Feldspars dislocations

Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. 

Numerous surface characteristics may play a role in determining mechanisms, rate-limiting steps, and rates of feldspar dissolution during weathering (as discussed above). These include A. Adhered fine particles B. Defects and dislocation outcrops C. 

The range of variability in feldspar weathering rates due to fine particles, strained surfaces, defects and dislocation outcrops, and surface layers is too small to account for the... 

At least a dozen slip systems have been identified by TEM in experimentally and naturally deformed feldspars (see Gandais and Willaime 1984). In many cases, the dislocations are dissociated, though the separation of the partial dislocations is usually small (<50 nm). The dissociation of dislocations of b = [100] gliding in (010) in experimentally deformed sanidine was first observed by Kovacs and Gandais (1980), who suggested the following reactions ... 

Olsen and Kohlstedt (1984) analyzed the dislocations in some naturally deformed intermediate plagioclase feldspars. All the known Burgers vectors except b = [100] were identified, and most, perhaps all, dislocations were dissociated by up to 20 nm. The microstructure was dominated by screw dislocations of b = [001], which had dissociated in (010) probably according to the reaction... 

Olsen, T. S., Kohlstedt, D. L. (1984). Analysis of dislocations in some naturally deformed plagioclase feldspars. Phys. Chem. Minerals, 11, 153-60. 

Yund R. A., Quigley J., and Tullis J. (1989) The effect of dislocations on bulk diffusion in feldspars during metamorphism. J. Metamorph. Geol. 7, 337-341. 

Fig. 4.6 Scanning electron micrograph showing square-shaped etch pits developed on dislocations in a feldspar from a southwestern England granite. Note that in places the pits are coalescing, causing complete dissolution of the feldspar. Scale bar= 10 pm. Photograph courtesy of ECC International, St Austell, UK.

Murphy, W.M. 1988. Dislocations and feldspar dissolution Theory and experimental data. Chem. Geology 70 163. 

Fig. 7. Present and original detrital composition of representative Lunde sandstones plotted on McBride s (1963) diagram A, considering albitized feldspars as diagenetic constituents (not in F pole)—observe the shift from the arkose (original composition white dots) to the subarkose field (present composition black dots) B, plotting albitized feldspars in F pole—a less substantial field dislocation is seen from original (white squares) to present composition (black squares).

Moreover, numerical solutions of Eq. 11 for calcite, quartz, and feldspars show that after a short transient period (a few minutes for calcite), the rate of dissolution remains roughly constant. Our calculations (see Table 4) indicate that during this steady-state period the overall dissolution rate should increase by a factor of only 3 or 4 at maximum due to two competing effects decreasing surface strain energy and increasing surface area, as dislocation cores dissolve ind widen. 

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. 

Analyses of dislocations in experimentally and naturally deformed feldspars by TEM indicate that the dominant slip plane is (010). The (010) plane may be favorable due to its low density of largely covalent Si(Al)-0 bonds (2 per unit cell, [57-59]). With this criterion, other possible slip systems include (001), (110), and (1 0 i) (with 4 Si(Al)-0 bonds per unit cell), (100), and (111) (with six 4 Si(Al)-0 bonds per unit cell). [001] is the dominant Burgers vector, which is supported by TEM analyses of experimentally and naturally deformed samples (see [60-62] but many other dislocations have also been identified. 

Experimental studies of alkali feldspars in Westerley granite with ordered microcline [63,64] and disordered sanidine [65-68] show that (010) [001] shp is active in both. Other dislocations derive from (010)[101] [Fig. 15(a)], (001) [110], and (121) [101] shp. In naturaUy deformed alkali feldspars, subgrain formation was observed, indicative of climb [357-360]. It has been suggested that shear-induced mechanical Albite and Pericline twinning in potassium feldspar may facilitate ordering [361] but this has been disputed [362]. Dislocations have no effect on diffusion in alkah feldspar [363]. 

The effect of dislocations on the reactivity of minerals has been investigated mainly for quartz with a few studies on feldspar and calcite, both experimentally and theoretically. The strain energy in a crystal lattice caused by dislocations can be described using ideal elastic behavior [4,453,454]. The increase in the internal energy of quartz caused by dislocations can be calculated by the following equation [453] ... 

An increase by 2 to 3 orders of magnitude in dislocation density has been shown to increase the dissolution rate by as much as a factor of 3 and this is highly significant for the stability and dissolution of quartz [458,459], of feldspars [363,460] and of carbonates [461]. Based on strain energy produced by dislocations in snbgrain boundaries, Twiss [462] developed a theory to use the recrystallized grain size as a paleopiezometer. 

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