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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.

Mineralogische und geochemische Untersuchungen an Proben des Leisniger Porphyrs sowie von Gangporphyren, Pechsteinen und Lithophysen (Hochtemperatur-Kristallisationsdomänen) im entsprechenden Verbreitungsgebiet wurden durchgeführt, um eine mineralogisch-petrographische Charakteristik dieser vulkanischen Bildungen und deren Zuordnung zu möglichen vulkanischen Edukten zu erarbeiten. Phasenanalytische Untersuchungen (Röntgendiffraktometrie, Mikroskopie, Kathodolumineszenz - KL) erbrachten eine monotone Mineralzusammensetzung, die von Quarz, den beiden Kalifeldspat-Phasen Sanidin und Orthoklas sowie Biotit dominiert ist. Anhand der geochemischen Charakteristika (Röntgenfluoreszenzanalyse - RFA) lassen sich die untersuchten Vulkanite überwiegend als Rhyolithe einstufen. Das Vorkommen von hypidiomorphen Quarzphänokristen, z.T. mit Einbuchtungen, gut erhaltenen Kalifeldspäten, sowie das Fehlen von Glas-Shards und Fiamme (Bimsfetzen) deuten darauf hin, dass es sich bei den vulkanischen Ablagerungen im Untersuchungsgebiet um keine Pyroklastite (Ignimbrite) handelt. Neuere Untersuchungen (Rehda 2018) gehen davon aus, dass es sich bei den Gesteinen nicht um klassische Fließlaven handelt, sondern um einen Lakkolith, der sich unter den Rochlitz-Ignimbrit eingeschoben hat. Damit müsste auch die bisherige stratigraphische Einordnung des Leisniger Porphyrs in die KohrenFormation korrigiert werden und eine Einordnung in die frühe Oschatz-Formation erfolgen. Die deutlichen Übereinstimmungen aller Gesteinstypen hinsichtlich mineralogischer Zusammensetzung, chemischer Charakteristik, Ausbildung der Phänokristen sowie deren KL-Eigenschaften deuten auf die Herkunft aus einem gemeinsamen Magmenreservoir hin. Auftretende Unterschiede sind im Wesentlichen auf sekundäre Alterationen zurückzuführen. Allerdings zeigen Geländebefunde und vertikal ausgerichtete Gefüge in Pechsteinen und Gangporphyren, dass diese vulkanischen Bildungen den eigentlichen Leisniger Porphyr wahrscheinlich zu einem späteren Zeitpunkt in Form vertikaler Gangstrukturen durchschlagen haben. Aufgrund der mineralogischen und geochemischen Ähnlichkeiten der Kristallisationsdomänen der Lithophysen und aller Gesteinstypen im Verbreitungsbereich des Leisniger Porphyrs kann bisher keine eindeutige Zuordnung der Lithophysen- und Achatbildungen zu einer speziellen Gesteinsgruppe erfolgen. Aus Geländebefunden ist eine Häufung in Kreuzungsbereichen von Gangporphyren und Pechsteinen mit dem eigentlichen Leisniger Porphyr zu beobachten, was allerdings im Anstehenden oder in Bohrproben nicht direkt nachgewiesen werden konnte. Die Bildung der Lithophysen im Bereich einer glasigen Fazies konnte ebenfalls bisher nicht bestätigt werden, da keine randlichen Anhaftungen von Pechsteinrelikten an den Lithophysen (wie im Fall des Kemmlitzer Porphyrs) beobachtet wurden. Auf Grund spezifischer textureller und mineralogischer Unterschiede kann allerdings eine Zuordnung der Lithophysen zum Seifersdorfer Porphyr im Liegenden bzw. zum Rochlitz-Ignimbrit im Hangenden ausgeschlossen werden. Mineralogical and geochemical investigations were carried out on samples of the Leisnig porphyry including certain vein-like porphyries, pitchstones and lithophysae (HTCD - high temperature crystallization domains) in the distribution area to get information about mineralogical and petrographical characteristics of the volcanic rocks and their origin. Analytical results of X-ray diffraction, microscopy and cathodoluminescence (CL) revealed a monotonous mineral composition dominated by quartz, orthoclase/sanidine and biotite. The volcanic rocks can be classified as rhyolite according to geochemical characteristics (X-ray fluorescence – XRF). Hypidiomorphic quartz phenocrysts, partially with embayments, well preserved K-feldspar, and the absence of glass shards and fiamme disclose a pyroclastic formation (ignimbrite) of the rocks of the Leisnig porphyry in contrast to the Rochlitz ignimbrite in the same area. Recent results of Rehda (2018) indicate that the volcanic rocks of the Leisnig porphyry were not formed by flowing lava, but represent a laccolith that emplaced below the volcanic rocks of the Rochlitz ignimbrite. This consideration would result in a stratigraphic position of the Leisnig porphyry in the early Oschatz Formation instead of the previously assumed position in the Kohren Formation. The common mineralogical and chemical composition of all rock types, as well as similar characteristics of phenocrysts including their CL properties, indicate an origin from the same magma chamber or at least a melt with similar composition. Detected differences can mainly be related to secondary alteration processes. Field observations and vertically oriented textures in pitchstones and vein porphyries show that these volcanic rocks subsequently penetrated the Leisnig porphyry as vertical vein structures. Up to now, no unambiguous assignment of the lithopysae and associated agates can be done to one of the rock types in the distribution area of the Leisnig porphyry because of their similarities in mineralogy and geochemistry with the crystallization domains of the lithophysae. Based on field observations, a preferred occurrence of lithophysae in cross areas of vein porphyries and pitchstones with the Leisnig porphyry is assumed. However, this assumption could not be proven directly in outcrops or drilling samples. Although the formation of lithophysae in a glassy facies is most likely (as it was proven for the Kemmlitz porphyry), this assumption could not be confirmed due to the lack of adherent pitchstone relics on the lithophysae. However, mineralogical and textural differences to the underlying Seifersdorf porphyry and the overlying Rochlitz ignimbrite disclose an assignment of the lithophysae in the area of the Leisnig porphyry to these volcanic rocks.

Large single crystals of GdCa₄O(BO₃)₃ (space group Cm) were grown by the Czochralski method. Dielectric, piezoelectric and elastic coefficients at room temperature as well as specific heat capacity, thermal expansion and cation disorder were studied employing a variety of methods including resonant ultrasound spectroscopy, differential scanning calorimetry, dilatometry and X-ray diffraction techniques. The electromechanical parameters (4 dielectric, 10 piezoelectric and 13 elastic stiffness coefficients) obtained on different samples are in excellent agreement indicating high internal consistency of our approach, whereas the values reported in literature differ significantly. The elastic behaviour of GdCa₄O(BO₃)₃ resembles the one of structurally related fluorapatite, i.e. the elastic anisotropy is relatively small and the longitudinal effect of the deviations from Cauchy-relations exhibit a pronounced minimum along the direction of the dominating chains of cation polyhedra. GdCa₄O(BO₃)₃ exhibits a maximum longitudinal piezoelectric effect of 7.67 × 10−12 CN−10, a value in the order of that of langasite-type materials. Significant changes of the calcium/gadolinium distribution on the 3 independent cation sites accompanied by characteristic anomalies of heat capacity and thermal expansion suggest processes of nonconvergent cation ordering above about 900 K in GdCa₄O(BO₃)₃.

Kurzfassung Mineralogische und geochemische Untersuchungen an Proben des Kemmlitzer Porphyrs sowie von Lithophysen (Hochtemperatur- Kristallisationsdomänen) im entsprechenden Verbreitungsgebiet wurden durchgeführt, um eine mineralogisch-petrographische Charakteristik dieser vulkanischen Bildungen und deren Zuordnung zu möglichen Muttergesteinen zu ermöglichen. Phasenanalytische Untersuchungen (Röntgendiffraktometrie, Mikroskopie, Kathodolumineszenz - KL) erbrachten eine monotone Mineralzusammensetzung, die von Quarz und den beiden Kalifeldspatphasen Sanidin und Orthoklas dominiert ist. Anhand der geochemischen Charakteristika (RFA) lassen sich die untersuchten Vulkanite als Rhyolithe einstufen. Das Vorkommen von hypidiomorphen Quarzphänokristen, z.T. mit Resorptionserscheinungen, gut erhaltenen Kalifeldspäten sowie das Fehlen von Glas-Shards und Fiamme (Bimsfetzen) deuten darauf hin, dass es sich bei den vulkanischen Ablagerungen im Untersuchungsgebiet um Laven und keine Pyroklastite (Ignimbrite) handelt. Aus den deutlichen Übereinstimmungen hinsichtlich mineralogischer Zusammensetzung, chemischer Charakteristik und textureller Ausbildung kann geschlussfolgert werden, dass alle untersuchten Proben einem vulkanischen Ereignis (Kemmlitzer Porphyr) zuzuordnen sind. Auftretende Unterschiede sind im Wesentlichen auf sekundäre Alterationen zurückzuführen. Das übereinstimmende Erscheinungsbild der Phänokristen sowie deren KL-Eigenschaften deuten auf eine Herkunft aus einer gemeinsamen Schmelze hin. Obwohl die mineralogischen, geochemischen und texturellen Ähnlichkeiten von Lithophysen und Kemmlitzer Porphyr auffällig sind, konnten bisher im massiven, porphyrisch ausgebildeten Vulkanit keine sphärolithischen Bildungen nachgewiesen werden. Randliche Anhaftungen von Pechsteinrelikten an den Lithophysen verschiedener Vorkommen lassen dagegen vielmehr auf eine Bildung im Bereich einer glasigen Fazies schließen. Abstract Mineralogical and geochemical investigations of the Kemmlitz porphyry and lithophysae (high-temperature crystallization domains) in the associated area were carried out to get information concerning mineralogical-petrographical characteristics of these volcanic formations and their relation to possible parent rocks. An integrated analytical study by X-ray diffraction, microscopy and cathodoluminescence (CL) revealed a monotonic mineral composition dominated by quartz, sanidine and orthoclase. The rocks can be classified as rhyolite based on the chemical composition (XRF). The predominance of hypidiomorphic quartz phenocrysts, partially with resorption features, well preserved K-feldspar as well as the lack of glass shards and fiamme indicate that the volcanic rocks derive from lava flows and are not related to ignimbrites. The investigated samples can probably be related to the same volcanic event (Kemmlitz porphyry) because of the similarities in mineral composition, chemical characteristics and texture. Detected variations mainly result from secondary alteration processes. The consistent appearance of phenocrysts and their CL characteristics indicate the origin from a common melt source. Despite these mineralogical, chemical and textural similarities between the Kemmlitz porphyry and the lithophysae, no spherulithic aggregates have been found within the massive porphyric volcanics. Marginal relics of pitchstone on some of the lithophysae instead indicate their formation in a glassy facies.

In this paper, zonation patterns of trace elements in apatite were visualized using four analytical techniques, namely back-scattered electrons (BSE) and cathodoluminescence (CL) imaging and electron probe micro-analysis (EPMA) and micro-proton-induced X-ray/gamma-ray emission (μPIXE/μPIGE) mapping. Each method demonstrates the in-grain compositional variations in a slightly different way. Both BSE and CL provide qualitative data, and the internal textures are displayed in most detail. Additionally, CL points to specific elements enriched in certain growth zones. Qualitative EPMA maps show detailed zonation patterns for specific elements (with high spatial resolution), which are in general correspondence with the patterns observed in BSE and CL images. The µPIXE/µPIGE maps are fully quantitative and the detection limits are relatively low compared to EPMA mapping. In present spot measurements µPIXE demonstrates lower detection limits than EPMA, however, the latter could be considerably improved by extending the acquisition times. There is no significant overlap of REE (rare earth elements) peaks in the acquired µPIXE energy spectra, however, when multiple REEs are present with sufficiently high concentrations, peak deconvolution may pose some difficulties. Spatial resolution of µPIXE/µPIGE images is not sufficiently high to reflect minor textural features, which also result from the greater interaction depth of the proton beam. However, major growth zones are distinguishable. Even though each method has their advantages and limitations, when applied together, they provide an almost complete characterization of compositional variability in trace-element-bearing minerals.

In the present study a first found of agates in Chile is reported from the Cordón de Lila region in northern Chile. The agates occur as veins and lenses in altered Permian volcanic rocks. The rock composition is rhyodacitic/dacitic consisting of a fi ne-grained K-feldspar-quartz groundmass with phenocrysts of plagioclase (An50-60) and pyroxene (augite). The volcanic host rocks show strong features of alteration and brecciation. Results of XRD, polarizing microscopy and cathodoluminescence (CL) microscopy and spectroscopy point to a late- to postvolcanic formation of the agates by hydrothermal fluids and SiO2 which was released during the alteration of unstable minerals and volcanic glass. Strongly varying agate micro-textures and the appearance of euhedral quartz crystals with pm-sized growth lamellae (so called Bambauer quartz) indicate fluctuations in the physico-chemical conditions (SiO2 concentration, pH) during alteration and agate formation. Another indication for the alteration processes is the occurrence of secondary calcite in the agate-bearing rocks.

„Bumble Bee Jasper" ist ein attraktiver Schmuckstein, welcher seit einigen Jahren auf dem Markt ist und vor allem durch seine intensive Farbgebung besticht. Dabei handelt es sich jedoch nicht um Jaspis; die Matrix wird durch Calcit aufgebaut. Die gelb-orange Farbe der Bänderung wird durch Realgar hervorgerufen. Zudem wurde in den dunkleren Bereichen Pyrit nachgewiesen, der z.T. in Form von Framboiden vorliegt.

Zircon and monazite from the Stolpen granite and related rhyolitic dyke show magmatic as well as post-magmatic textures. The magmatic textures are overprinted by post-magmatic fluid interaction which is indicated by elevated contents of Y, P, Th, and U in altered zircon domains. This can be observed both in the granite and rhyolite; however these processes were less intensive in the rhyolite. Patchy zoning in primary monazite from the granite also suggests fluid-induced alterations. High Ca, Pb, and Si contents and depletion in Th, as well as porous, spongy textures in monazite from the rhyolite are results of fluid-induced alterations that caused also crystallization of cheralite. Zircon and monazite from the granite are accompanied by fluorite, Y-rich and Nb-rich minerals. The alteration of primary, magmatic accessory minerals and formation of secondary minerals is interpreted as related to magmatic fluids, which origin cannot be defined precisely on the base of the present data, however their composition suggests some mantle contribution. The presence of fluorine and CO2 which promoted the transport of some HFSE and LILE accounts for their enrichment in the accessory minerals. Fluids that affected the accessory minerals from the rhyolite show different features (silica- and Ca-enrichment) and most probably came from another source

Zircon and monazite from the peraluminous Stolpen granite and related rhyolitic dyke show magmatic as well as post-magmatic textures. The magmatic textures represented mostly by subtle oscillatory zoning are overprinted by post-magmatic fluid interaction which is indicated by elevated contents of Y, P, Th, and U and dissolution textures in altered zircon domains. Late- to postmagmatic fluid-induced crystallization can be divided into two main stages separated by a strong dissolution event: 1st - the formation of Y-, P-, Th- and U-rich rims and 2nd - purification of these domains by trace element release which led to the formation of secondary xenotime and huttonite/thorite. This can be observed both in the granite and its aplitic parts and rhyolite. Patchy zoning in primary monazite from the granite also suggests fluid-induced alterations. Its formation could resulted from the coupled dissolution-reprecipitation processes. Dark patches are depleted in Th, U and Pb which is interpreted as a result of selective removal of these elements from the monazite structure. Beside modification of monazite composition, partial dissolution of monazite grains occurred as well indicating the aggressive character of the fluids. Monazite grains from the rhyolite show slightly different textures and composition. High Ca, Pb, and Si contents and depletion in Th, as well as porous, spongy textures are results of fluid-induced alterations that caused also crystallization of cheralite. Grains with pure monazite composition cannot be found. Zircon and monazite from the granite are accompanied by fluorite, Y-rich and Nb-rich minerals. High abundance of fluorite suggests elevated fluorine concentration in the fluids which facilitated both the dissolution of zircon and monazite grains and removal of certain elements from their structure. The absence of fluorite in the rhyolite indicates a different nature of the fluids involved in the alteration processes however the porous textures of zircon and monazite suggest their high reactivity as well. The alteration of primary, magmatic accessory minerals and formation of secondary minerals is interpreted as related to magmatic fluids, which origin cannot be defined precisely on the base of the present data. However their composition (high F-, Y- and Ca- activity) and the mineralogy of the accessory phases suggest that they weren’t derived only from the parental magma itself.