Elements refractory

The main problem in this technique is getting the atoms into the vapour phase, bearing in mind the typically low volatility of many materials to be analysed. The method used is to spray, in a very fine mist, a liquid molecular sample containing the atom concerned into a high-temperature flame. Air mixed with coal gas, propane or acetylene, or nitrous oxide mixed with acetylene, produce flames in the temperature range 2100 K to 3200 K, the higher temperature being necessary for such refractory elements as Al, Si, V, Ti and Be. 

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

The fractionation of these refractory elements is beheved to be the result of relative efficiencies of incorporation of condensed sohds rich in early high temperature phases into the meteorite parent bodies at different times and locations in the solar nebula. The data are taken from Reference 3. 

Advantages and drawbacks are iadicated by + and —, respectively. Thus, + implies iuexpensive, multielement capability, wide dynamic range, relative freedom from iaterferences, and the abiUty to analyze refractory elements. 

From a geochemical viewpoint, U is an incompatible lithophile and refractory element. U exists in three distinct oxidation states in nature (Galas 1979) but the most common are ([Rn] 5f ) and ([Rn]). The most reduced form (metal) is never found in natural environments. At the surface of the earth, U is dominantly in the form. However, in a reducing environment, it will be in the state where it is insoluble and therefore generally far less mobile than U(VI). In the mantle, U is thought to occur in the... 

Kurz A, Derry LA, Chadwick OA, Alfano MJ (2000) Refractory element mobility in volcanic soils. Geology 28 683-686... 

These facts would suggest that the electrolysis of molten alkali metal salts could lead to the introduction of mobile electrons which can diffuse rapidly through a melt, and any chemical reduction process resulting from a high chemical potential of the alkali metal could occur in the body of the melt, rather than being confined to the cathode volume. This probably explains the failure of attempts to produce the refractory elements, such as titanium, by electrolysis of a molten sodium chloride-titanium chloride melt, in which a metal dust is formed in the bulk of the electrolyte. 

There are a number of interferences that can occur in atomic absorption and other flame spectroscopic methods. Anything that decreases the number of neutral atoms in the flame will decrease the absorption signal. Chemical interference is the most commonly encountered example of depression of the absorption signal. Here, the element of interest reacts with an anion in solution or with a gas in the flame to produce a stable compound in the flame. For example, calcium, in the presence of phosphate, will form the stable pyrophosphate molecule. Refractory elements will combine with 0 or OH radicals in the flame to produce stable monoxides and hydroxides. Fortunately, most of these chemical interferences can be avoided by adding an appropriate reagent or by using a hotter flame. The phosphate interferences, for example, can be eliminated by adding 1 % strontium chloride or lanthanum chloride to the solution. The strontium or lanthanum preferentially combines with the phosphate to prevent its reaction with the calcium. Or, EDTA can be added to complex the calcium and prevent its combination with the phosphate. 

Magnesium. Corundum-hibonite associations are what eould be the first eondensates from a solar composition gas. Mg is not a refractory element and is strongly depleted in... 

Analytical and quality control details are summarised in Arne et al (2008). Gold was determined by fire assay, and major elements by ICP-OES following a four-acid digestion, with the exception of fresh drill core samples from Wildwood, which were analysed by lithium borate fusion and XRF. Trace elements were determined by ICP-MS. Refractory elements (W, Zr, Ba and Ti) were analysed by pressed powder XRF. 

Fig. 8.8. Imprisonment of chemical elements in dust grains. The elements precipitate out to varying degrees to form grains, depending on their affinity for the solid state. Volatile elements, with low condensation temperature, stay for the main part in the gaseous state. Refractory elements, with high condensation temperature, are mainly imprisoned within dust grains. Only atoms in the gaseous phase are detected by classic techniques analysing UV absorption spectra. The light source whose spectrum has been decoded here is the hot star f Ophiuchi.

These interference effects are far less common. Under this heading, some authors classify the enhancement of signals from several, otherwise refractory, elements by fluoride. The use of protective agents (e.g. EDTA for calcium or 8-hydroxyquinoline for aluminium or chromium) are also examples of this type of effect. 

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

Plots of uranium versus lanthanum (two refractory elements), and potassium versus lanthanum (a volatile element and a refractory element) for terrestrial and lunar basalts, HED achondrites (Vesta), and Martian meteorites. All three elements are incompatible elements and thus fractionate together, so their ratios remain constant. However, ratios of incompatible elements with different volatilities ( /La) reveal different degrees of volatile element depletion in differentiated bodies. After Wanke and Dreibus (1988). 

Uranium and thorium are actinide elements. Their chemical behavior is similar under most conditions. Both are refractory elements, both occur in nature in the +4 oxidation state, and their ionic radii are very similar (U+4 = 1.05 A, Th+4 = l.lOA). However, uranium can also exist in the +6 state as the uranyl ion (U02 2), which forms compounds that are soluble in water. Thus, under oxidizing conditions, uranium can be separated from thorium through the action of water. 

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