ALBERT

Description: Opal has long fascinated scientists. It is one of the few minerals with an amorphous structure, and yet, compared to silica glass, it is highly organized on the mesoscale. By means of inelastic neutron scattering (INS), we could document that in four samples of opal at low temperature an ice-like structure of water is present, with details depending on microstructural characteristics. While FTIR spectra for all samples are nearly identical and thus not very informative, INS shows clear differences, highlighting the significance of microstructures. Neutron diffraction at 100 K on one of the opal samples provides evidence for crystalline cubic ice.

Description: Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mossbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mossbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/{Sigma}Fe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

Description: Diagenetic carbonates, metamorphic carbonates, primary hydrothermal carbonates, and secondary remobilized carbonates (including sinters) from the Schwarzwald ore district in SW Germany formed in various tectonic settings and hydrothermal environments over a period of almost 300 Ma. They were investigated in order to define sources of carbon, dispersion of carbon during fluid-rock interaction processes and, where possible, to specify geochemical fingerprints for carbonates formed during different processes and in different geochemical and tectonic environments. For this purpose, 335 samples of calcite, ankerite, dolomite, siderite, and strontianite from 92 localities in 46 mining areas in the Schwarzwald were analyzed for their carbon and oxygen stable isotope, radiogenic strontium isotope, and trace element (including REE) concentrations and compared to analyses from all potential carbon sources available in this region. These include graphite and rare marbles of the crystalline basement, Permian calcrete from redbed sedimentary rocks (Rotliegend) overlying the crystalline basement, and Triassic carbonates from sediments of higher stratigraphic levels (Muschelkalk). Hydrothermal carbonates mostly formed due to fluid-mixing of hot ascending brines with cool sediment-sourced formation water. Fluid inclusions record temperatures of formation between 100 and 150 °C for most primary calcites. The mixed fluid from which they formed was a highly saline brine of around 25 wt.% salinity, containing NaCl and CaCl 2 in similar proportions. Before mixing, the deep brine was in equilibrium with graphite of the basement and contained, as main carbon species, H 2 CO 3 of very low C-isotopic values [around –16, Vienna Pee Dee Belemnite (V-PDB)], whereas the sediment-sourced formation water contained HCO 3 – with higher C-isotopic values (around +2, V-PDB). We find that graphite and Triassic carbonates in variable proportions (which are mainly related to variations during the fluid mixing process) are the carbon sources for primary calcite, dolomite of the Permian calcrete for primary ankerite, and the Triassic carbonate sediments for the primary ankerite mineralization of the area between Waldkirch and Feldberg. At some localities, remobilization and reprecipitation appears to have taken place without addition of external solutes, as Sr and C show no difference in their isotopic composition between primary and secondary carbonates. The oxygen isotopic composition of secondary carbonates is invariably more positive than that of primary ones, reflecting lower formation temperatures. One very conspicuous type of secondary calcite, which forms olive-green stubby scalenohedra, was dated for the first time using the U-Pb isochron method. Its Neogene age represents uplift and erosional denudation of the Schwarzwald and corresponds well with its remobilized C and O isotope signature. The carbonates in the Schwarzwald hence reflect discontinuous addition of carbon from surface sediments to the crystalline basement through time involving fluid-rock interaction and fluid mixing processes.

Description: The Cretaceous Mont Saint-Hilaire complex (Quebec, Canada) comprises three major rock units that were emplaced in the following sequence: (I) gabbros; (II) diorites; (III) diverse partly agpaitic foid syenites. The major element compositions of the rock-forming minerals, age-corrected Nd and oxygen isotope data for mineral separates and trace element data of Fe–Mg silicates from the various lithologies imply a common source for all units. The distribution of the rare earth elements in clinopyroxene from the gabbros indicates an ocean island basalt type composition for the parental magma. Gabbros record temperatures of 1200 to 800°C, variable silica activities between 0·7 and 0·3, and f O2 values between –0·5 and +0·7 (log FMQ, where FMQ is fayalite–magnetite–quartz). The diorites crystallized under uniform a SiO2 ( a SiO2 = 0·4–0·5) and more reduced f O2 conditions (log FMQ ~ –1) between ~1100 and ~800°C. Phase equilibria in various foid syenites indicate that silica activities decrease from 0·6–0·3 at ~1000°C to 〈0·3 at ~550°C. Release of an aqueous fluid during the transition to the hydrothermal stage caused a SiO2 to drop to very low values, which results from reduced SiO 2 solubilities in aqueous fluids compared with silicate melts. During the hydrothermal stage, high water activities stabilized zeolite-group minerals. Fluid inclusions record a complex post-magmatic history, which includes trapping of an aqueous fluid that unmixed from the restitic foid syenitic magma. Cogenetic aqueous and carbonic fluid inclusions reflect heterogeneous trapping of coexisting immiscible external fluids in the latest evolutionary stage. The O and C isotope characteristics of fluid-inclusion hosted CO 2 and late-stage carbonates imply that the surrounding limestones were the source of the external fluids. The mineral-rich syenitic rocks at Mont Saint-Hilaire evolved as follows: first, alkalis, high field strength and large ion lithophile elements were pre-enriched in the (late) magmatic and subsequent hydrothermal stages; second, percolation of external fluids in equilibrium with the carbonate host-rocks and mixing processes with internal fluids as well as fluid–rock interaction governed dissolution of pre-existing minerals, element transport and precipitation of mineral assemblages determined by locally variable parameters. It is this hydrothermal interplay between internal and external fluids that is responsible for the mineral wealth found at Mont Saint-Hilaire.

Description: Recrystallized zircons extracted from intermediate granulites of the Letseng-la-Terae Diamond Mine in northeastern Lesotho provide evidence that the Tugela Terrane of the Mesoproterozoic high-grade Namaqua-Natal mobile belt may extend ~200 km farther west than previously thought. The rock-forming minerals in the investigated lower crustal granulites contain garnet, clinopyroxene, plagioclase and quartz or garnet, wollastonite, plagioclase and quartz. Calculated values for peak granulite-facies metamorphism are ~13 kbar and ~850°C. Thermal overprint and therein granulite-facies metamorphism occurred during the late Mesoproterozoic and is documented by SHRIMP metamorphic zircon ages of 1018 ± 18, 1126 ± 10 and 1127 ± 6 Ma. These ages are in good agreement with the timing of other events in the Namaqua-Natal belt. High-grade metamorphism was probably caused by crustal thickening resulting from continent-arc collision, followed by underplating of hot mantle melts.

Description: Tridacna derasa shells show a crossed lamellar microstructure consisting of three hierarchical lamellar structural orders. The mineral part is intimately intergrown with 0.9 wt% organics, namely polysaccharides, glycosylated and unglycosylated proteins and lipids, identified by Fourier transform infrared spectrometry. Transmission electron microscopy shows nanometre-sized grains with irregular grain boundaries and abundant voids. Twinning is observed across all spatial scales and results in a spread of the crystal orientation angles. Electron backscatter diffraction analysis shows a strong fibre texture with the [001] axes of aragonite aligned radially to the shell surface. The aragonitic [100] and [010] axes are oriented randomly around [001]. The random orientation of anisotropic crystallographic directions in this plane reduces anisotropy of the Young's modulus and adds to the optimization of mechanical properties of bivalve shells.

Description: We report results of a laser-ICP-MS investigation of trace element contents in the main constituent minerals of an amphibole-bearing migmatite from the Variscan orogen in northeastern Sardinia. The migmatite is associated with migmatised orthogneiss and Al-silicate-bearing pelitic migmatites. The protolith of the amphibole-bearing migmatite was a mid-Ordovician igneous rock of intermediate composition characterised by a biotite + plagioclase + quartz assemblage. The migmatite consists of mesosomes and tonalitic (or, less frequently, granodioritic) leucosomes, characterised by amphibole crystals (potassian ferropargasite) up to 2 cm in size. The tonalitic leucosomes are made up of quartz, plagioclase, ±K-feldspar, biotite, ±amphibole, garnet. The mesosomes are foliated rocks made up of the same minerals with different modal proportions. In leucosomes, amphibole is the most abundant mafic mineral, occurring as euhedral crystals rich in plagioclase, quartz, and small garnet inclusions. Garnet occurs as corroded and fractured grains in the matrix or within the amphibole. Zircon forms euhedral bipyramidal grains up to a few hundreds of micrometres in size. Some amphibole rims have higher REE and negative Eu anomalies whereas cores exhibit lower REE and positive Eu anomalies. Garnet has strongly fractionated REE patterns with chondrite-normalised abundances up to 2000 for HREE. Plagioclase has flat REE patterns with pronounced positive Eu anomalies. Zircon displays fractionated REE patterns with HREE enrichment, LREE depletion, positive anomalies for Ce and negative ones for Eu. Monazite shows high REE abundances, LREE enrichment, HREE depletion and negative Eu anomalies. Garnet is mostly a restitic phase, as indicated by significant variation in HREE concentrations between grains in the mesosome, the absence of a noticeable Eu anomaly, and Y depletion in the leucosomes as compared to the mesosomes. In the leucosomes and mesosomes, the cores of zoned amphibole are characterised by positive Eu anomalies: these crystallised from or in the presence of melt produced by anatexis of the original Bt + Pl + Qtz protolith. Adjacent rims with negative Eu-anomalies developed in coexistence with a Eu-depleted melt that had experienced plagioclase fractionation.

Description: Vaterite and aragonite polymorphs in freshwater cultured pearls from mussels of the genus Hyriopsis (Unionidae) were structurally and compositionally characterized by Raman spectroscopy, Micro computer tomography, high resolution field emission scanning electron microscopy, electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry. The appearance of vaterite in pearls is related to the initial stages of biomineralization, although we demonstrate that vaterite can not be a precursor to aragonite. It is not related to a particular crystal habit and therefore does not have a structural functionality in the pearls. Larger contents of elements typically bound to organic molecules, such as P and S in vaterite, as well as larger total organic contents in vaterite as opposed to aragonite in conjunction with larger concentrations of Mn2+ and Mg2+, imply a stabilizing role of organic macromolecules and X2+ ions for biological vaterite. Distribution coefficients between aragonite and vaterite for provenance-independent elements, such as Mn and Mg (0.27 and 0.04, respectively) agree very well with those observed in fish otoliths.