Albite disorder

Figure 5,54 (A) Cationic occupancies in tetrahedral positions in case of complete disorder (monoclinic structure upper drawing) and complete order (triclinic structure lower drawing). (B) Condition of complete order in microcline and low albite with AliSi = 1 3, compared with cationic ordering in anorthite (AhSi = 2 2). Note doubling of edge c in an-orthite.

According to equation 5.170, the completely disordered form of the NaAlSi303 end-member (i.e., monalbite, = 0) has an enthalpy about -1-2.6 kcal/mole higher than that of the fully ordered form (low albite, = 1) as a result of simple substitutional effects. 

Salje (1985) interpreted overlapping (displacive plus Al-Si substitutional) phase transitions in albite in the light of Landau theory (see section 2.8.1), assigning two distinct order parameters Q n and to displacive and substitutional disorder and expanding the excess Gibbs free energy of transition in the appropriate Landau form ... 

Coefficients a, B, and C in equation 5.175 have the usual meanings in the Landau expansion (see section 2.8.1) and for the (second-order) displacive transition of albite assume the values = 1.309 cal/(mole X K) and B, = 1.638 kcal/mole (Salje et al., 1985). is the critical temperature of transition = Bla = 1251 K). The corresponding coefficients of the ordering process are = 9.947 cal/(mole X K), B = -2.233 kcal/mole, = 10.42 kcal/(mole X K), and = 824.1 K. With all three coefficients being present in the Landau expansion relative to substitutional disorder it is obvious that Salje et al. (1985) consider this transition first-order. A is a T-dependent coupling coefficient between displacive and substitutional energy terms (Salje et al., 1985) ... 

Note that, displacive disorder being associated with a first-order transition, exhibits a jump at whose magnitude decreases from fully ordered albite (i.e., = 1) to... 

Helgeson et al. (1978) have shown that the enthalpy of disordering in sanidine is similar to that observed in albite—i.e.. 

As a matter of fact, the volumes of NaAlSi308-KAlSi308 mixtures are not ideal and have positive excess terms. Deviations from simple Vegard s rule proportionality are mainly due to edge h Kroll and Ribbe (1983) provide three polynomial expressions relating the volumes of mixtures to the molar fractions of the potassic component Xor) valid, respectively, for monoclinic symmetry (complete disorder), triclinic symmetry of perfect order (low albite and maximum micro-cline), and the intermediate structural state ... 

Reactions 7 and 8. Thermobarometric estimates are complicated by order-disorder transition in albite and by the difficulty of evaluating the activity of jadeite and Ca-Tschermak in natural clinopyroxenes (cf Essene, 1982). 

It is observed that in the case of simultaneous saturation of two or more phases, the phase that forms first is often the least stable, or the most disordered, especially at room temperatures. For example, in aqueous solutions, opal (disordered) often forms but the more stable quartz rarely forms. Over a very long time, opal may "mature" to become quartz. The same is true for the formation of calcite (as compared dolomite), and analbite (as compared to albite). From the vapor phase, phosphorous vapor condenses first to yellow phosphorus (high entropy), instead of the more stable red phosphorous (low entropy)... 

Sanidine is monoclinic (space group C2/m), and there is complete disorder in the occupation of the tetrahedral (T) sites by the A1 and Si atoms. Over geological time, ordering takes place. In low (or maximum) micro-cline, the ordering is complete (all A1 in TiO sites), and the symmetry is reduced to triclinic (CT). There are four main orientational variants in this structure two orientations related by the albite twin law (rotation of 180° about b ) and two orientations related by the pericline twin law (rotation of 180° about b). The composition planes of these two twins are, respectively, (010) and the rhombic section which is parallel to b and approximately normal to (001). Thus, the characteristic cross-hatched pattern observed in (001) sections between crossed-polarizers in the optical microscope has, for many years, been simply interpreted as intersecting sets of albite and pericline twin lamellae formed at the monoclinic-to-triclinic transformation. However, TEM observations indicate that this model is too simple. Because these observations, collectively, also constitute an excellent example of the application of the principal modes of operation of TEM to a specific mineralogical problem, we discuss them in some detail. 

Having discussed some of the background to the models that have been used to underpin studies of order-disorder in minerals, let us finally turn to a few of the main results from studies of strain-order-disorder coupling that have been reported for a number of phases. It is worth first considering the case of the high-temperature behaviour of albite (NaAlSi308), as this mineral provides, in many respects, the archetypal example... 

A further consequence of the interplay between the kinetically controlled Qod and spontaneous strain in albite is that, in an ordering or disordering experiment out of equilibrium, a distribution of Qod states exists which can be observed in the evolution of diffraction peaks susceptible to changes in the y angle (Carpenter and Salje 1989). Furthermore, in kinetically disordered crystals there is a tendency to form spatially non-uniform distributions of the order parameter, and modulated tweed microstructures typically develop. Albite has the distinction of being one of the first... 

Hard mode infrared spectroscopy indicates that Pi character is absent even at the microscale when Ca contents fall below Ange (Atkinson et al. 1999). Moreover, the character of the transition changes in response to anjncreasing albitic component. Endmember Ca-anorthite transforms tricritically from Pi to / I symmetry, but with an increase in Na+Si, the transition becomes second-order (Redfem 1990). Interestingly, Redfern et al. (1988) and Redfern (1992) present evidence that an increasing albitic component acts to increase critical temperatures of anorthitic solid solutions when samples are equally disordered with respect to Al and Si. Th effects of Al-Si order/disorder are addressed by Redfern in a chapter in this volume. 

Yang W-H, Kirkpatrick RJ, Henderson, DM (1986) High-resolntion Si A1, and Na NMR spectroscopic stndy of Al-Si disordering in armealed albite and oligoclase. Am Mineral 71 712-726... 

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]. 

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