Tschermaks mineralogische und petrographische Mitteilungen

Ilmenite in alkali feldspar quartz syenite from Cape Ashizuri contains up to 4.4 wt.% Nb2O5. Niobium substitutes for Ti in the octahedral site of the ilmenite structure. Substitution of Nb for Ti may involve a coupled exchange to maintain charge balance, and an exchange of 2Nb + 2Fe3+ = 3Ti + 2Fe2+ is advocated. An Fe-Ti oxide geothermometer obtained from mineral pairs of granular and lamellar intergrowths indicates a subsolidus re-equilibration temperature of 510–640 °C andfO2 between the FMQ and MW-buffers, implying that it is very undersaturated with respect to water. On the other hand, Zn-ilmenite, containing up to 5.4 wt.% ZnO, occurs in miarolitic cavities in peralkaline rhyolite which cuts the quartz syenite. The Zn-ilmenite is one of the last crystalline phases of the Ashizuri magmatic activities under volatile-rich conditions. Nb-oxides, such as fergusonite, samarskite, columbite and a pyrochlore-like mineral, are Ta- and Mn-poor, which corresponds to those of less-fractionated rocks of anorogenic alkali granite and pegmatitic granite in the continental situation. Ta- and Mn-poor Nb-oxides in F- and Li-rich alkaline felsic magmas such as the Ashizuri syenites are unusual; this may be related to a rapid emplacement and cooling of mantle-derived small-volume magma in the island are situation.

Die Notiz behandelt ein neues Mottramitvorkommen aus Bolivien. Neue chemische Analysen von Mottramit- und Cuprodescloizitvorkommen, die letzteren aus dem Otavilande in Südwestafrika, werden mitgeteilt und die A. Diefenbachschen Untersuchungen durch kristallographische Messungen am Mottramit (C. Hlawatsch) erweitert. Für den Mottramit von Bolivien wird die Formel 3 (Pb, Cu, Zn)3 V2O8 (Pb, Cu, Zn) (OH)2 aufgestellt.

In eleven Alpine clefts of the western Alps, in-situ dating of monazite-(Ce) and xenotime-(Y) has been attempted to gain insights on possible disturbances of the geochronological U-Th-Pb systems and age interpretations in hydrothermal conditions. In most clefts, monazite-(Ce) in-situ 208Pb/232Th dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) yields well-resolved ages (with errors typically <0.3 Ma, quoted at the 2σ level), indicative of a short duration monazite-(Ce) growth. However, monazite-(Ce) dating demonstrates two successive episodes of growth separated by several million years within two single clefts. Only in one cleft, complex age mixture in a porous and complex zoned monazite-(Ce) suggests disturbance of the 208Pb/232Th ages due to replacement by dissolution-precipitation processes. While some U-Pb ages are coherent with the 208Pb/232Th ages, U-Pb ages are generally disturbed by significant 206Pb excess in monazite-(Ce) with high Th/U ratio (>100). Xenotime-(Y) has remarkably high Th/U ratios and U-Pb dating is also disturbed by 206Pb excess, whereas 208Pb/232Th dating gave well-resolved ages (34.9 ± 0.5 Ma), close to but higher than the monazite-(Ce) age obtained in the same cleft (32.3 ± 0.3 Ma). Correlation of the monazite-(Ce) U-Th-Pb age dataset with other geochronological data suggests for monazite-(Ce) precipitation at periods of high tectonic activity. In the external massifs, monazite-(Ce) dating confirms a polyphased transpressive regime with activity periods around 13–11 Ma and 8–6 Ma. Older monazite-(Ce) ages in the Argentera massif (20.6 ± 0.3 Ma) are consistent with the regional diachronism in the western external Alps. In the 2 clefts of the internal massifs, monazite-(Ce) dating provides first ages of hydrothermal activity: the monazite-(Ce) age at 32.3 ± 0.3 Ma coincides with the exhumation along the Penninic front, but the monazite-(Ce) age at 23.3 ± 0.2 Ma is complex to attribute to a specific deformation stage.

Limestone represents the main raw material for ordinary Portland cement clinker production. In this study eight natural limestones from different geological environments were chosen to prepare raw meals for clinker manufacturing, aiming to define a parameter controlling the burnability. First, limestones were characterized by X-Ray Fluorescence, X-Ray Powder Diffraction and Optical Microscopy to assess their suitability for clinker production and their petrographic features. The average domains size and the microstrain of calcite were also determined by X-Ray Powder Diffraction line profile analysis. Then, each limestone was admixed with clay minerals to achieve the adequate chemical composition for clinker production. Raw meals were thermally threated at seven different temperatures, from 1000 to 1450 °C, to evaluate their behaviour on heating by ex situ X-Ray Powder Diffraction and to observe the final clinker morphology by Scanning Electron Microscopy. Results indicate the calcite microstrain is a reliable parameter to predict the burnability of the raw meals, in terms of calcium silicates growth and lime consumption. In particular, mixtures prepared starting from high-strained calcite exhibit a better burnability. Later, when the melt appears this correlation vanishes; however differences in the early burnability still reflect on the final clinker composition and texture.

¶The epithermal, low sulfidation Emperor gold telluride deposit in Fiji, hosted by Late Miocene-Early Pliocene shoshonitic rocks, is spatially related to a low-grade porphyry Cu system on the western flank of the Tavua Caldera. Gold is largely in the form of “invisible” gold in arsenian pyrite but 10 to 50% of gold is in the form of precious metal tellurides. Gold mineralization occurs in steeply dipping dikes and faults, flat-dipping structures (<45°), referred to locally as “flatmakes,” and at the intersection of two or more structures referred to as “shatter zones.” Petrographic, electron microprobe, and scanning electron microscope analyses of ores from some of the more recently discovered orebodies, Matanagata, Matanagata East, and R1 reveal that tellurium-bearing minerals, sylvanite, calaverite, krennerite, petzite, hessite, coloradoite, melonite, native tellurium, and rare benleonardite, formed during various hydrothermal stages, hosted in quartz, and to a lesser extent arsenian pyrite and tetrahedrite group minerals. Sylvanite followed by krennerite are the two most common tellurides in these orebodies. These tellurides show no systematic spatial distribution within flatmakes but there appears to be a higher concentration of tellurides where the flatmake intersects steep structures. Gold-rich tellurides preceded the formation of silver-rich tellurides and were constrained at 250 °C in log f S2 and log f Te2 space at −12.7 to −10.1 and −9.4 to −7.8, respectively, based on sulfide and telluride stabilities, and the composition of sphalerite. Ore forming components, such as Au, Ag, Te, Cu, V, and S, were likely derived from Late Miocene-Early Pliocene monzonites in and adjacent to the Tavua caldera.

Some types of Allanite (Orthite) are here described. They are found as assessory minerals in the rocks of the Hohe Tauern (penninic zone of the Eastern Alps) and in the hercynian foreland (Bohemian Massif). We distinguish Allanite I, II and III. The Allanite I (Hohe Tauern) is not isotropisized, but currently idiomorphic, with a xenomorphic rim of Clinozoisite. Where Allanite I is bordering Quartz or Potassium feldspar there is generally no rim of Clinozoisite. In the tectonites one observes granulated Allanite I and transformation of Allanite I to finegranied aggregates of Clinozoisite. Allanite II (Hohe Tauern) is found in intensely radioactive rocks. It is currently isotropisized, xenomorphic and surrounded by xenomorphic rims of Pistacite. Allanite III (Bohemian Massif) may be the oldest one and it is mostly isotropisized. The problems of the genesis and the age of crystallisation of Allanite during the history of our polymetamorphic rocks are shortly touched.

Mit Hilfe eines Einkreisgoniometers mit senkrechter Drehachse wurde an Schliffebenen verschiedener Materialien (Glas, Minerale) der Brewstersche Winkel bestimmt. Das mit Polarisationsprismen ausgerüstete Instrument besaß einen sowohl für teleskopischen wie für mikroskopischen Strahlengang verwendbaren Tubus. Eine mechanische Einrichtung bewirkte, daß die reflektierende Fläche in jedem Augenblick senkrecht zur Winkelhalbierenden von Fernrohr und Kollimator stand. Die Erreichung des Polarisationswinkels wird teleskopisch am Auftreten eines dunklen Balkens, mikroskopisch an der minimalen Helligkeit erkannt. Diese Bestimmungsart ist von der Größe des Brechungsquotienten grundsätzlich unabhängig. Im ungedeckten Dünnschliff kann man die mittlere Lichtbrechung der Minerale annähernd bestimmen. Gleichzeitig ergibt sich eine Art Dunkelfeldbeobachtung. Mit Hilfe eines angeschlossenen Photoelementes wurden auch quantitative Messungen durchgeführt. Die erhaltenen Intensitätskurven haben ihr Minimum beim Polarisationswinkel. Die aus diesen Kurven graphisch und rechnerisch abgeleiteten Minimumwerte sind weniger genau als die durch Einstellen des dunklen Balkens bestimmten. Störeffekte entstehen unter anderem durch den Poliervorgang, insbesondere bei Trockenpolitur. Bei Kristallen ist der Fehler im allgemeinen gering, bei Gläsern hingegen meist beträchtlich. Die scheinbar höhere Lichtbrechung von Gläsern könnte vielleicht als Maß für den erzielten Hochglanz verwendet werden. Bei doppeltbrechenden Kristallen ist der Polarisationswinkel gemäß der Höhe der Doppelbrechung für die entsprechenden Schwingungsrichtungen verschieden groß.