An XRD, TEM and Raman study of experimentally annealed natural monazite

 The healing of radiation damage in natural monazite has been experimentally studied in annealing experiments using XRD, TEM, Raman microprobe and cathodoluminescence analysis. The starting material was a chemically homogeneous monazite from a Brazilian pegmatite with a concordant U–Pb age of 474 ± 1 Ma and a U–Th/He age of 479 Ma. The monazite shows nm-scale defects induced by radioactive decay. The X-ray pattern of the unheated starting material revealed two distinct monazite “phases” A and B with slightly different lattice parameters. Monazite A shows sharp reflections of high amplitudes and slightly expanded lattice parameters (1% in volume) compared to a standard monazite. Phase B exhibits very broad reflections of low amplitudes. Two sets of experiments were performed. First, dry monazite powder was annealed at 500, 800 and 1000 °C for 7 days. Each run product was analysed by X-ray diffractometry. Second, monazite grains were hydrothermally annealed at temperatures from 500 to 1200 °C for 5 to 15 days. TEM observations show that partial healing of the monazite lattice already occurred at 500 °C and increased gradually with temperature, so that after 10 days at 900 °C complete healing was achieved. The observations are interpreted accordingly: the starting material has a mosaic structure consisting of two domains, A and B, which are basically two monazite crystals with different lattice parameters. We suggest that the A domains correspond to well-crystallised areas where helium atoms are trapped. The accumulation of He causes expansion of the A monazite lattice. Diffraction domains B are interpreted as a helium-free distorted monazite crystal lattice, which can be referred to old alpha-recoil tracks. These B domains are composed of “islands” with an expanded lattice, induced by the presence of interstitials, and “islands” of a compressed monazite lattice, induced by presence of vacancies. Both the islands will pose stress on the lattice in the vicinity of the islands. The broadening of the B reflections is due to the expanded or compressed diffraction domains and to the different amount of the distortion. With increasing temperature the unit-cell volume of monazite A decreases, i.e. the position of the A reflections shifts towards smaller d _hkl values. This was interpreted as a relaxation of the monazite lattice due to helium diffusion out of the monazite lattice. Simultaneously, the nm-sized defect domains B are healed. At 900–1000 °C only a monazite with well-crystallised lattice and minimum unit-cell volume is observed. Received: 7 May 2001 / Accepted: 11 October 2001     

Synthesis of tourmaline solid solutions in the system Na2O–MgO–Al2O3–SiO2–B2O3–H2O–HCl and the distribution of Na between tourmaline and fluid at 300 to 700 °C and 200 MPa

Tourmaline has been synthesized hydrothermally at 200 MPa between 300 and 700 °C from oxide mixtures with Mg-Al ratios for the end members dravite NaMg₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH) and Mg-foitite &ding6F;(Mg₂Al)Al₆ (Si₆O₁₈)(BO₃)₃(OH)₃(OH). Six different Na concentrations were investigated to determine the distribution of Na between tourmaline and fluid in the SiO₂-saturated system Na₂O-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O-HCl. Synthetic tourmaline ranges from X-site vacant (&ding6F;) tourmaline (Mg-foitite) to nearly ideal dravite with Na=0.95 apfu. There are small, but significant, amounts of proton deficiency and negligible tetrahedral Al. Chemical variation is primarily caused by the substitutions Al&ding6F;Mg_-1Na_-1 and minor AlMg_-1H_-1. Varying amounts of Na and &ding6F; determine the Mg/Al ratios. Besides tourmaline and quartz, additional Mg-Al phases are chlorite and, at 700 °C, cordierite. Albite is also present at high Na concentrations in the bulk composition. The c dimension of the tourmaline crystals increases with Na in tourmaline. The amount of Na in the X-site depends strongly on the bulk concentration of Na in the system as well as on the temperature. These factors in turn control the phase assemblage and the composition of the fluid phase. For the assemblage tourmaline + quartz + chlorite/cordierite + fluid, a linear relationship exists between Na concentration in the fluid (quenched after the run) and tourmaline with temperature: T °C [ᆭ °C]=(Na_fluid/Na_tur)앾.878-14.692 (r²=0.96). For the assemblage tourmaline + albite + quartz + fluid, it is: T °C [ᆣ °C]=(Na_fluid/Na_tur)욝.813-6.231 (r²=0.95), where Na_fluid is the concentration of Na⁺ in the final fluid (mol/l) and Na_tur is the number of Na cations in the X-site of tourmaline. The equations are valid in the temperature range of 500-715 °C. Our experiments demonstrate that the occupancy of the X-site in combination with the changing concentrations of Al and Mg can be used to monitor changes in the fluid composition in equilibrium with a growing tourmaline crystal. Currently, this relation can be applied qualitatively to natural tourmaline to explain zoning in Na- and Al/(Al+Mg).     

Potassium-fluor-richterite in metacherts from the Bufa del Diente contact-metamorphic auréole,NE-México

Summary Potassium-fluor-richterite is reported from wollastonite-rimmed metachert bands embedded in marbles of the Bufa del Diente aureole, NE-Mexico. K-F-richterites were generated via reaction of previously formed contact-metamorphic diopside with infiltrated hypersaline brines of magmatic origin that used the metachert bands as metamorphic aquifers. Their formation occurred at peak metamorphic conditions of 500 to 600 °C at 1 kbar according to the generalised reaction 10 CaMgSi₂O₆ + 8 SiO₂ + 2 KCl_aq + 6 NaCla_aq + 4 H₂O + 4 HF diopside + quartz = 2 KNaCaMg₅Si₈O₂₂(F)₂ + 4 Ca₂NaH(SiO₃)₃ + 8 HCl_aq K-F-richterite + pectolite Compositional ranges in two samples are K-Ri_58–69Ri_16–25Tri_12–21 and K-Ri_58–66Ri_32–40Tr_0–5. Mg/(Mg + Fe²⁺) ranges from 0.81 to 0.95, F/(F + OH) from 0.55 to 0.85, and the K-F-richterites are Cl-free.Microthermometrically determined K/(K + Na)-ratios of hypersaline brines trapped at 500°C to 600°C range from 0.50 to 0.64, resulting in distribution coefficients K _DK-Na ^Amph-F1 = X _{K(in A)} ^Amph /X _{N(in A)} ^Amph · X _Na ^F1 /X _K ^F1 of 0.85 to 1.3 for the Na-K amphibole-fluid exchange reaction. Very low Ca-contents in bulk fluid leachates indicate that Cacontaining fluids cannot coexist with richterite and that Ca is incorporated into amphibole as a tremolite component. Amphibole compositions in Al-free systems can predict Na-K-Ca-ratios of metasomatic brines that equilibrated with them. Despite the hypersaline nature of the brines, Cl is not incorporated into the Mg-rich K-F-richterites because of the Mg-Cl and Fe-F avoidance rules in amphiboles.

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Kalium-Fluor-Richterit in kontaktmetamorphem Quarziten der Bufa del Diente Aureole, NO-México

Zusammenfassung In die Marmore der Kontaktaureole des Bufa del Diente-Alkalisyenits sind 5-15 cm mächtige Quarzitbänder eingeschaltet, die mehrere cm-dicke Wollastonitsäume aufweisen. Die Metaquarzite enthalten Kalium- und Fluor-reiche Richterite. Die K-FRichterite bildeten sich durch Reaktion von zuvor gebildetem, kontaktmetamorphem Diopsid mit infiltrierenden hochsalinaren Fluiden magmatischen Ursprungs. Die Metaquarzite dienten als metamorphe Aquifere für diese Fluide.Die K-F-Richterite wurden bei den Maximaltemperaturen der Kontaktmetamorphose von 500-600°C bei 1 kbar nach der folgenden schematischen Reaktion gebildet: 10 CaM9Si₂O₆ + 8 SiO₂ + 2 KCl_aq + 6 NaCl_aq + 4 H₂O + 4 HF Diopsid + Quarz = 2 KNaCaMg₅Si₈O₂₂(F)₂ + 4 Ca₂NaH(SiO₃)₃ + 8 HCl_aq K-F-Richterit + Pektolit Der Zusammensetzungsbereich von zwei Proben aus unterschiedlichen Metaquarzitbändern liegt bei K-Ri_58–69Ri_16–25Tr_12–21 und K-Ri_58–66Ri_32–40Tr_0–5. K-FRichterite in beiden Proben haben F/(F + OH)-Verhältnisse von 0.55 bis 0.85 und enthalten kein Cl.Mikrothermometrisch bestimmte K/(K + Na)-Verhältnisse der bei 500°-600°C in Flüssigkeitseinschlüssen okkludierten, hochsalinaren Fluide betragen 0.50–0.64. Daraus lassen sich Verteilungskoeffizienten K _DK-Na ^Amph-Fi = X _{K(in A)} ^Amph /X _{N(in A)} ^Amph · X _Na ^F1 /X _K ^F1 von 0.85-1.3 für die Amphibol-Fluid Austauschreaktion in tremolitfreien Amphibolen ableiten. Wässrige Extrakte der Gesamtfluidpopulationen enthalten sehr geringe Ca-Konzentrationen im Vergleich zu K und Na. Dies zeigt, daß Fluide mit signifikanten Ca-Konzentrationen nicht mit Richteriten koexistieren können und Ca bevorzugt als Tremolitkomponente eingebaut wird. Amphibolzusammensetzungen in Al-freien Systemen eignen sich als Indikatoren für K-Na-Ca-Verhältnisse von koexistierenden metasomatischen Fluiden. Die K-F-Richterite sind Cl-frei, obwohl sie mit hochsalinaren Fluiden koexistiert haben. Mg-reiche Amphibole können kein CI einbauen und Fe-reiche Amphibole kein F.

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With 4 Figures     

One-year temperature records in the atmospheric surface layer above sea ice and open water

Temperature observations of three buoys drifting in the Weddell Sea for one year and covering the ice-water-ice cycle from July 1986 to July 1987 are presented. Significant differences between winter and summer are shown to be a consequence of the air-sea heat exchange being drastically modified by the sea ice cover. Over ice, prevailing variance is in the synoptic scale (periods 3 to 5 days) with amplitudes of 25 °C, whereas over water, the diurnal wave dominates with amplitudes of less than 1 °C.     

Zur Frühjahrs-Windumstellung in Stratosphäre und Mesosphäre

Zusammenfassung An Hand der bisher vorliegenden Ergebnisse des amerikanischen Raketennetze (MRN) wird der Auswirkung der sechsundzwanzigmonatigen Schwingung bei der Frühjahrsumstellung in der Mesosphäre nachgegangen. Das Fehlen der Möglichkeit zur Zeichnung zirkumpolarer Mesosphärenkarten erweist sich als Hauptnachteil. Ob die Asymmetrie der Märzdruckverteilung in gewissen Jahren, dieScherhag fand, bis in die Mesosphäre reicht, kann auf Grund des spärlichen Raketenmaterials früherer Jahre noch nicht entschieden werden. Der Zeitpunkt der Frühjahrsumstellung erweist sich — neben der selbstverständlichen Abhängigkeit von der geographischen Lage — stark höhenabhängig. Mehrere Beispiele zur Illustration der mesosphärischen Frühjahrsumstellung über dem Gebiet des MRN werden gegeben.

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Summary Based upon the hitherto available results of the American Rocket Network (MRN), the effect of the twenty-six months' oscillation on the spring transition in the mesosphere is investigated. Lack of the possibility to construct circumpolar charts for the mesosphere proves the major disadvantage. Whether the asymmetry in the March pressure distribution of certain years found byScherhag extends up into the mesosphere has not yet been determined owing the scarcity of rocket material in previous years. The date of the vernal transition is — apart from its automatically depending on geographical location — found to be strongly height-dependent. Several examples illustrating the mesospheric spring transition over the MRN area are given.

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Diese Arbeit wurde unterstützt vom US-Department of the Army, European Research Office, unter Kontrakt Nr. DA-91-591-EUC-3765.