Solid Solutions & Phase Diagrams

These flashcards cover key vocabulary related to solid solutions and phase diagrams, including concepts from thermodynamics, crystallization processes, and mineral stability.

Gibbs Free Energy

A way to express the relative stability of a mineral in specific pressure (P), temperature (T), and composition (X) spaces.

ΔGrxn

The Gibbs free energy of a reaction, calculated as the difference between the Gibbs free energy of the products and reactants.

Negative ΔGrxn

Indicates that a reaction will proceed to the right.

Positive ΔGrxn

Indicates that a reaction will proceed to the left.

Aragonite

A polymorph of CaCO3, which can transform to calcite under specific conditions.

Calcite

A mineral form of CaCO3 that can exist alongside aragonite when Gibbs free energy (ΔG) is zero.

Gibbs free energy of formation (∆G˚f)

The amount of energy released when pure elements react to form one mole of a mineral.

Free energy difference (ΔGf)

The difference between the free energy of an element in standard state and when it is bonded in a mineral structure.

Stable form

The mineral with the lowest free energy under specified P,T conditions.

Solid solution

A homogeneous crystalline phase where two or more elements can substitute for one another without changing the crystal structure.

Liquidus

The temperature above which a rock or mineral is completely molten.

Solidus

The temperature below which a rock or mineral is completely solid.

Fractional crystallization

A process where crystals that form are separated from the melt, preventing further chemical interaction.

Equilibrium crystallization

Occurs when a melt and its crystallizing solid remain in continuous chemical communication.

Metastable phases

Phases that can persist under certain conditions even if they are less stable.

Polymorphs

Different structural forms of the same mineral, such as α-quartz and β-quartz.

Reconstructive transformation

A transformation where bonds are broken and new ones are formed, requiring an energy barrier to overcome.

Phase diagram

A graph representing the stable phases for a given composition under specific pressure and temperature.

Lever rule

A rule used to determine the relative amounts of phases in a two-phase system based on segment lengths of a tie-line.

Tie-line

A line connecting the compositions of two phases on a phase diagram.

Mafic magma

Magmas that are high in magnesium and iron content.

Felsic magma

Magmas that are high in silica and aluminum content.

Continuous solid solution

A range of compositions between two end-member minerals, like Forsterite and Fayalite in olivine.

Pressure-temperature (P-T) conditions

Conditions that influence mineral stability and transformation.

Phase boundary

The P-T conditions where two phases can coexist.

Thermodynamic tables

Reference tables that provide Gibbs free energy values for various minerals.

Mineral substitution

The process by which one element in a mineral can be replaced by another without changing the overall structure.

Crystallization

The process of forming solid crystals from a liquid or gas.

Olivine

A mineral that commonly crystallizes first in basaltic magmas.

Silica-rich melt

Melt that has a higher proportion of silica after early-forming minerals are removed.

Chemical equilibrium

A state in which the composition of a crystallizing solid remains balanced with that of the liquid.

Crystals

Solid materials made up of atoms arranged in an orderly repeating pattern.

Dynamic equilibrium

An ongoing state in which conditions remain balanced over time.

Thermodynamic stability

A measure of a mineral's tendency to maintain its form under specific conditions.

Broader geological environments

Different physical and chemical settings where minerals can form or exist.

Phase behaviour

The study of how different phases interact and transform under varying conditions.

Crystallization path

The sequence of mineral formation as a melt cools.

Residual melt

The remaining melt after some minerals have crystallized out.

Incompatible elements

Elements that do not fit well into the structure of certain minerals and remain in the melt.