Volatile elements condensation temperatures

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.

As exhaust gases and fly ash particles are vented from the furnace, they quickly begin to cool, leading to the condensation and adsorption of volatilized elements onto the surfaces of fly ash particles entrained in the gas stream (Kaakinen etal. 1975). Under high-temperature combustion conditions certain elements, including S, are enriched on the surface of particles (Davison et al. 1974 Smith 1980). A vaporization-condensation process is the primary mechanism... 

As was previously mentioned, trace elements that sublime at temperatures below those attained during coal combustion (e.g., As, Se, Hg, Zn), and are associated with thermally unstable solid phases (in particular organic matter and sulphide minerals), are subject to vaporization into furnace gases. Once these gases, and fly ash particles entrained in the gases, are vented from the combustion furnace they quickly cool, leading to the condensation of volatilized elements onto the... 

A cosmochemical periodic table, illustrating the behavior of elements in chondritic meteorites. Cosmic abundances are indicated by symbol sizes. Volatilities of elements reflect the temperatures at which 50°/o of each element would condense into a solid phase from a gas of solar composition. As in Figure 1.2, the chemical affinities of each element, lithophile for silicates and oxides, siderophile for metals, and chalcophile for sulfides, are indicated. Some of the most highly volatile phases may have remained uncondensed in the nebula. Stable, radioactive, and radiogenic isotopes used in cosmochemistry are indicated by bold outlines, as in Figure 1.2. Abundances and 50% condensation temperatures are from tabulations by Lodders and Fegley (1998). 

Vapor-solid and vapor-liquid transformations (condensation of a gas, or its reverse, evaporation) can fractionate elements and sometimes isotopes. Each element condenses over a very limited temperature range, so one would expect the composition of the condensed phase and vapor phase to change as a function of the ambient temperature. Many of the chemical fractionations that took place in the early solar system are due, in one way or another, to this phenomenon. It is convenient to quantify volatility by use of the 50% condensation temperature, that is, the temperature by which 50% of the mass of a particular element has condensed from a gas of solar composition. Table 7.1 lists the 50% condensation temperatures for the solid elements in a gas of solar composition at a pressure of... 

The volatile element depletions among the various classes of chondrites were once considered to be the result of equilibrium condensation, with accretion of the different classes of meteorites taking place at different temperatures before the missing volatiles could... 

Volatile element abundances in CV chondrites (normalized to Cl chondrites and silicon) lie along a linear array on semi-log plots versus their 50% condensation temperatures. This depletion pattern persists, whether the elements are siderophile, lithophile, or chalcophile. 

The process of condensation of minerals in the early solar nebula has long been invoked to explain the chemistry and mineralogy of primitive chondritic meteorites (e.g. Cameron 1963). Their observed bulk compositions show volatile-element depletions that are clearly smooth functions of calculated condensation temperature in a gas of solar composition (Davis 2006). Despite this success in explaining the bulk composition of chondrites, the diverse mineralogy of these bodies is not reproduced well in the condensation sequence calculations. To date, there is no incontrovertible evidence for direct condensation of rocky meteoritic material in the... 

Their abundances are in most cases below solar, i.e., they have lower element/silicon ratios than the Sun or Cl chondrites, they are depleted (see below). In Figure 2, abundances of moderately volatile elements in CVS meteorites relative to those in Cl meteorites are plotted. Increasing depletions correlate with decreasing condensation temperatures but are independent of the geochemical properties of the elements. Depletions of moderately volatile elements in meteorites are produced by incomplete condensation. The amount and the relative abundances of these elements in meteorites are probably the result of removal of volatiles during condensation (Palme et al., 1988). 

Figure 2 Abundances of volatile elements in CVS chondrites (e.g., Allende) normalized to Cl chondrites and Si. There is a continuous decrease of abundances with increasing volatility as measured by the condensation temperature. The sequence contains elements of very different geochemical character, indicating that volatility is the only relevant parameter in establishing this pattern (source Palme, 2000).

Figure 8 Abundances of elements along the line of sight towards Oph Ophiuchus), a moderately reddened star that is frequently used as standard for depletion studies. The ratios of Oph abundances to the solar abundances are plotted against condensation temperatures. The abundances of many of the highly volatile and moderately volatile elements up to condensation temperatures of around 900 K are, within a factor of 2, the same in the ISM and in the Sun. At higher condensation temperatures a clear trend of increasing depletions with increasing condensation temperatures is seen. It is usually assumed that the missing refractory elements are in grains (source Savage and...

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