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1.
Martin R. Lee Mark E. Hodson Maureen MacKenzie Caroline L. Smith 《Geochimica et cosmochimica acta》2008,72(20):4962-4975
Our ability to identify thin non-stoichiometric and amorphous layers beneath mineral surfaces has been tested by undertaking X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) work on alkali feldspars from pH 1 dissolution experiments. The outcomes of this work were used to help interpret XPS and TEM results from alkali feldspars weathered for <10,000 years in soils overlying the Shap Granite (north-west England). The chemistry of effluent solutions indicates that silica-rich layers a few nanometers in thickness formed during the pH 1 experiments. These layers can be successfully identified by XPS and have lower Al/Si, Na/Si, K/Si and Ca/Si values than the outermost ∼9 nm of unweathered controls. Development of Al-Si non-stoichiometry is coupled with loss of crystal structure to produce amorphous layers that are identifiable by TEM where >∼2.5 nm thick, whereas the crystallinity of albite is retained despite leaching of Na to depths of tens to hundreds on nanometers. Integration of XPS data over the outermost 6-9 nm of naturally weathered Shap feldspars shows that they have stoichiometric Al/Si and K/Si ratios, which is consistent with findings of previous TEM work on the same material that they lack amorphous layers. There is some XPS evidence for loss of K from the outermost couple of nanometers of Shap orthoclase, and the possibility of leaching of Na from albite to greater depths cannot be excluded using the XPS or TEM results. This study demonstrates that the leached layer model, as formulated from laboratory experiments, is inapplicable to the weathering of alkali feldspars within acidic soils, which is an essentially stoichiometric reaction. 相似文献
2.
An Ordovician geanticlinal marine sequence is exposed in theCentral West of New South Wales, Australia. The sequence containingbasic lavas, labile sedimentary, and proclastic rocks, has undergoneextensive adjustment during regional burial metamorphism. Thebasic lavas have a patchy coloration due to the formation ofalternation minerals of the prehnite-pumpellyite facies whichapperar to have formed during the metamorphism. The hererogeneousappearance of the basic lavas reflects a pronouced chemicaland mineralogical departure from the original composition. Twodiverging trends result. One leads to the production of a Ca-enrichedlithology, the other, which by far dominates in area at eachouterop, leads to a spilitic lithology. The alterationdomains in many cases crosscut all primary textural featureand their chemical variation does not correspond to likely variationsin the original lava composition. 相似文献
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Caroline M. Isaacs 《Geo-Marine Letters》1985,5(1):25-30
Although best known as a deposit rich in silica and organic matter, the Miocene Monterey Formation actually had long-term
rates of silica and organic Matter accumulation equal to or lower than values in the underlying and overlying detritus-rich
formations in the Santa Barbara coastal area. The factor Mainly influencing the composition of the Monterey Formation in this
area was the slow accumulation of terrigenous debris. In contrast, with greater than average silica abundance within the Monterey
Formation probably resulted from rapid short-term silica accumulation (of hundreds to tens of thousands years duration) rather
than from especially slow accumulation of diluting debris. 相似文献
5.
Fragmentation, or the "coming apart" of magma during a plinian eruption, remains one of the least understood processes in
volcanology, although assumptions about the timing and mechanisms of fragmentation are key parameters in all existing eruption
models. Despite evidence to the contrary, most models assume that fragmentation occurs at a critical vesicularity (volume
percent vesicles) of 75–83%. We propose instead that the degree to which magma is fragmented is determined by factors controlling
bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time. Bubble coalescence in
vesiculating magmas creates permeability which serves to connect the dispersed gas phase. When sufficiently developed, permeability
allows subsequent exsolved and expanded gas to escape, thus preserving a sufficiently interconnected region of vesicular magma
as a pumice clast, rather than fully fragmenting it to ash. For this reason pumice is likely to preserve information about
(a) how permeability develops and (b) the critical permeability needed to insure clast preservation. We present measurements
and calculations that constrain the conditions (vesicularity, bubble size distribution, time, pressure difference, viscosity)
necessary for adequate permeability to develop. We suggest that magma fragments explosively to ash when and where, in a heterogeneously
vesiculating magma, these conditions are not met. Both the development of permeability by bubble wall thinning and rupture
and the loss of gas through a permeable network of bubbles require time, consistent with the observation that degree of fragmentation
(i.e., amount of ash) increases with increasing eruption rate.
Received: 5 July 1995 / Accepted: 27 December 1995 相似文献
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7.
Sustainability: A View From The Paddock 总被引:1,自引:0,他引:1
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The Canadian Certified Reference Materials Project announces the availability of reference uranium tailings sample UTS-1, -2, -3 and -4. Eighteen laboratories participated in the interlaboratory program for total iron, titanium, aluminium, calcium, barium, uranium, thorium, total sulphur, sulphate and arsenic. Eight laboratories participated in the inter-laboratory program for thorium-230, radium-226, lead-210 and polonium-210 in all four samples and for thorium-232, radium-228 and thorium-228 in UTS-1 and UTS-2. 相似文献
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