

MAGIA is a mission approved by the Italian Space Agency (ASI) for Phase A study. Using a single large-diameter laser retroreflector, a large laser retroreflector array and an atomic clock onboard MAGIA we propose to perform several fundamental physics and absolute positioning metrology experiments: VESPUCCI, an improved test of the gravitational redshift in the Earth?CMoon system predicted by General Relativity; MoonLIGHT-P, a precursor test of a second generation Lunar Laser Ranging (LLR) payload for precision gravity and lunar science measurements under development for NASA, ASI and robotic missions of the proposed International Lunar Network (ILN); Selenocenter (the center of mass of the Moon), the determination of the position of the Moon center of mass with respect to the International Terrestrial Reference Frame/System (ITRF/ITRS); this will be compared to the one from Apollo and Lunokhod retroreflectors on the surface; MapRef, the absolute referencing of MAGIA??s lunar altimetry, gravity and geochemical maps with respect to the ITRF/ITRS. The absolute positioning of MAGIA will be achieved thanks to: (1) the laboratory characterization of the retroreflector performance at INFN-LNF; (2) the precision tracking by the International Laser Ranging Service (ILRS), which gives two fundamental contributions to the ITRF/ITRS, i.e. the metrological definition of the geocenter (the Earth center of mass) and of the scale of length; (3) the radio science and accelerometer payloads; (4) support by the ASI Space Geodesy Center in Matera, Italy. Future ILN geodetic nodes equipped with MoonLIGHT and the Apollo/Lunokhod retroreflectors will become the first realization of the International Moon Reference Frame (IMRF), the lunar analog of the ITRF.     

Effect of pressure, temperature, and oxygen fugacity on the metal-silicate partitioning of Te, Se, and S: Implications for earth differentiation

We have measured liquid Fe metal-liquid silicate partitioning (D_i) of tellurium, selenium, and sulfur over a range of pressure, temperature, and oxygen fugacity (1-19 GPa, 2023-2693 K, fO₂ −0.4 to −5.5 log units relative to the iron-wüstite buffer) to better assess the role of metallic melts in fractionating these elements during mantle melting and early Earth evolution. We find that metal-silicate partitioning of all three elements decreases with falling FeO activity in the silicate melt, and that the addition of 5-10 wt% S in the metal phase results in a 3-fold enhancement of both D_Te and D_Se. In general, Te, Se, and S all become more siderophile with increasing pressure, and less siderophile with increasing temperature, in agreement with previous work. In all sulfur-bearing experiments, D_Te is greater than D_Se or D_S, with the latter two being similar over a range of P and T. Parameterized results are used to estimate metal-silicate partitioning at the base of a magma ocean which deepens as accretion progresses, with the equilibration temperature fixed at the peridotite liquidus. We show that during accretion, Te behaves like a highly siderophile element, with expected core/mantle partitioning of >10⁵, in contrast to the observed core/mantle ratio of ∼100. Less extreme differences are observed for Se and S, which yielded core/mantle partitioning 100- to 10 times higher, respectively, than the observed value. Addition of ∼0.5 wt% of a meteorite component (H, EH or EL ordinary chondrite) is sufficient to raise mantle abundances to their current level and erase the original interelement fractionation of metal-silicate equilibrium.     

Adjustability of exposed elements by updating their capacity for resistance after a damaging event: application to an earthquake–tsunami cascade scenario

Natural Hazards - The 2011 Great East Japan earthquake and tsunami remind us once again that these types of cascade event can occur and cause considerable damage. The scientific community realizes...     

The potential‐energy tensors for subsystems. A special case of two homogeneous ellipsoids bounded by a heterogeneous homeoid

The potential‐energy tensors for subsystems are evaluated in the special case of two homogeneous and coaxial ellipsoids, one lying completely within the other, bounded by a heterogeneous homeoid, where the isodensity surfaces are similar and similarly placed with respect to the outer ellipsoid. Two particular density profiles, related to the perfect ellipsoid and the isothermal sphere, respectively, are examined with more detail.     

Petrographic and geochemical characterization of Archaic‐Hellenistic tableware production at Solunto,Sicily

A selected assortment of Archaic‐Hellenistic tableware samples from Solunto, a Phoenician‐Punic site located 20 km east of Palermo (Sicily), has been subjected to thin‐section petrography and chemical analysis (XRF). In this settlement several ceramic kilns remained operative over a long time period (7th to 3rd century B.C.). The main goal of this analytical study is to distinguish the ceramics manufactured locally from regional and off‐island imports. Analytical results were matched to similar data concerning local natural clay sources and to coeval tableware productions from other sites in the same area. The ceramic pastes used by the ancient craftsmen of Solunto in the case of this class of pottery could be differentiated clearly by their petrochemical characteristics. We conclude that ceramics were locally produced far beyond satisfying just internal consumption needs, indicating interaction of Solunto with neighboring Greek colonies, indigenous people, and Phoenician‐Punic colonies of Sicily. © 2009 Wiley Periodicals, Inc.     

Characterization of an Area Polluted by Copper and Zinc:the Relation between Soil Texture,Mineralogy and Pollutant Concentration

Abstract: Twenty-four soil samples were collected at three depths from an approximately 2.5 acre contaminated site in southern Piedmont (Italy) and then analyzed. The main soil parameters determined were: pH, Cation Exchange Capacity (CEC), particle size distribution, total organic carbon (TOC) content and retained metal concentration. The mineral phases were identified by X-Ray Powder Diffraction (XRPD). All of the samples contained Zn and Cu resulting from industrial contamination during the last century, and those obtained at depths of 20-40 cm consistently showed the highest levels. To determine which size fraction was most active in the retention process, the samples were separated into four fractions (≤2 mm, ≤63 μm, ≤30 μm and ≤2 μm) and the amount of pollutant measured in each. It was found that metal retention was the highest in the clayey fraction, whose clay minerals were identified by XRPD after K+ and Mg2+ saturation, glycerol treatment and heating to 550°C. The clayey fraction was also the richest in TOC, and a direct correlation between TOC amount and metal retention was observed.