Dissolution etch pits

The theoretical model developed to take account of these factors, Eqs. (31)—(35), is consistent with the experimental data presented in Figure 21. This model indicates that —20 lattice layers of the crystal surface are removed in each current surge [given the deduced value of N = 12.5 X 10 9 mol cm-2, and the density of Cu2+ in the (100) surface of —5.3 X 10 10 mol cm-2 (51)]. The value of Ccrit = 1.25 X 10 6, is some five orders of magnitude smaller than the value typically required for the nucleation of observable dissolution etch pits at dislocation sites (3,52,53). Intuitively, this indicates that the measured value of Ccrit is consistent with dissolution from a dislocation-free area. 

Inclusions readily form in a crystal that has been subjected to dissolution rapid growth occurs on the partially rounded surfaces and entraps mother liquor. The rapid healing of dissolution etch pits will do the same. These regeneration inclusions which usually lie in lines, i.e. along the former crystal edges, are characteristic of crystals grown from seeds. 

The formation of etch pits and tunnels on n-Si during anodization in HF solutions was reported in the early 1970 s. It was found that the solid surface layer is the remaining substrate silicon left after anodic dissolution. The large current observed on n-Si at an anodic potential was postulated to be due to barrier breakdown.5,6 By early 80 s7"11 it was established that the brown films formed by anodization on silicon substrate of all types are a porous material with the same single crystalline structure as the substrate. 

Gratz, A. J., S. Manne, and P. K. Hansma (1991), "Atomic Force Microscopy of Atomic-Scale Ledges and Etch Pits Formed During Dissolution of Quartz", Science 251,1343-46. 

A common phenomenon in the dissolution of silicate minerals is the formation of etch pits at the surface (90-91.,93-94). When this occurs, the overall rate of mineral dissolution is non-uniform, and dissolution occurs preferentially at dislocations or defects that intercept the crystal surface. Preferential dissolution of the mineral could explain why surface spectroscopic studies have failed... 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Dissolution kinetics at etch pits. If an etch pit opens up at a... 

Low temperature etching. Our data suggests that, under hydrothermal conditions the rate of pit formation is dramatically reduced, although perhaps not completely stopped, at C = Ccrjt. Etch pits on a natural, hydrothermally-etched quartz surface therefore indicate extended dissolution times, but not necessarily etching at C < Ccrit This is because the rate of etch pit formation even above Ccr t can be significant at elevated temperatures (as shown by crystal R9). However, at low temperatures, formation of etch pits when C > C would be less likely, and natural surfaces etched at low temperature should record the saturation state of the etching fluid. 

Ives and Hirth (6) report similar data for etch pits in LiF, in which the rate of pit widening drops to 0 at 0.22CQ and 32°C in dilute ferric fluoride solution, where the 2.5 ppm FeJ acts as a dissolution poison. They argue that, when no poison is present, the rate should decrease linearly with increasing C/CQ, until the rate equals 0 at C CQ. In the presence of a poison adsorbed to a kink, however, dissociation of a molecule should be slowed, reducing the local equilibrium concentration to a value of C < CQ. If poison molecules completely cover the kink sites, the rate of pit widening could reach 0 at C rather than CQ. The contribution of poisons to... 

Several refinements of our experiments could test these theories further. By measuring etch pit densities as well as pit dimensions on sequentially-etched crystals, nucleation rate data and pit growth data could be collected, yielding information about the rate-limiting steps and mechanisms of dissolution. In addition, since the critical concentration is extremely dependent on surface energy of the crystal-water interface (Equation 4), careful measurement of Ccrit yields a precise measurement of Y. Our data indicates an interfacial energy of 280 + 90 mjm- for Arkansas quartz at 300°C, which compares well with Parks value of 360 mJm for 25°C (10). Similar experiments on other minerals could provide essential surface energy data. 

The formation of etch pits by chiral additives present in solution during dissolution of the crystal. 

Scientists have long recognized that partial dissolution of crystals in the presence of impurities may result in the formation of well-shaped etch pits (60-62). When crystals of calcite, for example, were partially dissolved in the... 

The effect of thr and allothr on the racemic crystals of ser is further manifested independently by dissolution experiments. When rhomblike crystals of ser grown in the presence of (/ ,S)-thr are dissolved in the presence of (fl)-thr, well-formed etch pits develop only at the (011) and (Oil) faces. By virtue of symmetry, similar etch pits form only at the (011) and (Oil) faces when (S)-thr is present... 

Etchants for defect and junction delineation are usually composed of HF and an oxidizing agent such as HN03 [Dal, Gr4, Ka4, Nel], K2Cr207 [Se5] or Cr03 [Sil, Jel, Sc7, Ya4, Me5]. Alkaline solutions are rarely used for defect delineation [Mal2], An etch pit will form on a silicon surface if the dissolution rate is enhanced locally. Enhancement of the etch rate may occur for various reasons ... 

The question arises as to what effects are responsible for the generation of the very first tiny etch pits on the surface. However, this question is misleading in some respects. A flat n-type surface anodized in HF is in an unstable condition. Tiny inhomogeneities of flatness in the electrode surface, inevitably produced by the dissolution process, are amplified, because the concave areas will focus the electric field and thereby become more efficient in collection holes than flat areas. An increase in anodization bias accelerates pore initiation at flat electrodes. 

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