Metastable persistence

Figure 7,6 Phase stability relations in pseudobinary system NaAlSi30g-KAlSi30g, after Waldbaum and Thompson (1969). (A) High-T relations loop of metastable persistency of sanidine like a Roozeboom type III. (B) Expanded T-range with decreasing T, a solvus held opens downward.

Of the various factors that cause redox disequilibria, the most effective are biologic activity (photosynthesis) and the metastable persistence of covalent complexes of light elements (C, H, O, N, S), whose bonds are particularly stable and difficult to break (Wolery, 1983). For the sake of completeness, we can also note that the apparent redox disequilibrium is sometimes actually attributable to analytical error or uncertainty (i.e., difficult determination of partial molalities of species, often extremely diluted) or even to error in speciation calculations (when using, for instance, the redox couple Fe /Fe, one must account for the fact that both Fe and Fe are partly bonded to anionic ligands so that their free ion partial molalities do not coincide with the bulk molality of the species). 

Figure 12.14 Diagram showing relationships among aqueous sulfur species that are either thermodynamically stable or have significant metastable persistence. Reprinted from Geochim. et Cosmochim. Acta, 56, M. A. Williamson and J. D. Rimstidt, Correlation between structure and thermodynamic properties of aqueous sulfur species, 3867-80, 1992, with permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, U.K.

These studies have provided direct information on the phase boundaries associated with the sl, sll and sH phases. The formation of the sll phase in methane hydrates, as evidenced by the metastable persistence of si phase, may be kinetically or compositionally controlled. Further investigations are required to obtain complete (i.e., compositionally dependent) phase relations in the methane hydrate system in this /"-T range. 

Tridymite. Tridymite is reported to be the siUca form stable from 870—1470°C at atmospheric pressure (44). Owing to the sluggishness of the reconstmctive tridymite—quart2 conversion, which requites minerali2ers such as sodium tungstate, alkah metal oxide, or the action of water under pressure, tridymite may persist as a metastable phase below 870°C. It occurs in volcanic rocks and stony meteorites. 

The hydroxides as precipitated are amorphous, but if they are refluxed ia a neutral or slightly acidic solution they convert to a mixture of cubic and monoclinic hydrous zirconia crystaUites on continued refluxing, only the monoclinic form persists (196). If the refluxing is conducted in an alkaline solution, metastable cubic zirconia is formed (197). 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

The presence of metastable decompression paths having transient pressures temporarily much smaller than the temperature equilibrium values could not be absolutely confirmed or refuted. If present, they persist for only 1 msec and do not seriously affect the decompression time of the driver tube, and the duration is independent of vessel size. 

Time-resolved spectroscopy establishes that the 25-ps laser irradiation of the relatively persistent charge-transfer complex of p-bromoanisole with iodine monochloride generates the contact ion pair (see Fig. 15b) in which the metastable ICP undergoes mesolytic fragmentation to form the reactive triad, i.e.,... 

A given phase may persist beyond the point at which transition to another phase should properly occur. On the T5 isotherm, for instance, it is possible to compress the vapour in clean conditions beyond point V, as shown by the dots, without condensation occurring. This vapour is supercooled, and Pv > Pl- When disturbed, the supercooled vapour condenses at once. In the same way, clean liquid may be superheated without boiling, in which case Pl > Pv Supercooled or superheated phases are metastable. They appear to be stable, but are thermodynamically unstable, since another state of lower chemical potential exists. 

Only 15 was sufficiently stable for isolation and chemical and structural characterization (33). Compounds 12-13 persist for several hours in chloroform at room temperature and for 2-3 weeks at 251K, particularly when water is carefully excluded. All three react readily with PR3, forming oxorhenium(V) compounds that in these cases, unlike those with dithiolates, are metastable. The fastest reaction occurs between P(p-MeOC6H4)3 and 13, with a rate constant of 2.15 x 102Lmol 1s 1 in chloroform at 298 K. Other reactions of PAr3 upon applying the Hammett equation yield p =— 0.7. This is the same value of the reaction... 

Because transitions between the metastable level and the ground or excited levels are so slow, we sometimes say they are forbidden. Once an electron enters the metastable level, it remains there until it can make a forbidden transition, in which case a photon is released. The time of residence for the electron in the metastable level determines the length of time that phosphorescence persists. 

Glow-in-the-dark molecules possess such metastable quantum states. The reason why the light release persists is that demotion of an electron from the metastable state to the ground state can only... 

The visible consequence of such a metastable state is phosphorescence straightforward irradiation of such a material readily causes excitation to an excited state and partial demotion to the metastable state. But the electrons are then trapped in the metastable states. The faint glow demonstrates that a few electrons demote per second the glow persists for as long as it takes for all the electrons to reach the ground state. 

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