System plagioclase

Fig. 2.37. Phase diagram for Ca0-Na20 Si02-(Al203)-H20 system in equilibrium with quartz at 400°C and 400 bars. Plagioclase solid solution can be represented by the albite and anorthite fields, whereas epidote is represented by clinozoisite. Note that the clinozoisite field is adjacent to the anorthite field, suggesting that fluids with high Ca/(H+) might equilibrate with excess anorthite by replacing it with epidote. The location of the albite-anorthite-epidote equilibrium point is a function of epidote and plagioclase composition and depends on the model used for calculation of the thermodynamic properties of aqueous cations (Berndt et al., 1989).

Hidaka H, Ohta Y, Yoneda S, DeLaeter JR (2001) Isotopic search for live Cs in the early solar system and possibility of Cs- Ba chronometer. Earth Planet Sci Lett 193 459-466 Hinton RW, Bischofif A (1984) Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites. Nature 308 169-172... 

Figure 7.13 Compositional trends in plagioclase binary system during crystallization in open or closed systems.

The system is not strictly ternary, because of the limited solubility of AI2O3 in diopside, but we will neglect this minor complication here. Because the maximum number of phases in the system is three (diopside plus plagioclase mixtures plus liquid) and the components also number three, the minimum variance of the system is 1, at constant P. 

Drake M. J. and Holloway I R. (1978). Henry s law behavior of Sm in a natural plagioclase/ melt system Importance of experimental procedure. Geochim. Cosmochim. Acta, 42 679-683. 

Seek H. A. (1971). Koexisterend Alkali-feldspate und Plagioclase im System NaAlSi30g-KAlSi308-CaAl2Si208-H20 bei temperaturen von 650°C bis 900°C. N. Jahrb. Min. 

Muncill G.E. and Lasaga A.C. (1987) Crystal-growth kinetics of plagioclase in igneous systems one-atmosphere experiments and application of a simplified growth model. Am. Mineral. 72, 299-311. 

Tsuchiyama A. (1985a) Dissolution kinetics of plagioclase in the melt system of diopside-albite-anorthite and origin of dusty plagioclase in andesites. Contrib. Mineral. Petrol. 89, 1-16. 

Figure 2. pK for equilibrium between Ca-beidellite and kaolinite plotted as a function of Na content of Sierra Nevada spring waters. Arrow ABC is the path of water evolution calculated for the reaction from Na0 62Ca038 plagioclase to kaolinite in a closed system with an initial dissolved C02 of 0.0006 moles per liter. Arrow ABD is the expected path if evolution is also controlled by the two-phase equilibrium kaolinite-montmorillonite... 

Jessberger, E. K., Ostertag, R. (1982) Shock effects on the K-Ar system of plagioclase feldspar and the age of inclusions from North-eastern Minnesota. Geochim. Cosmochim. Acta, 46, 1465-71. 

More recently, Montardi and Mainprice (1987) made a detailed TEM study of dislocations in naturally deformed calcic plagioclases (An6 -7o)-As in the specimens studied by Olsen and Kohlstedt (1984), the microstructure was dominated by the slip system (010) [001]. The [001] dislocations dissociated according to the reaction just given, the separation being about SO nm. The pair of gliding partial dislocations left behind a fault characterized by fringes of low contrast, as previously discussed. Image... 

Pb-Pb age of LEW and ADOR (Nyquist et al., 2003). The same authors found Mg in plagioclase and constructed a two-point Al-Mg isochron that implies A1/ A1 = (2.3 0.8) X 10 at crystallization. This low value is reasonably close to that expected based on A1 decay from the canonical ratio given an absolute age 4,561 Ma. If these preliminary results hold, they will represent a new point of concordance between the Al-Mg and Mn-Cr systems in igneously differentiated meteorites. 

---