Binary Phase Diagrams in Crystallisation

Questions and Answers

What is the phase rule for binary systems when pressure is constant?

F = C + P + 1

F = C - P + 1 (correct)

F = C + P - 1

F = C - 2 + 1

What is the composition of the crystals in equilibrium with the melt at Tliquidus?

An65

An75

An85

An90 (correct)

Which of the following best describes a binary system?

A system defined using only 3 components

A system composed of exactly 2 components (correct)

A system that cannot change its phases

A system with a single component over varying temperatures

What does the equilibrium relationship in a binary phase diagram indicate about plagioclase and its coexisting melt?

Plagioclase is always more calcic than the melt. Signup and view all the answers

What factors are kept constant while constructing binary phase diagrams?

Pressure and temperature Signup and view all the answers

In a binary phase diagram, which axes are typically represented?

Temperature and Composition Signup and view all the answers

At Tsolidus, what is the composition of the melt in equilibrium with the crystals?

An50 Signup and view all the answers

During the crystallization of magma in a binary phase diagram, which of the following occurs?

The crystallization sequence affects the composition of the remaining melt. Signup and view all the answers

Which component interaction is represented in the plagioclase system example?

Complete solid solution occurs between two components Signup and view all the answers

How many phases can a binary system typically consist of?

2 to 4 phases Signup and view all the answers

What is the overall trend observed in the plagioclase system concerning solid and liquid composition?

Solid compositions are more calcic than liquid compositions. Signup and view all the answers

What does the axis labeled 'X' in a binary phase diagram typically represent?

Percentage of one of the components Signup and view all the answers

What is a characteristic of a one-phase binary system?

It can show high variance in composition Signup and view all the answers

Which phase has the highest compositional value of An in the given plagioclase system?

Plag (An90) Signup and view all the answers

What is the significance of Tsolidus in the context of the plagioclase system?

It represents the temperature at which a solid begins to fully melt. Signup and view all the answers

In the plagioclase system, what does it mean that 'plag is always more calcic than coexisting melt'?

Plag maintains a higher content of calcium than the melt. Signup and view all the answers

At which point is the melt (An75) observed in the plagioclase system?

Tliquidus Signup and view all the answers

What does Tliq/sol represent in the context of the binary phase diagram?

The temperature at which solid and liquid co-exist. Signup and view all the answers

What does zoned plagioclase in granite indicate about the cooling process of magma?

The evolution of crystal and melt compositions occurs as magma cools. Signup and view all the answers

What is implied by oscillatory zoning in plagioclase crystals?

Fluctuations in composition occur from core to rim, indicating changes during cooling. Signup and view all the answers

Which parameters might change during cooling to explain reversals in zoning patterns?

Both pressure and temperature may influence zoning changes. Signup and view all the answers

What is a characteristic of the extinction angle in zoned crystals?

It varies according to the changing composition of the crystal. Signup and view all the answers

How does zoning in rocks like andesite relate to magma cooling?

Zoning can reveal the temperature and composition changes of the melt during cooling. Signup and view all the answers

At what temperature does the melt solidify when producing An75 plagioclase?

1100°C Signup and view all the answers

Which composition represents the final plagioclase composition during equilibrium crystallization?

An75 Signup and view all the answers

What happens to the composition of plagioclase as it continuously reacts with melt?

It becomes zoned. Signup and view all the answers

What is the initial melt composition when starting the crystallization process?

An75 Signup and view all the answers

During equilibrium crystallization, the composition of plagioclase crystals is equal to what?

Bulk composition Signup and view all the answers

At what crystallization stage does plagioclase become An0?

Tfinal Signup and view all the answers

Which phase is present at the liquidus temperature during the crystallization process?

Plagioclase at An90 Signup and view all the answers

What characterizes the plagioclase composition in relation to the coexisting melt?

It is always more calcic. Signup and view all the answers

Study Notes

Binary Phase Diagrams Overview

Binary systems involve only two components and typically exhibit a small number of phases (2 to 4).
Relevant to crystallization processes from magma, focusing on melt (L) and crystals (Xls).

Phase Rule and Diagrams

Given composition (C) = 2, the maximum variance (F) can be 3 in a single phase system.
Pressure is kept constant; changes are made to temperature and composition.
The phase rule simplifies to F = C - P + 1 under constant pressure conditions.

Plagioclase System Example

The plagioclase system demonstrates complete solid solution where components mix thoroughly.
Major components: Na+, Ca2+, and Al3+.
Evolving compositions in plagioclase from melt lead to zoned crystals in igneous rocks.

Equilibrium vs. Fractional Crystallization

In equilibrium crystallization, plagioclase continuously reacts with the melt, maintaining composition balance.
Final plagioclase composition equals bulk and initial melt composition, solidifying at ~1100°C.
Fractional crystallization can cause zoned plagioclase due to variations in melt and crystal compositions as cooling progresses.

Crystallization Phases and Composition

Initial phase: melt (An75) transitions to various phases as temperature decreases.
As temperature decreases, compositions change:
- Liquidus: melt (An75), plagioclase (An90)
- Tliq/sol: melt (An65), plagioclase (An85)
- Tsolidus: melt (An50), plagioclase (An75)
- Final composition at Tfinal: plagioclase (An75).

Zoning in Plagioclase Crystals

Zoning is characterized by variable extinction angles reflecting different compositions.
Oscillatory zoning indicates fluctuations in An (anorthite) and Ab (albite) percentage during crystallization.
Changes in other parameters like composition (X) or pressure (P) during cooling explain fluctuations in crystal formation.

Application and Importance

Understanding binary phase diagrams aids in predicting mineral stability and composition in geological processes.
Key in igneous and metamorphic petrology for interpreting rock formation and evolution.

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Description

Explore the complexities of binary phase diagrams and their significance in crystallisation from magma. This quiz focuses on systems with two components and the variance in phases. Understand how to illustrate these systems through three-dimensional diagrams.