Mineralization homogeneous nucleation

Several investigators studied R-12. Holt and Muenker (1972) and Rausch and Levine (1973) made simple spills of this cryogen into water. The highest water temperature used by both teams was —342 K and weak explosions were noted. From Table XVI, it can be seen that this water temperature was barely within the range of the superheat-limit temperature, so no or only minor explosions might have been expected. Henry et al. (1974) spilled R-12 on top of a hot mineral oil. For oil temperatures less than about 409 K, there was little interaction except rapid boiling. Above 409 K, explosions resulted. Henry et al. state that this oil temperature would lead to an interface temperature [see Eq. (1)] close to the expected homogeneous nucleation temperature (—345 K) so that the explosions were to be expected. 

At some distance close to the mineral surface, the saturation index (SI, IAP/Xsp) of pyromorphite must be greater than zero to allow local super saturation and precipitation. The question is whether this is a heterogeneous nucleation/ surface precipitation reaction or a dissolution homogeneous nucleation reaction. 

For many of the more abundant elements, such as Al, Fe, and Mn, precipitation of mineral forms is common and may greatly influence or even control their solubility. For most trace elements, direct precipitation from solution through homogeneous nucleation appears to be less likely than adsorption-desorption, by virtue of the low concentration of these metals and metalloids in soil solutions in well-aerated dryland soils. When soils become heavily polluted, metal solubility may reach a level to satisfy the solubility product to cause precipitation. Precipitation may also occur in the immediate vicinity of the phosphate fertilizer zone, where the concentration of heavy metals and metalloids present as impurities may be sufficiently high. Precipitation of trace metals as sulfides may have a significant role in metal transformation in reduced environments where the solution sulfide concentration is sufficiently high to satisfy the solubility product constants of metal sulfides (Robert and Berthelin, 1986). 

The first is that calcium, and probably phosphate ions as well, are concentrated by hard tissue cells, and subsequently secreted in matrix vesicles. In this way the (calcium x phosphate) ion product may be boosted above the solubility level of the more soluble, but spontaneously nucleable, calcium phosphates, such as amorphous calcium phosphate. Under appropriate conditions, this substance may undergo solid phase transformation to biological apatite, which cannot itself be formed by homogeneous nucleation. This type of mineralization may occur in extrafibrillar situations at an early stage of the process, e.g. calcospherites may be formed where maximum ordering of the inorganic phase has not yet been reached. 

The prineipal eonstituent of marine particles are sea salt, non-sea-salt sulfate (nss sulfate), and mineral dust (38). The nss sulfate has both a continental and marine souree and is derived from homogeneous nucleation and diffusive mass transport proeesses of aerosol precursor gases, such as SO2 and NH3. Numerous studies have shown that the accumulation mode in clean marine air is predominately composed of nss sulfate. Its concentration decreases from coastal regions of the continents to the remote areas of the oceans. Single-particle analysis by electron microscopy showed that most of the particles are morphologically similar to (NH4)2S04 (39,40). Also a small number of H2SO4 particles were found. 

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

F.K. LeGoues, H.I. Aaronson, Y.W. Lee, and G.J. Fix. Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation. I. The classical theory regime. In International Conference on Solid—>Solid Phase Transformations, pages 427-431, Warrendale, PA, 1982. The Minerals, Metals and Materials Society. 

Dissolution and minerogenesis (or crystallization, precipitation) are two interconnected and oppositely directed processes. They always accompany one another and compete with each other. This competition is especially important at the moments minerals initiate, when in a liquid medium of the solution appears the solid medium of a crystal. It is customary to identify two stages in the competition of the described processes before and after nucleation, i.e., before and after a homogeneous solution converts into a heterogeneous medium. 

Before the nucleation, dissolution and minerogenesis processes nm actually in a homogeneous medium, without the formation of a solid substance. At this stage, a solution maybe oversaturated, and for stable existence of a mineral it is necessary to overcome the resistance of surface tension. Here, mineral micro-particles, nuclei, as if form and straight away dissolve, not having reached significative size. Such a relatively stable state of oversaturated solution without minerogenesis is called a metastable state. 

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