Solution/reprecipitation

The second type of transformation, the reconstructive transformation involves dis-solution/reprecipitation the initial phase breaks down completely (dissolves) and the new phase precipitates from solution (for a review see Blesa Matijevic, 1989). There is, therefore, no structural relationship between the precursor and the product. In contrast to the solid-state transformation, the reconstructive process is... 

The temperature dependence of the stability of the ass could explain why the ass — jSss transformation takes place at 1350-1700 K [123, 128, 138, 188] in samples with compositions on the boundary of the ass region or in two-phase ass// ss mixtures. The transformation is a solution-reprecipitation process which often is kinetically hindered, and the presence of a liquid or low viscosity glassy phase is necessary [188],... 

Silicon nitride ceramics are mostly produced from oc-rich Si3N4 powders which transform during sintering by a solution reprecipitation process to / solid... 

Concentrated sulfuric acid Silver nitrate Chlorides, Ch Evolution of hydrogen chloride gas (pungent odor) White precipitate of silver chloride, which is soluble in ammonia solution (reprecipitate with HN03)... 

Harris W.H. and Matthews R.K. (1968) Subaerial diagenesis of carbonate sediments Efficiency of the solution-reprecipitation process. Science 160, 77-79. 

Hellmann R., Penisson J.-M., Hervig R., Thomassin J. H., and Abrioux M.-F. (2002) Solution-reprecipitation responsible for altered near-surface zones during feldspar dissolution do leached layers really exist Goldschmidt Conference Abstracts, A320. 

In deep-sea sediments, opal-CT commonly occurs as platy, blade-shaped crystals that are often arranged in spheroidal rosettes a few /zm in diameter. These rosettes are known as lepispheres (Wise and Kelts, 1972). The nearly euhedral crystal habit of natural lepisphere crystals indicates that the opal-CT precipitated in situ as an authigenic mineral this conclusion is supported by results of experimental studies (Oehler, 1973) in which completely euhedral lepisphere crystals were synthesized from amorphous silica. Thus, it appears that the conversion of biogenic silica (opal-A) to opal-CT occiurs principally, if not exclusively, through a solution-reprecipitation mechanism. 

The conversion of opal-CT to quartz has been studied experimentally with conflicting results. Mizutani (1966) concluded that the reaction takes place through a solution-reprecipitation mechanism. Ernst and Calvert (1969) concluded that the mechanism is by solid-solid conversion. In a re-evaluation of the data presented by Ernst and Calvert (1969), Stein and Kirkpatrick (1976) concluded that the evidence more closely supports a solution-reprecipitation mechanism. At present, it is unclear which of these two processes, if either, dominates in the marine sedimentary environment. 

A final factor influencing the rates of diagenetic reactions (solution, reprecipitation, recrystallization) in marine siliceous deposits is the porosity and permeability of the sediment. In studies of radiolarian cherts from Italy, Thurston (1972) found that the effects of diagenetic processes were least in... 

The COH ] must be maintained at a certain minimum value (pH 10) in order to synthesize crystalline pyrochlore from solution. This implies that a minimum solubility is required for crystallization and that this crystallization of the pyrochlore directly out of alkaline solution may involve a solution-reprecipitation mechanism. Once the restriction of minimum pH has been satisfied, COH ] does not seem to have a significant effect on crystallinity as long as oxidizing conditions are maintained. The solubility of lead rapidly increases with hydroxide concentration therefore, when all else is held constant, the lattice parameter of the product pyrochlore decreases as the pH of the synthesis medium increases. 

One additional parameter that affects the solubility of lead ruthenate pyrochlores in alkali is the extent of lead substitution on the B-site. The greater the substitution (i.e. the larger x is in formula 1), the higher the solubility of the pyrochlores is in alkali 2). If it is assumed that a solution-reprecipitation mechanism of synthesis is operative, the stoichiometry-dependent solubility could explain why it becomes significantly more difficult to crystallize lead ruthenate directly out of alkaline solution when x <0.3. 

Small amounts of weathered solutes reprecipitate in lower soil horizons. Examples include clay accumulation in the B horizon, silica pans (impermeable layers of soil particles indurated with silica), and the wide-spread caliche horizons of CaCC>3 accumulation in arid regions. Most solutes, however, reach the sea, where precipitation of other secondary minerals removes most of the weathered solutes except Na+, Cl-, and Mg2+. Marine sediments, in turn, are slowly convened into igneous, metamorphic, or sedimentary rocks, which form new soil parent material. Such element recycling has circulated ions many times from land to sea during the earth s histoiy. 

The complex anion hexaazidorhodate(III), [Rh (N3)6] is known as tetraphenylarsonium salt [139] and as tetrabutylammonium salt [162]. For preparation of the former, see Table IX. The tetrabutylammonium hexa-azidorhodate(III) is obtained from the [RhCle] " complex by ligand exchange 0.2 g Na3RhCl6 I2H2O is dissolved in 25 ml water and added to a solution of 1.5 g sodium azide in 50 ml water. The mixture is kept at 40°C for 1 hr and then precipitated with 1 g tetrabutylammonium bromide. The faintly red crystals are washed with alcohol and, from a methylene chloride solution, reprecipitated with ether [162]. 

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