Cathodoluminescence (CL) microscopy and spectroscopy are luminescence techniques with widespread applications in geosciences. Many rock-forming and accessory minerals show CL characteristics, which ...can be successfully used in geoscientific research. One of the most spectacular applications is the visualization of growth textures and other internal structures that are not discernable with other analytical techniques. In addition, information from CL imaging and spectroscopy can be used for the reconstruction of processes of mineral formation and alteration to provide information about the real structure of minerals and materials, and to prove the presence and type of lattice incorporation of several trace elements. In the present article, an overview about CL properties of selected minerals is given, and several examples of applications discussed. The presented data illustrate that best results are achieved when luminescence studies are performed under standardized conditions and combined with other analytical techniques with high sensitivity and high spatial resolution.
Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline ...quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups. Most agate occurrences worldwide are related to SiO2-rich (rhyolites, rhyodacites) and SiO2-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (>90 ppm), B (>40 ppm) and U (>20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors.
Textures and whole-rock chemistry, as well as mineral composition, were analyzed in megaspherulites (high-temperature crystallization domains HTCDs) that formed in different geographical and ...geotectonic contexts and during different geological periods (Silver Cliff, CO, USA—Paleogene; El Quevar, Argentina—Miocene; Meissen Volcanic Complex, Germany—Late Carboniferous). All of these megaspherulites have formed exclusively in rhyolitic lava, and their mineral composition is dominated by K-feldspar (sanidine) and SiO
2
phases (quartz, cristobalite, tridymite). All megaspherulites represent composite HTCDs, comprising three zones: inner domain (ID), outer domain (OD), and a marginal domain (MD). Early evolution of megaspherulites is characterized by either central cavities and sector- to full-sphere spherulites or dendritic quartz-sanidine domains. The latter consist of bundles of fibrils each radiating from a single point reflecting relatively high growth rates. A common feature of OD and MD of all three megaspherulite occurrences is autocyclic banding. It mainly comprises fibrous (≤ 100 μm length), radially oriented sanidine and quartz, which formed at a temperature close to glass transition temperature (
T
g
). The termination of megaspherulite growth is marked by centimeter-sized sector-sphere spherulites on the surface. Megaspherulite formation requires limited nucleation, which is probably related to the low phenocryst content of the hosting lava. Latent heat from overlying crystallizing lithoidal rhyolite maintained low undercooling conditions keeping nucleation density low and facilitating high diffusion and growth rates. Late megaspherulite growth and its termination under low diffusion conditions is controlled by cooling close to
T
g
. Calculations based on literature data suggest that the megaspherulite growth presumably lasted less than 60 years, perhaps 30 to 40 years.
The present study aimed to investigate the genesis and characteristics of some of the world-famous agate deposits in the state of Chihuahua, Mexico (Rancho Coyamito, Ojo Laguna, Moctezuma, Huevos del ...Diablo, Agua Nueva). Geochemical and textural studies of host rocks showed that all the studied deposits are related to the same rock type within the geological unit of Rancho el Agate andesite, a quartz-free latite that shows clear indications of magma mixing. As a result of their large-scale distribution and various differentiation processes, as well as transport separation, different textures and local chemical differences between rocks of different localities can be observed. These differences have also influenced the properties of SiO2 mineralization in the rocks. The mixing of near-surface fluids from rock alterations with magmatic hydrothermal solutions led to the accumulation of various elements in the SiO2 matrix of the agates, which were, on the one hand, mobilized during secondary rock alteration (Fe, U, Ca, K, Al, Si) and, on the other hand, transported with magmatic fluids (Zn, Sb, Si, Zr, Cr). Different generations of chalcedony indicate a multi-stage formation as well as multiple cycles of filling the cavities with fluids. The hydrothermal fluids are presumably related to the residual solutions of a rhyolitic volcanism, which followed the latitic extrusions in the area and probably caused the formation of polymetallic ore deposits in the Chihuahua area. The enrichment of highly immobile elements indicates the involvement of volatile fluids in the agate formation. The vivid colors of the agates are almost exclusively due to various mineral inclusions, which consist mainly of iron compounds.
The occurrence and formation of silicified wood from five late Paleozoic basins in Central Europe was investigated. Fossil wood from diverse geological settings was studied using field observations, ...taphonomic determinations as well as mineralogical analyses (polarizing microscopy, cathodoluminescence (CL) microscopy and spectroscopy). The results indicate that silicification is either a monophase or multiphase process under varying physico-chemical conditions. In particular, CL studies revealed complex processes of silica accumulation and crystallization. The CL characteristics of quartz phases in silicified wood can mostly be related to blue (390 and 440 nm), yellow (580 nm), and red (650 nm) emission bands, which may appear in different combinations and varying intensity ratios. Yellow CL is typical for initial silicification, reflecting quick precipitation under oxygen-deficient conditions caused by initial decay of the organic material. Blue CL is predominantly of secondary origin, resulting from replacement of precursor phases by a secondary hydrothermal quartz generation or subsequent silicification of wood. The red CL can be related to a lattice defect (non-bridging oxygen hole center—NBOHC).
We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone ...specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g−1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g−1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.
Nous présentons et proposons à la communauté le zircon M257 qui pourrait devenir un matériau de référence pour la détermination des âges U‐Pb sur zircon par spectrométrie de masse à ions secondaires (SIMS). Ce spécimen scié d'une gemme brun clair, sans defauts, provenant du Sri Lanka, pèse 5.14 g (25.7 carats). Les moyennes des rapports isotopiques (et les incertitudes associées 2s) mesurées par TIMS sur ce zircon M257 sont 0.09100 ± 0.00003 en 206pb/238U et 0.7392 ± 0.0003 en 207pb/235U. Son âge 206pb/238U est de 561.3 ± 0.3 Ma (moyenne non pondérée, incertitude: 95% intervalle de confiance); le système U‐Pb y est concordant, aux erreurs sur les constantes de désintégration près. Le zircon M257 contient ∼ 840 μg g−1 U (Th/U ∼ 0.27). Ce matériel montre de très faibles hétérogénéités, avec une quasi absence de textures internes, même a I'examen par cathodoluminescence. Le degrê de dommage lié aux radiations (estimé par I'expansion des paramètres de la cellule unitaire, I'élargissement des paramètres des spectres Raman et la densité) est modéré et uniforme. II se corrèle bien, (sur I'alignement “Sri Lanka”) avec les concentrations en actinides, I'âge U‐Pb et la fluence calculée de 1.66 × 1018 g−1 Ceci, coupléà son âge (U+Th)/He de 419 ± 9 Ma (2s), nous permet d'exclure I'existence d'événement thermique passé, ou de traitement par la chaleur, qui aurait potentiellement pu affecter le processus de rétention de Pb radiogénique. Le rapport isotopique de I'oxygène de ce zircon est 13.9%o VSMOW, suggérant une génèse par métamorphisme, au sein d'un marbre ou d'un skarn calco‐silicaté.
Quartz of metamorphic rocks from the Kaoko belt (Namibia) representing metamorphic zones from greenshist to granulite facies were investigated by cathodoluminescence (CL) microscopy and spectroscopy ...to characterize their CL properties. The samples cover P-T conditions from the garnet zone (500 ± 30 °C, 9 ± 1 kbar) up to the garnet-cordierite-sillimanite-K-feldspar zone (750 ± 30 °C, 4.0–5.5 kbar). Quartz from 10 different localities and metamorphic environments exclusively exhibits blue CL. The observed CL colors and spectra seem to be more or less independent of the metamorphic grade of the host rocks, but are determined by the regional geological conditions. Quartz from different localities of the garnet-cordierite-sillimanite-K-feldspar zone shows a dominant 450 nm emission band similar to quartz from igneous rocks, which might be related to recrystallization processes. In contrast, quartz from different metamorphic zones in the western part of the central Kaoko zone (garnet, staurolite, kyanite, and kyanite-sillimanite-muscovite zone) is characterized by a heterogeneous blue-green CL and a dominant 500 nm emission band that strongly decreases in intensity under electron irradiation. Such CL characteristics are typical for quartz of pegmatitic and/or hydrothermal origin and indicate the participation of fluids during neoformation of quartz during metamorphism.
Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming ...processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl-silicate complexes), in the agates confirm their contemporaneous supply with SiO2 and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO2 with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm−1 and moganite at 502 cm−1, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply.
The growth in volatile renewable energy (RE) generation is accompanied by an increasing network load and an increasing demand for storage units. Household storage systems and micro power plants, in ...particular, represent an uncertainty factor for distribution networks, as well as transmission networks. Due to missing data exchanges, transmission system operators cannot take into account the impact of household storage systems in their network load and generation forecasts. Thus, neglecting the increasing number of household storage systems leads to increasing forecast inaccuracies. To consider the impact of the storage systems on forecasting, this paper presents a new approach to calculate a substation-specific storage forecast, which includes both substation-specific RE generation and load forecasts. For the storage forecast, storage systems and micro power plants are assigned to substations. Based on their aggregated behavior, the impact on the forecasted RE generation and load is determined. The load and generation are forecasted by combining several optimization approaches to minimize the forecasting errors. The concept is validated using data from the German transmission system operator, 50 Hertz Transmission GmbH. This investigation demonstrates the significance of using a battery storage forecast with an integrated load and generation forecast.
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