Crystallographic etch pits

One proposal for IGC is related to crystallographic etch pit formation at grain boundaries. According to this theory, grain boundary susceptibility is related to the crystallographic facet exposed on the boundary and the different rates of dissolution of such facets. Bicrystal studies as... 

Figure 1.53 shows diagrammatically various types of pits that can range from hemispherical with a polished surface, in which crystallographic etching has been completely suppressed, to crystallographic pits whose sides are composed of the crystal planes that corrode at the slowest rate. Pits formed on Ni during anodic polarisation in an acetic acid-acetate buffer of pH 4-6 are shown in Fig. 1.54. 

Figure 19. Schematic drawing of cross sections of two types of pits developing in pitting corrosion of passive metals (a) geometric pit (b) crystallographic or etch pit.

Wilson (28) noted the presence of etch pits (crystallographically controlled voids or features of negative relief, or "negative crystals") on some soil feldspars, and reviewed similar observations from earlier studies. Some examples of etch pits on naturally weathered feldspars are shown in Figure 3- Etch pits... 

We have seen above that the kinetics of mineral dissolution is well explained by transition-state theory. The framework of this theory and kinetic data for minerals have shown that dissolution is initiated by the adsorption of reactants at active sites. Until now these active sites have been poorly characterized nevertheless, there is a general consensus that the most active sites consist of dislocations, edges, point defects, kinks, twin boundaries, and all positions characterized by an excess surface energy. Also these concepts have been strongly supported by the results of many SEM observations which have shown that the formation of crystallographically controlled etch pits is a ubiquitous feature of weathered silicates. 

Recent advances in applying transition state theory to geochemical kinetics (SQ, SD have emphasized the interaction of the activated complex with specific surface reaction sites. The rate of reaction is assumed to be a function of the surface reaction site density. A correspondence is also observed between surface dissolution features such as etch pits, and crystallographically controlled extended defect features such as edge and screw dislocations (S2). Based on these lines of evidence, the reactive surface area has been proposed to be proportional to the defect density within minerals... 

Fig. 4.8 Etch pits on the a-b surface plane of a tetracene crystal, after etching with oleum (H2S2O7). The edges of the pits correspond to the crystallographic directions 110 and 110. The number density is ca. 10 cm . From ). Niemax, Dissertation,...

The quartz, albite and anorthite samples underwent dissolution, often along preferred crystallographic directions to form etch pits visible by SEM. The phyllosilicate reactants were too fine grained for any dissolution textures to be visible by SEM. New calcium aluminosilicate phases precipitated abundantly in the anorthite, chlorite and muscovite samples. The resolution of the SEM analytical facility was not sufficiently high to obtain quantitative chemical data for these phases, whereas good quality chemical data were obtained for them by ATEM (Tables 3 and 4). 

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