Phase Rule | Geology Optional Notes for UPSC PDF Download

5.2 PHASE RULE

• Understanding System Components

  • System refers to a part of the universe that can be isolated for study. There are three types of systems:
  • Open System: Allows transfer of energy and matter to and from the surroundings.
  • Closed System: Permits exchange of only energy like heat with the surroundings.
  • Isolated System: Prevents transfer of both energy and matter with the surroundings.

• Defining Phases and Surroundings

  • Phase is a homogenous part of a system separated by interfaces from other phases.
  • Surroundings are the immediate area outside the system being studied.

• Explaining Variance and Degrees of Freedom

  • Variance (Degrees of Freedom) is the minimum variables needed to define a system's equilibrium state.
  • A system's state involves properties like temperature, pressure, composition, mass, and volume.

• Introducing Gibbs Phase Rule

  • Developed by J. Willard Gibbs, this rule determines the variance (f) of a system.
  • It's applicable to heterogeneous reversible reactions in equilibrium.
  • Other reversible reactions are studied using the "law of action."

Phase Rule

• Phase Rule governs the number of phases in equilibrium in a system.
• Expressed as F = C - P or F + P = C
• Where:
  • P = Number of phases
  • F = Degree of Freedom
  • C = Components
• Phases are physically distinct in a system based on composition, structure, and state.
• Examples:
  • Matter
  • Ice
  • Ice + water
  • Two pieces of ice

Components and Phases

• Components are the minimum chemical species needed to define a system and its phases.
• Components are usually expressed as proportions of oxides.
• Example: Ice and water have 1 component (H2O) and 2 phases.
• Number of components depends on system behavior and study conditions.

Phase Diagram

• Graphical representation of pressure and temperature conditions for phases.
• Shows areas, curves, lines, and points indicating phase stability.
• Phase diagrams illustrate relationships between phases under different conditions.
• They show stability fields and phase relationships based on pressure, temperature, and composition.
• Used to study crystallization and melting behaviors of a magma under specific conditions.

Igneous Petrology: Phase Equilibrium Diagrams

Phase Equilibrium Diagrams

• Phase equilibrium diagrams illustrate the constitution of alloys under equilibrium conditions concerning temperature.
• These diagrams are vital in understanding the behavior of substances at different temperatures.

Components of Phase Diagrams

• Melting points of components such as A and B are critical in determining the temperature at which transitions occur.
• The diagrams showcase the solid-liquid-gas behavior of substances, delineating fields for solid, liquid, and gas phases.

Understanding Phase Boundaries

• At high temperatures, the system predominantly exists in the molten state, separated by the liquidus line.
• The liquidus line marks the boundary between liquid and liquid-solid phases, where 100% melt is present.
• Conversely, the solidus line denotes the boundary at low temperatures, where the system is entirely solid.
• Between the solidus and liquidus, equilibrium phases of both liquid and solid crystals coexist.

Phase Diagram Illustrations

• Phase diagrams visually represent the behavior of a unicomponent system, indicating critical and triple points.
• Temperature is typically depicted on the Y-axis, while composition is shown on the X-axis.

By comprehending these phase equilibrium diagrams, geologists and scientists can interpret the transformations and stability of materials at varying temperature conditions effectively.

Unit 5: Crystallisation of Magma

Crystallisation of Magma Overview

• The crystallisation of magma involves a system where temperature (T °C) is plotted on the X-axis and pressure (P atm) on the Y-axis.
• Phases like solid, liquid, and gas are separated by three distinct curves.
• Triple point signifies the temperature and pressure at which solid, liquid, and vapor phases of a pure substance coexist in equilibrium.
• The critical point denotes the endpoint of a phase equilibrium curve, where two phases of a substance become indistinguishable.

Earth's Interior and Magma Generation

• Geologists have limited direct access to the Earth's interior, relying on indirect sources like melts of subsurface materials, geophysical studies, and high-pressure experiments for information.
• P and T conditions influencing magma formation and modification are inferred from real mineral melting experiments conducted in controlled laboratory settings.

Condensed Phase Rule

• When either pressure (P) or temperature (T) is held constant, the condensed phase rule can be applied.
• The formula for the condensed phase rule is F = C - P + 1, where the total number of variables reduces to 1 as only one intensive property (P or T) can vary.
• This rule aids in comprehending isobaric T (temperature) - X (composition) or isothermal P (pressure) - X (composition) diagrams and experimental geochemistry.
• Experimental scenarios where either pressure or temperature is fixed help analyze how a system depends on the other intensive variable.