Model Answer
0 min readIntroduction
The albite-anorthite system (NaAlSi<sub>3</sub>O<sub>8</sub> - CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>) is a fundamental binary silicate system widely used in petrology to understand the crystallization behavior of plagioclase feldspars in igneous rocks. It’s a classic example of a simple eutectic and cotectic system exhibiting solid solution between the two end members. Understanding the changes in crystallized solid composition during cooling is vital for interpreting the formation history of igneous rocks. This question asks us to describe the changes occurring in a liquid of An<sub>50</sub> composition as it cools from 1500°C at 1 atm pressure, focusing on the evolving mineral assemblage and their compositions.
Initial State and Liquidus Temperature
At 1500°C, the composition An50 exists entirely as a homogeneous liquid. As the temperature begins to decrease, the liquid reaches the liquidus temperature, which is the temperature at which the first crystals begin to form. For an An50 composition, the liquidus temperature is approximately 1380°C. At this temperature, crystals of both albite (An0) and anorthite (An100) simultaneously nucleate from the melt.
Crystallization and Cotectic Point
As cooling continues below 1380°C, the amount of crystals increases. The composition of the coexisting crystals is not fixed but changes systematically with decreasing temperature. This change is governed by the cotectic point, which is the lowest temperature at which a liquid can coexist with two solid phases. In the albite-anorthite system, the cotectic point is at approximately 1175°C. The tie-line, a horizontal line on the phase diagram connecting the compositions of the coexisting solid phases at a given temperature, plays a crucial role in determining these compositions.
Tie-Line Movement and Compositional Changes
As the temperature decreases from the liquidus to the cotectic point, the tie-line shifts towards the anorthite end member. This means that the albite crystals become progressively more calcium-rich (increasing An content), while the anorthite crystals become progressively more sodium-rich (increasing Ab content). The overall effect is a gradual shift in the compositions of both solid phases towards an intermediate composition.
Detailed Changes with Decreasing Temperature:
- 1380°C - 1250°C: Initial crystallization of both albite and anorthite. The albite crystals are relatively sodium-rich (low An content), and the anorthite crystals are relatively calcium-rich (high An content). The tie-line is positioned such that the compositions of the coexisting crystals are significantly different.
- 1250°C - 1175°C: Continued crystallization, with the tie-line moving towards the anorthite end member. The albite crystals become increasingly calcium-rich, and the anorthite crystals become increasingly sodium-rich. The difference in composition between the coexisting crystals decreases.
- At 1175°C (Cotectic Point): The liquid is completely consumed, and the remaining solid assemblage consists of albite and anorthite with compositions defined by the cotectic point. The compositions of the final crystals are approximately An30 for albite and An70 for anorthite.
Phase Diagram Representation
Imagine a phase diagram with temperature on the y-axis and composition (An content) on the x-axis. Starting at An50 and 1500°C, a vertical line representing cooling intersects the liquidus line, initiating crystallization. As the line moves down, it intersects a series of tie-lines, each representing the compositions of the coexisting albite and anorthite crystals at that temperature. The tie-lines progressively shift towards the anorthite side until the cotectic point is reached, where the liquid disappears.
Impact of Cooling Rate
The cooling rate can influence the texture of the resulting rock. Slow cooling allows for greater equilibration between the liquid and the crystals, resulting in larger, well-formed crystals. Rapid cooling can lead to disequilibrium textures, such as zoned crystals or the retention of a glass phase.
Conclusion
In summary, cooling a liquid of An<sub>50</sub> composition from 1500°C at 1 atm pressure results in the simultaneous crystallization of albite and anorthite. The compositions of these coexisting phases change systematically with decreasing temperature, driven by the movement of the tie-line towards the anorthite end member. At the cotectic point (1175°C), the liquid is completely consumed, leaving a solid assemblage of albite (An<sub>30</sub>) and anorthite (An<sub>70</sub>). Understanding this process is fundamental to interpreting the petrogenesis of plagioclase-bearing igneous rocks.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.