Quartz, dislocation etch pits

Brantley S. L., Crane S. R., Crerar D., Hellmann R., and Stallard R. (1986) Dissolution at dislocation etch pits in quartz. Geochim. Cosmochim. Acta 50, 2349-2361. 

Dissolution at dislocation etch pits in quartz. Geochimica et Cosmochimica Acta, 50, 2349-2361. 

Dislocation etch pits in quartz. In Geochemical Processes at Mineral Surfaces, eds. Davis, J.A., Hayes, K.F. American Chemical Society, Washington D.C., pp. 635-649. 

For C crit t 161"6 ls a double root to the maximization equation, and there is an inflection point in the AG function (curve D on Figure 1). Since there is no activation barrier to opening up the etch pit, any pit nucleated at a dislocation should open up into a macroscopic etch pit. Similarly, for C < Ccr t, there are no real solutions and no maxima and minima in the A G function, and nucleated pits open up into etch pits. At 300°C, the calculated Ccr t for quartz equals 0.6CQ. 

A very accurate measurement of Ccrjt would allow back-calculation of the surface energy for a given crystal. Because Ccrjt is dependent on the square of Y, such a measurement could be a very sensitive method of measuring interfacial energy at dislocation outcrops. The calculated interfacial energy from our experiments is 280+ 90 mJm- for the rhombohedral face of quartz at 300°C. Parks (10) estimated 25°C value of 360 + 30 mJm is well within the experimental error of our measurement. The best way to determine the value of Ccrjt would be to measure etch pit nucleation rate on... 

In general, the shape and character of etch pits may reveal information about the impurity content of the crystal. "Beaked pits (pits with curved apexes, see 12) can indicate impurity haloes. Some forms of the arcuate etching we observed in quartz (16) may be examples of beaking. Very shallow pits can form at aged dislocations while very deep pits form at new dislocations. "Aging" may be related to impurity diffusion in the crystal lattice. 

Fig. 7-10. Graphical results of a Monte Carlo simulation of progressive dissolution and etch pit formation around a screw dislocation in quartz. Etch profiles illustrate the removal of high energy material adjacent to the dislocation with time (reprinted wilh permission from Blum and Lasaga, 1987 Copyright 1987, John Wiley Sons.)...

These findings, together with the observation that etch pits are developed in a similar manner on both deformed and undeformed samples of feldspar and calcite (e.g., see Murphy, 1989), indicate that etch pits may only be weakly related to dislocations. Probably, the dense etch pitting observed in natural samples of quartz and silicates must reflect their aqueous chemical environment (i.e., the presence of ligands, which considerably enhance dissolution) and the presence in these solids of localized chemical impurities such as aluminium, which favor the specific adsorption of F and organic ligands as oxalate, silicilate, and similar. This specific adsorption on chemical impurities may result in localized enhancements of dissolution as illustrated by Figure 17. 

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