Obsidian – das Glas der Natur Sicker, Dieter
Chemie in unserer Zeit,
08/2022, Letnik:
56, Številka:
4
Journal Article
Recenzirano
Reisen Sie gern? Oft hat man dabei unerwartete Erlebnisse, die irgendwie mit Chemie zu tun haben und chemisches Lehrbuchwissen wird ungeahnt erweitert. Es verknüpft sich mit Geschichte und ...Geschichten, unbekannten Anwendungen, praktischen Handgriffen und dem realen Leben anderer Menschen. Über solche kleinen Wunder soll diese Serie berichten. Diesmal führt uns die Suche zu Obsidian, vulkanischem Glas, nach Guatemala.
This paper traces and reviews the conduct of obsidian studies in California, with emphasis on the results of provenance studies and hydration dating over the past few decades. Review of substantive ...results reveals temporal and spatial change in the distribution of certain obsidians in different regions within the geopolitical boundaries of the state, as well as evidence for variation in source-use commensurate with different socio-ceremonial contexts in prehistory. Perspectives and lessons on obsidian use, derived from California studies, can be extended more generally to obsidian studies elsewhere in the world.
In this study, the obsidian (Ob) that is a natural glass were doped with and Cerium oxide (CeO2) and Erbium Oxide (Er2O3) in different amount (0.5%, 1%, 3%, and 5%). Obsidian was grinded and glass ...powder was doped with CeO2 and Er2O3 via mechanical alloying. The doped glass samples were prepared with melting-quenching method. Radiation shielding properties of obsidian doped with CeO2 and Er2O3 were determined at 22.16–59.54 keV photon energies by using an HPGe detector with a resolution of 182 eV at 5.9 keV. It was observed that the radiation absorption tended to increase with the increasing doping amount to obsidian samples. The mass attenuation coefficients both of CeO2 and Er2O3 doped obsidian glass decrease with increasing photon energy. The Er2O3 doped obsidian glasses are better the radiation shielding material than CeO2 doped obsidian glasses.
•Doped obsidian glasses are better radiation shielding materials than ordinary obsidian glass.•µm and Zeff values increase whereas MFP, HVL and TVL parameters decrease with the increasing doping concentration.•The doping with Er2O3 of the obsidian glass increases the effectiveness of the radiation shielding.
A geochemical analysis of 382 obsidian samples from different archeological sites in Central Chile (32°–35° Lat. South) has identified the preferential use of three known sources in the Andean ...mountain range, Arroyo Paramillos and Laguna del Diamante located in the Maipo Volcano area, and Las Cargas located ca 120 km further south. The analysis of the circulation and use of this raw material from the beginning of the Archaic period until the arrival of the Inka to this territory reveals differences in how obsidian from these three main sources was used, both spatially and temporally. The hunter-gatherers occupying the Andean mountain range preferred the obsidian source from the Maipo Volcano area, while the hunter gatherer and horticulturalist groups from the central valley used more frequently the obsidian from Las Cargas source. These differences are linked to the quality of the obsidian, its suitability for the intended use and the distance of the users from the source.
•Rhyolite lavas do not degas passively, but do so in an explosive manner.•Batched degassing links the chemistry of tephra, bombs, tuffisites, to lava.•Tuffisites bleed off volatiles from magma and ...also permit buffering.•Silicic lavas are the healed remains of explosively fragmented magma.
A long-standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava. A widely cited idea is that the explosive-to-effusive transition manifests a two-stage degassing history whereby lava is the product of non-explosive, open-system gas release following initial explosive, closed-system degassing. Direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive, as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent. This explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava. Here we use precise H-isotope and magmatic H2O measurements and textural evidence to demonstrate that effusion results from explosion(s)—lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels (tuffisites) that repetitively and explosively vent from effusing lava. Our measurements show, specifically that D/H ratios and H2O contents of a broad suite of explosive and effusive samples from Chaitén volcano (hydrous bombs, Plinian pyroclasts, tuffisite veins, and lava) define a single and continuous degassing trend that links wet explosive pyroclasts (∼1.6 wt.% H2O, δD=−76.4‰) to dry obsidian lavas (∼0.13 wt.% H2O, δD=−145.7‰). This geochemical pattern is best fit with batched degassing model that comprises small repeated closed-system degassing steps followed by pulses of vapour extraction. This degassing mechanism is made possible by the action of tuffisite veins, which, by tapping already vesicular or brecciated magma, allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed-system domains and large tracts of conduit magma by giving them long-range connectivity. Even though tuffisite veins render magma degassed and capable of effusing, they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut. Thus the effusion of silicic lava, traditionally thought to be relatively benign process, presents a particularly hazardous form of explosive volcanism.
Previous research suggests that the complex symbolic, technological, and socio-economic behaviors that typify
had roots in the middle Pleistocene <200 ka, but data bearing on human behavioral origins ...are limited. We present a series of excavated Middle Stone Age sites from the Olorgesailie Basin, southern Kenya, dated ≥295 to ~320 ka by
Ar/
Ar and U-Series methods. Hominins at these sites made prepared cores and points, exploited iron-rich rocks to obtain red pigment, and procured stone tool materials from ≥25-50 km distance. Associated fauna suggests a broad resource strategy that included large and small prey. These practices imply significant changes in how individuals and groups related to the landscape and one another, and provide documentation relevant to human social and cognitive evolution.
Obsidian-tempered ceramics represent a typical production of the Chalcolithic period in the southern Caucasus. Previous studies have already assessed the viability of LA-ICP-MS analysis to identify ...the provenance of the obsidian temper contained in the ceramic paste. In this article the results of the analyses of the obsidian-tempered ceramics and of the obsidian lithic artefacts from the sites of Aratashen (Armenia) and Mentesh Tepe (Azerbaidjan) are compared. The aim of this comparison is to define analogies and differences in the modalities of acquisition of the obsidian as they are revealed by ceramic and lithic production at two sites that were localised at different distances from primary and secondary sources of obsidian. The results of this comparison allowed us to highlight different modalities of provisioning and exploitation of the same raw material by knappers and potters.
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