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Alkali Content (alkali + content)

Distribution by Scientific Domains

Earth and Environmental Science	50%

Selected Abstracts

The Cornubian Batholith: an Example of Magmatic Fractionation on a Crustal Scale

RESOURCE GEOLOGY, Issue 3 2006
Bruce W. Chappell
Abstract. The Cornubian Batholith comprises six major and several smaller bodies of S-type granite in southwestern England. These late-Variscan granites comprise two-mica granites, and much less abundant Li-mica granites that are restricted to one of the major bodies (St Austell) and smaller bodies. Some of these intrusive rocks are associated with major Sn mineralization. This paper is concerned with the geochemistry of the two-mica granites, which are felsic, strongly peraluminous, and have a high total alkali content and low Na:K. Rocks with very similar compositions to these granites occur elsewhere, including the Variscan granites of continental Europe, and in southeastern Australia. In detail all of the major plutons of this batholith have distinctive compositions, except for Bodmin Moor and Carnmenellis which cannot be discriminated from each other compositionally. A comparison with experimental data shows that the granites attained their major element composition under conditions of crystal-liquid equilibrium, with the final melt being saturated in H2O, at temperatures close to 770d,C and pressures about 50 MPa. That temperature estimate is in good agreement with values obtained from zircon saturation thermometry. The specific minimum-temperature composition excludes the possibility of widespread transfer of elements during hydrothermal alteration. Minor elements that are relatively very abundant are Li, B, Cs and U, while F, Ga, Ge, Rb, Sn, Ta, W and Tl are quite abundant and P is high for felsic rocks. Sr, Ba, and the trace transition metals Sc to Zn, are low, but not as low as they commonly are in very felsic granites. These trace element abundances, and the EL2O-saturation, resulted from the fractional crystallization of a melt derived by the partial melting of feldspathic greywackes in the crust. The Cornubian granites have compositions very similar to the more felsic rocks of the Koetong Suite of southeastern Australia, where a full range of granites formed at the various stages of magmatic fractionation postulated for the Cornubian granites, can be observed. The operation of fractional crystallization in the Cornubian granites is confirmed by the high P abundances in the feldspars, with P contents of the plagioclase crystals correlating with Ab-con-tent Most of the granites represent solidified melt compositions but within the Dartmoor pluton there is a significant component of granites that are cumulative, shown by their higher Ca contents. The Cornubian plutons define areas of high heat flow, of a magnitude which requires that fractionated magmas were transported laterally from their sources and concentrated in the exposed plutons. The generation of these granite plutons therefore involved magmatic fractionation during the stages of partial melting, removal of unmelted material from that melt, and fractional crystallization. During the later stages of those processes, movement of those magmas occurred on a crustal scale. [source]

Besonderheiten bei Alkali-Kieselsäure-Reaktionen in Betonfahrbahndecken

BETON- UND STAHLBETONBAU, Issue 8 2007
Rolf Breitenbücher Prof. Dr.-Ing.
In den letzten Jahren sind Rissbildungen in Betonfahrbahndecken mehrfach mit einer Alkali-Kieselsäure-Reaktion (AKR) in Verbindung gebracht worden. Die Risse sind nur in den seltensten Fällen einzig auf eine AKR zurückzuführen. Vielmehr ist davon auszugehen, dass sie durch Überlagerung mehrerer Spannungsquellen hervorgerufen wurden. Dennoch sind Alkali-Kieselsäure-Reaktionen bei Betonfahrbahndecken nicht zu vernachlässigen. Als Besonderheit kommt hier einer externen Alkalizufuhr eine wesentliche Bedeutung zu. Die in den Wintermonaten aufgestreuten alkalihaltigen Taumittel werden durch den nachfolgenden Verkehr mehr oder weniger intensiv in den Beton eingewalkt. Dabei kann das Eindringen der Alkalien durch Vorschädigungen (Risse) nachhaltig begünstigt werden. Unter diesen Randbedingungen sind bei Betonfahrbahndecken zur Vermeidung von schädigenden Alkali-Kieselsäure-Reaktionen weiterreichendere Maßnahmen als bei herkömmlichen Betonbauteilen notwendig. Solche wurden zwischenzeitlich vom BMVBS mit einem einschlägigen Rundschreiben erlassen. Die darin enthaltenen Vorgaben beziehen sich insbesondere auf die Bewertung von kritischen Gesteinskörnungen und auf den zulässigen Alkaligehalt der Zemente. Die Regelungen dieses Erlasses beruhen zunächst auf allgemeinen empirisch gewonnenen Erkenntnissen. Der tatsächliche Einfluss der Alkali-Kieselsäure-Reaktion auf die Rissbildung in Betonfahrbahndecken wird derzeit in Forschungsprojekten untersucht. Characteristics of Alkali Silica Reactions in Concrete Pavements In the last few years cracking in concrete pavements was often associated with an alkali silica reaction (ASR). Even if relevant reaction products were detected in appropriate samples, cracks result only in the rarest cases from an ASR solely. Rather it is assumed, that cracks were caused by a superposition of several stress impacts. Beside restraint stresses due to disabled thermal and hygric self-deformations, concrete pavements are also stressed by traffic. However, the influence of the alkali silica reaction in concrete pavements can not be neglected. Here especially the infiltration of alkalis from external resources is of a substantial importance. Alkaline deicing agents applied in the winter months are more or less intensively penetrating into the concrete by the following traffic, which leads to a continuous increase of the alkali potential in the concrete. The penetration of the alkalis is forwarded by already existent damages (cracks). Within these circumstances more extensive measures are necessary to avoid damaging alkali silica reactions for concrete pavements in comparison to conventional concrete constructions. In the meantime such measures were decreed by the BMVBS in a relevant circular. The contained requirements refer in particular to the evaluation of critical aggregates and to the permitted alkali content of the cements. The regulations of this decree are initially based on general empirically revealed findings. The actual influence of the alkali silica reaction referring to cracking in concrete pavements is examined in current research projects. [source]

Glass and Mineral Chemistry of Northern Central Indian Ridge Basalts: Compositional Diversity and Petrogenetic Significance

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2009
Dwijesh RAY
Abstract: The glass and mineral chemistry of basalts examined from the northern central Indian ridge (NCIR) provides an insight into magma genesis around the vicinity of two transform faults: Vityaz (VT) and Vema (VM). The studied mid-ocean ridge basalts (MORBs) from the outer ridge flank (VT area) and a near-ridge seamount (VM area) reveal that they are moderately phyric plagioclase basalts composed of plagioclase (phenocryst [An60,90] and groundmass [An35,79]), olivine (Fo81,88), diopside (Wo45,51, En25,37, Fs14,24), and titanomagnetite (FeOt,63.75 wt% and TiO2,22.69 wt%). The whole-rock composition of these basalts has similar Mg# [mole Mg/mole(Mg+Fe2+)] (VT basalt: ,0.56,0.58; VM basalt: ,0.57), but differ in their total alkali content (VT basalt: ,2.65; VM basalt: ,3.24). The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the ,REE of magma also increased with fractionation, without any change in the (La/Yb)N value. Glass from the VM seamount shows more fractionated characters (Mg#: 0.56,0.57) compared to the outer ridge flank lava of the VT area (Mg#: 0.63,0.65). This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure >10 kbar, followed by Fe-rich olivine at <10 kbar pressure. [source]

In Situ CO2 Capture in a Dual Fluidized Bed Biomass Steam Gasifier , Bed Material and Fuel Variation

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2009
G. Soukup
Abstract The AER (absorption enhanced reforming) process, based on the dual fluidized bed steam gasification process, uses a bed material which, apart from its function as heat carrier, acts as a CO2 transporting agent and selectively transfers CO2 from the gasification reactor to the combustion reactor. Thus, a hydrogen-rich gas is produced (H2 55...70 vol.-%db, CH4 8...13 vol.-%db, CO 5...11 vol.-%db as well as CO2 7...20 vol.-%db). Seven different CaO containing bed materials were tested for their principal applicability to the AER process with special focus on their influence on tar production. The amount of tars was generally found to be five times lower than with conventional gasification (at 850,°C) despite the low gasification temperatures (600,700,°C). Due to these low gasification temperatures, biomass with high alkali contents (e.g., straw), indicating a low ash melting point (< 800,°C), can be used under AER conditions. In summary, the AER process is very effective owing to the integration of the heat of the chemical reaction of CO2 with CaO and the water-gas shift reaction, which are both exothermic, into the gasification and the internal reforming of primary and secondary tars, which cuts off the formation of higher tars. [source]