Rolf D. Neuser

Crystallographic texture and microstructure of terebratulide brachiopod shell calcite : An optimized materials design with hierarchical architecture

Abstract We analyzed the microstructure, microchemistry, and microhardness variations across the architectural elements of the shells of the brachiopod species Megerlia truncata and Terebratalia transversa with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS), and Vickers microhardness indentation (VMHI). The brachiopod valves consist of two principal layers of distinct calcite biomineralization: a thin, nanocrystalline, outer, hard protective layer with VMHI values exceeding 200 HV and a much thicker, inner, secondary layer of a hybrid organic-inorganic fiber composite material. The secondary layer is further structured into two sublayers, an outer part with VMHI values varying between 110 and 140 HV, and a softer inner part (70 < HV < 110). Whereas the size of the calcite crystals within the primary layer varies between a few tens of nanometers and 2 μm, calcite crystals within the secondary layer are fibrous, commonly reaching lengths exceeding 150 μm. Cross sections of these fibrous crystals are spade shaped, their dimensions being about 5 × 20 μm. The fibers are aligned parallel to each other. They are single crystals with their morphological fiber axes pointing almost parallel to the shell vault. The crystallographic orientation of the morphological fiber axes, however, is arbitrary within the a-b plane of the calcite lattice, whereas the c-axis (hexagonal unit-cell setting) is perpendicular to the morphological fiber axes and thus parallel to the radial vector of the valve vault. This morphology strongly indicates that fibrous growth is controlled by confinement within a cell in an organic matrix and not by attachment of biomolecules to specific crystallographic faces. We observe inhomogeneous Sr2+ and Mg2+ concentrations in the shell calcite within the 0.1.0.9 wt% range. Design of the shell appears to be highly optimized for mechanical performance. Crystal morphology and orientation as well as incorporated organic matter are structured hierarchially at different length levels forming a hybrid organic-inorganic fiber composite architecture.

Nature and Nurture: Environmental Isotope Story of the River Rhine

The total dissolved load of the Rhine river increases downstream due to the combined impact of natural and pollution loads. The natural background, controlled by geology, is soon swamped by pollution, particularly from salt and coal mining activities in Alsace and the Ruhr area. The Rhine is also a net source of CO2 from oxidation of excess organic productivity that is fuelled by nutrient pollution, a problem even more serious for some of its tributaries.

LOW LIMIT OF MN2+-ACTIVATED CATHODOLUMINESCENCE OF CALCITE : STATE OF THE ART

In the literature, the lower limit for Mn2+-activated cathodoluminescence (CL) of calcite is variously reputed to over a very wide range of values above 10 ppm Mn. Our spectroscopic investigations of the CL response in natural calcite reveal that below 10 ppm manganese content Mn2+-activation is also present. Using the Quantitative High Resolution Spectral analysis of CL (QHRS-CL) an activation by Mn2+ in the range of 700 ppb is proved, which cannot be determined visually. So, if not quenched, the minimum Mn2+ content for Mn2+-activation is one atom in the irradiated calcite crystal lattice volume. As the intrinsic (background blue) luminescence is used to determine non-altered biogenic calcite, the limit of Mn2+-activation plays an important role in the interpretation of diagenetic processes. Our results of spectroscopic analyses require a revision of current opinions about the diagenesis of calcite as revealed by CL investigation.

UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan New insights using cathodoluminescence petrography

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

REE-activated cathodoluminescence of calcite and dolomite: high-resolution spectrometric analysis of CL emission (HRS-CL)

Abstract Cathodoluminescence (CL) investigations of Phanerozoic marine limestones, sinter calcites and saddle dolomites of hydrothermal veins indicate that in contrast to previous knowledge rare earth element (REE)-activated CL in sediments is common. The methods of high-resolution spectrometric analysis of CL emission (HRS-CL) combined with ‘hot-cathode’ CL microscopy are able to prove qualitatively some REE below the detection limits of electron microprobe and proton-induced X-ray emission analysis (PIXE). Our investigations document Sm3+, Dy3+, Tb3+ and probably Ho3+-activated CL in calcite and Eu3+-activated CL in a saddle dolomite of hydrothermal veins. Sometimes the REE-activated CL is hidden by dominant Mn 2+ emission. In such cases the REE emission spectrum may be obtained by subtracting a spectrum of luminescence produced only by Mn2+. The occurrence of only one REE in natural samples is uncommon. In all observed samples REE are present in groups.

Hierarchical fibre composite structure and micromechanical properties of phosphatic and calcitic brachiopod shell biomaterials – an overview

Abstract Brachiopods are a phylum of shell-forming sessile marine invertebrates which have existed since the early Cambrian. Two very different biomaterial design strategies for their shells evolved early in their history. Both strategies use hybrid fibre composites, however, one is based on mineral fibres embedded in ~2 wt.% of organic biopolymer sheaths and the inorganic fibres are calcite single crystals. In the second strategy the fibres are biopolymers and are reinforced with Ca-phosphate nanoparticles to form a fibrous nanocomposite. Here the organic component (chitin) dominates. The Ca-phosphate nanoparticle-reinforcement strategy is not unlike that in vertebrate bone, however, the microscale structure is laminated with alternating laminae which have a different degree of mineralization. The calcitic shells feature an outer compact layer of calcite micro- and nanoparticles protecting the inner fibrous layer from the outside. Transmission electron microscopy of the fibrous layer reveals intercrystalline and intracrystalline biopolymers. The calcitic shell material is stiff with nano-indentation E-moduli of 63±8 GPa and relatively hard (Vickers microhardness up to 400 HV 0.0005/10 and nanohardness 4±0.5 GPa). Compared to inorganic calcite the microhardness is doubled and the nanohardness increases by 60%. We attribute this increased hardness to intracrystalline biopolymers. The nano-indentation E-moduli of the chitinophosphatic shells range from 3 to 55 GPa as a result of the varying degree of mineralization between their laminae, and similarly their nanohardness varies between 0.1 and 3 GPa. For brachiopods burrowing inside the sediment, the alternation of non-mineralized laminae with thin, more strongly mineralized laminae provides abrasion-resistance, hardness and longitudinal stiffness while it preserves the flexibility provided by the organic component for bending movements.

Calcite morphology, texture and hardness in the distinct layers of rhynchonelliform brachiopod shells

We have investigated the texture and shell microstructure together with the individual hardness distribution patterns of recent calcitic brachiopods of the species Kakanuiella chathamensis, Liothyrella uva and Liothyrella neozelanica. One of the most distinctive features of the studied species is the number of layers that compose the shell. Kakanuiella chathamensis is built entirely of nano- to microcrystalline primary layer calcite. Liothyrella uva contains a nanocrystalline outer primary layer and an inner fibrous secondary layer. Liothyrella neozelanica is composed of three layers, a nanocrystalline outer primary layer, a columnar secondary and an innermost fibrous tertiary shell layer. Even though Kakanuiella chathamensis consists only of primary layer material we observe some textural features and a pattern in the distribution of hardness within the shell. The texture of Liothyrella uva and of Liothyrella neozelanica is significantly more defined than that of Kakanuiella chathamensis. Within the valves calcite crystal c-axes are perpendicular to and rotate accordingly with the shell vault. In contrast to the valves, a multimodal c-axis distribution pattern is present within the hinge region. The hardness distribution in Liothyrella neozelanica and Liothyrella uva is such that the outermost part of the shell is hard while the innermost shell portion is soft. In general, Liothyrella uva is significantly harder than Liothyrella neozelanica and Kakanuiella chathamensis, even though Kakanuiella chathamensis contains only primary layer calcite.

High resolution cathodoluminescence combined with SHRIMP ion probe measurements of detrital zircons

Abstract Cathodoluminescence (CL) microscopy and spectroscopy combined with SHRIMP ion probe measurements were carried out on detrital zircons from the Cretaceous Weferlingen quartz sand (Germany) to distinguish and characterize different zircon populations. Investigations by CL microscopy, SEM-CL and BSE imaging show that there are three main types of zircons (general grain sizes of 100-200 μm): (1) apparently weakly zoned, rounded grains with relict cores, (2) well rounded fragments of optically more or less homogeneous zircon grains showing CL zoning predominantly parallel to the z-axis, and (3) idiomorphic to slightly rounded zircon grains typically showing oscillatory euhedral CL zoning. A fourth type of low abundance is characterized by well-rounded grain fragments with an irregular internal structure showing bright yellow CL. High-resolution CL spectroscopic analyses reveal that blue CL is mainly caused by an intrinsic emission band centered near 430 nm. Dy3+ is the dominant activator element in all zircons, whereas Sm3+, Tb3+, Nd3+ have minor importance. Yellow CL (emission band between 500 and 700 nm) is probably caused by electron defects localized on the [SiO4] groups (e.g. related to oxygen vacancies) or activation by Yb2+ generated by radiation. Variations of the integral SEM-CL intensity are mainly controlled by the intensity of the broad bands and the Dy3+ peaks. SHRIMP analysis provides in situ high-resolution U-Pb dating of single zircon grains and confirms different ages for the evaluated different zircon types. The measurements show that the U-Pb ages of the zircons from Weferlingen scatter over a wide range (340 to 1750 Ma), backing up earlier conclusions that the quartz sand from Weferlingen is quite heterogeneous in terms of provenance.

Cement stratigraphy in Triassic and Jurassic limestones of the Weserbergland (northwestern Germany)

Abstract Cement stratigraphy of the Korallenoolith (Oxfordian) and Trochitenkalk (Upper Muschelkalk) Formations, southern Lower Saxony Hills of Germany, is based on investigation of 232 carbonate samples by cathodoluminescence (CL). This enables subdivision of cements into four main generations: Generation 1, consisting of fine columnar, equant and syntaxial cements with blotchy CL and/or microdolomite inclusions, that is interpreted as originally submarine Mg calcitic precipitate. Generation 2, found in Jurassic samples only. These cements show intrinsic to non-luminescent CL with thin bright orange subzones and they are interpreted as meteoric phreatic calcites. Generation 3.1, characterized by zoned calcites with variable CL colours. These cements precipitated under shallow burial conditions, in a phreatic environment during an incipient stage of late diagenesis. Generation 3.2, with relatively uniform orange CL, indicative of precipitation in a stable chemical environment. This cement generation is interpreted as late diagenetic deep burial calcites. Generation 3.3, partially strongly zoned and with a characteristically intense CL contrast. It has been identified in only 17 of the 82 Trochitenkalk samples. Deformation twins end within this generation, indicating a post-tectonic genesis for the subsequent younger cement zones. The cements were probably precipitated in a near-surface environment characterized by reducing or variable Eh conditions. Generation 4, consisting of calcites that show intrinsic CL with bright orange subzones comparable to generation 2, but lacking deformation twins. These cements are interpreted as post-tectonic late diagenetic products of a meteoric phreatic environment. Based on detailed CL petrography, fifteen diagenetic events (three calcite cements, three dolomite generations, three fissure generations, two stages of dedolomitization, aragonite dissolution, matrix recrystallization and two stages of HMC → LMC transformation) can be distinguished and timed relative to cement zonation. The progressively decreasing δ18O values for cement generations 1 to 3.2 likely reflect increasing temperatures caused by burial. The low δ18C values of cement generation 4 are probably a reflection of a meteoric diagenetic environment. A time—burial—cementation pathway can be modelled combining CL patterns, cement isotopic data and the subsidence history of the study area. The main features of this model are: 1. (1) Generation 1 cements are submarine precipitates and their age therefore coincides with sedimentation. 2. (2) Cement generation 3.1 of the Trochitenkalk commenced precipitation at a depth of about 120 m, reached during Late Triassic time. In case this minimum depth is also the formation depth of generation 3.1 in the Korallenoolith Formation, then precipitation of these cements started during Late Jurassic time. 3. (3) Cement generation 3.2 of both the Trochitenkalk and the Korallenoolith Formation, precipitated at assumed depths > 1000 m, reached during the Doggerian and at the time of Jurassic/Cretaceous transition, respectively. 4. (4) Cement generation 4 precipitated after the sediments had undergone telogenesis, and must thus be younger than the Intra-Senonian (Sub-Hercynian) tectonic phase and possibly even younger than the Paleocene tectonic pulse (Laramian Phase).

Micro-PIXE and quantitative cathodoluminescence spectroscopy: Combined high resolution trace element analyses in minerals

Abstract We combined high resolution Cathodoluminescence (CL)-spectroscopy and micro-PIXE to study the correlation of the activator concentration and the CL-intensity. Based on these results the Quantitative High Resolution Spectral analysis of Cathodoluminescence (QHRS-CL) is developed. Micro-PIXE and the new method (QHRS-CL) have been used to investigate trace elements in minerals. Using micro-PIXE and related methods the crystal lattice site and charge state of the analysed elements cannot be determined. This can be analysed exactly by using QHRS-CL. So the combination of micro-PIXE and QHRS-CL is a powerful tool for analysing trace element concentration above 100 ppb, the charge state and the lattice site of these elements in crystal structures.